How to setup and bias a Class A amp: the Vox AC10
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- čas přidán 15. 08. 2020
- Modern mass-produced amps need a bit of attention to unleash their true sound potential. This Chinese-made Vox AC10 is one of my favourites. During this video, I will show you how to check the dissipation at idle of the power stage of a Class A amp and we will see why modern manufacturers invariably set the bias too hot, and what we can do to tame them.
If you agree with my partner who glazes over when I start “nerding out”, you can skip to one of these scenes:
3:00 How does Cathode bias work?
6:50 Power supply, Filter Capacitors, and the myth of rectifier Sag
9:45 How to calculate the power dissipation of an EL84 tube
14:50 Kafe and Paximathi
15:30 Matching tubes for Plate current and Transconductance
25:20 Reduce the gain with a 5751 preamp tube
27:00 Reduce the gain even more with a 12AY7 preamp tube
30:00 The Hunt for the Rattle
36:00 The Weber DT10P speaker option
38:00 The Sound Check
BEWARE THE GRIM REAPER: Before you try this on your amp, know that there will be parts of our beloved amp that will be intentionally trying to kill you. Sure, the villains will look innocent enough, but touch them and that foul demon will throw you across the room with deadly force.
PS. If you need your amp repaired in Sydney Australia, give me a call. My details are on my website guitaramptech.com.au/ - Hudba
So interesting to see the behind the scenes... loving the sound of my AC10! Thanks again Chris :)
Grids and Dots, I am so happy you liked what I did with your little gem 😁. You gave me a good brief, so I knew where I had to go with it.
I own my AC10 for almost 7 years now. It just so happened that it started making humming noises when idling and I thought it could be a broken Tube so I looked it up on CZcams.
Found your video and now I’m mind blown 🤣
I need to find an amp tech in Germany to do that kind of work for me.
You addressed exactly the things I noticed on my amp 🤯
Im kinda sad I can’t give the video more than one thumbs up.
Thanks a lot for this very entertaining and interesting video. I’ll make sure to come back for more ☺️
vielen danke Meik
You’re welcome ☺️
Excellent video, as usual. Whilst I enjoy various videos about vintage amp repair, it's nice to see work on a PCB-based amp as well. Strong work!
Thank you Andy. Not many people, including me, like working on PCBs, but it is the way of the future, and present. So, we have to come to terms in knowing that there will be a lot of dismantlying with fragile connectors waiting to trap the unwary.
This is one of the best tech videos I've seen for my taste. Keep the vids coming!
Wow! Thank you. I will try to be more regular, but the videos are fitting in between my customer's repair work. Maybe I'm too fussy with editing ... and sound .. and lights ..
Brilliant! Followed all the way through. Thank you for a clear and concise tutorial. Liked and subbed.
Gee, thanks Archtops. That is very kind of you.
Absolutely invaluable work, thank you! Will consider these adjustments if I end up getting the AC10.
Thank you for your kind words, Patrick, I’m glad you got something from the videos. The AC10 is one of my favorites. The Weber speaker is a great addition if it’s in your budget. Let me know how you get on.
Hi Chris, The voxAC10 is turning out to be a great little amp, thanks for the upgrade tips. Tragically my aftermarket Webber speaker stopped working for reasons as an electronics Layman such as myself cant determine, that's why I only watch your vids for the humor.
I do know that Webber speakers are relentlessly tested in their Indiana factory and they very, very rarely blow. I was amazed how quick Webber agreed to send me a replacement Webber speaker including o/seas postage from the US to WA. I am actually quite stunned and would just like to note on your Vox AC 10 vid comments section here because you recommended the Webber in this upgrade, that I have the utmost respect and love for the service and sounds of the Ted Webber Speaker Company, RIP Ted.
thanks
Adam Cherry, WA
Awesome work. I only just came across your channel the other day. Looking forward to more vids.
Thank you for your kind words, Mikulas. I’m glad you got something positive from watching the video.
Well finally completed the mod! I don't know any of the technical jargon. But what I do know. It sounds great! To me, it's a huge improvement in tone. Thank you very much.
I am so glad to hear that Fred. Thank you for letting me know
Thanks, mate! Don't get to say that much in the States. What a gift that you share your talent and experience with us, so we can sop up a bit of the grease. Those of us in the arid west, especially up at a mile above sea level, would argue that a homemade flour tortilla and some cowboy coffee are tough to beat. Of course, whiskey and high voltage don't play well together. Keep these vids coming in whatever flavor you find in the shop. They're a dose of sanity in a world sorely lacking it.
Thank you so much for your kind words, Bondoloso. I am so happy that you are getting some benefit from my videos.
I have been blessed to do many shows in the USA in my younger days and I love that country, and the wide range of foods in each region. Well, now the coffee...um...I won't say anything, but if you ever get to Sydney or Melbourne, you'll be in for a treat. LOL :-)
Great vid matey. Certainly correct about a lot of these modern amps running the power tubes super hot.
Well said, Stefan. I think it’s irresponsible of manufacturers who set up their amps to clip at low store volumes.
@@theguitaramptech 100%
I mean, it’s nice and all buying a 10W amp that’s pushing out 14W of music power output. But I’d rather they have 10W as advertised and not have to do these sort of maintenance and repairs for years.
My Bassman 50 head is 45 years old. It’s on its second set of tubes and second set of filter caps since new. I bought it from a mate who bought it new in 1975. No one has ever complained about a Bassman. Built well, biased within reasonable range from the factory.
In my experience I’ve found only Laney doing their more recent LT20 and other Lion Heart amps to be running their amps at reasonable plate voltages and bias. Their LT20 is running at 20W with four EL84’s. As we all should be aware, each EL84 is rated to give 5W of music power. Need I do the math. Those amps are the only modern amp I’ve seen thus far that are running tubes at their designed spec. It’s as if the amp designer actually opened the tube data sheet and respected it. 😂
@@whatsstefon very true mate. Those old fender amps like your Bassman were designed for working musicians. Today’s amps are designed by accounts.
@@theguitaramptech ain’t that a fact.
fascinating stuff! reminds me of all the stuff i learned and forgot in my electrical engineering class way back in my college days. Sure wish you lived next door man, my vintage Music Man from the mid 70's could use biasing. i replaced the tubes after all those years and it seems to have lost something. way too intimidating for me to even consider working on, and i'm sure they'll charge me an arm and a leg and my first born if i took it to a shop these days... still sound great but its like a car that can use new plugs and distributor..
thanks again for the informative vids!
Hi Raj. Please be careful with those old MusicMan amps. They have 700v on the plates. Biasing is OK if yours has a tube phase inverter, but if it uses solid state drivers into the cathodes you'd better take it to someone who knows what they are doing.
@@theguitaramptech thank you for the prompt reply my friend. And, oh, no way i'm working on it. I know i'd end up either electricuting myself or screwing up my vintage amp. i probably will do nothing for now. i put the original tubes back in and it sounds good, but not where it should be. the replacement tubes sound ok, but too little headroom, as it distorted at a lower volume. i'll look around for someone who knows these amps well. the biasing seems to be a bit more effort on this amp and i no longer feel confident myself on a job like this. let me know if you're ever in the Southern California USA area🙃
Just got an AC10 and I'm excited to give this a go. I have a nice electrcal supply house near me and now so need to make a shopping list. Sounds great at the end.
Cool! Have fun Bradford and enjoy the end result. Please be sure to discharge the capacitors and unplug the power before you start.
@@theguitaramptech Of course. Not in the mood to feel the tingles from an amp. Haha
@@bciecko1 Sorry, Bradford. Because I don't know you, I felt compelled to keep you safe. Please accept my apologies if my comment offended your actual knowledge.
@@theguitaramptech No offense taken at all! I appreciate the looking out. So many people don't know the punch and amp can carry. You're all good. Warn everyone. Haha
Fantastic advice! Many thanks! 👏👏👏👏👏👏👏
Thanks Solomon! Much appreciated.
This is such a great video thank you
Thank you, Zeshan
The tweaks you made to my AC10 turned it into a very versatile 10 watt home amp. The Webber speaker and upgraded tubes have turned the amp into a great sounding but very versatile Vox. Put a bit of Strymon punch in the pre-amp and it’s a bit of tonal heaven. Clearly I am a very happy customer.
Thank you for your very kind words, Rob. I also enjoyed our mutual journey along the path to Vox Nirvana.
Great, well done videos. Thanks for the good info. By the way, Coffee and Loukoumades are my tasty treat.
YUM!!!
Thanks for this, very informative. FYI Celestion G10 Vintage fits and sounds much better behaved than stock speaker - bit louder, bass holds together better when driven and top end less harsh
That’s excellent feedback. Thanks Johnny
i've done the opposite after some time. Took out the Greenback 10 it came with and put back the factory speaker...
Great information
Thanks, mate.
Very cool guide to cooling a cathode biased amp. However I would have loved a before and after comparison!
That is an excellent suggestion, Gretschen. Unfortunately it comes down to time. My videos take me approximately 2 days to record and edit each video. I earn a few cents per video so I am looking for ways to make producing videos more economic. If I can achieve a shorter time, then I will certainly incorporate your recommendation. Thank you for taking the time to raise the idea.
fun and informative!
Thank you Carlos. Glad you think so!
I changed the V1 to a 12AU7 - did the trick for me ;-)
Oooh, I bet that would be very nice, Christian. I might give that a try, too.
I bought the version which has a Celestion Creamback and JJ tubes. The clean tone sucks, not because it has no headroom but because it simply doesn't sound as good as American amp cleans. The only thing this is good for is hitting it hard with a well tuned Les Paul at full gain and volume with the treble all the way down, the bass at half and the reverb barely on at all. It sounds glorious and a different amp is necessary for good cleans!
Thank you for your email. I understand totally what you are saying. I may have an easy solution for you. The problem is that the first preamp tube is a 12AX7. This has way too much gain for the solid-state Reverb stage that your signal passes through and you know that solid-state clipping is not a pretty thing. Your signal will pass through this Reverb stage regardless of whether you are using the Reverb or not. Order a 12AY7 tube and replace the first 12AX7 (V2). That will help the situation significantly. To further help this situation, I keep the Master high and the Gain lower.
I got that exact amp so thank you sir. Subscribed!
Thank you Mr Bum. I am glad you enjoyed the video. Its a great little amp. I bought one, too, and put in the Weber DT-10P. Expensive to get to Australia, but worth the cost.
@@theguitaramptech by the way the gain knob is on zero during your testing and demo correct (its volume only on 6 or 7)? Thank you!
@@Strumbum01 Hmmm... got me there. I don't recall, but it sounds right. If I am setting up an amp, I will always set it up for maximum Clean so I can assess the bias for clipping or crossover distortion. Good observation, Mr Bum.
@@theguitaramptech just asked cuz I’ll probably try these mods down the road. Thank you for the video and for your responses!
@@Strumbum01 my pleasure
I didn't realize that a pair of push-pull finals would be biased so heavily, I can see why you call it Class A. Also interesting to see modern Chinese cost-cutting and tubes all in one place, and that you are able to tweak it for more manageable control of the gain. I like the EL84s...
Agreed. I like the sound of EL84s. At first, I was a bit concerned about the design and layout of the AC10, but I have to say that it is an excellent compromise between cost and the actual sounds that it produces..you just have to upgrade that speaker.
Great video! Much lot of the lingo went over my head but your passion for electronics is contagious!
With the modifications you made, how much louder is the final result?
I've got one of these AC10s and I'm looking at making it louder for rehearsal context (3 guitars/bass and drums).
Right now it's not quite loud enough, breaks up too early and feedbacks too much.
Are the gains with these modifications + speaker change marginal, or is it significant enough to compete with a rhythm guitarist, lead guitarist, bassist and drummer?
The other option I'm considering is to hook it up to an external cabinet.
Experience? Thoughts?
Thank you Lucas. For me, my AC10 is loud enough for a rehearsal. My improvements may make a small improvement in clean volume...but not enough for you, I think. The sound quality is definitely better. It may be best to save your money and buy an AC15.
Hi. Great Video!!! You need to reference changing the resistor to 5 Watt 150 Ohm 17:13
Thanks Brewer. I appreciate your comment, but I apologise for not understanding what you are suggesting. Did I not make it clear that changing these resistor will change the Bias? That would be a big oversight!
@@BrewerShettles that sounds sensational. What a sound!
Very well done , here are my thoughts why the Valve amps today are running so hot. most Valve amps need a certain output to sound good, so they let the valves work harder today to get decent distorted tones at low Volume and the Valves need to have certain Temperature, but as soon you run them at max Volume this is a no go I use a booster to get more gain so my Valve amp would cry for help using the original setup like on the AC10 you are tweaked.I have an AC15 C2 and at low Volumes it doing great the booster does what he has to do more gain at lower Volumes great tones btw, also I am controlling the gain with my Guitar (BrianMay) style , again great at lower Volumes. At high Volumes the clean sound and slightly distorted sounds changes to quick , I had no change to hear the amp on max Volume yet but I know the valves are getting really hot good for cold Winter days, it warms your heart . But I would like to do the tweaks on the Bias and Input gain stage . Any Ideas how to do it, or can I use the same Method you shown here. Thank you for doing those footage. Oh and I use only the normal Chanel without the Tone controls . I am a simple Man I use only normal Chanel and Master Volume at max everything flat out . As I stated now I can't do this so easy anymore at home. Have y nice Day.
What an entertaining email! Yes, you can use exactly the same method to give your EL84s a more comfortable life and reward you with a little more cleans.
@@theguitaramptech Thank you for your answer. I'm glad I could entertain you in those Days. Have a pleasant Day .
Hi Chris finally a man who knows his valve amps I wonder if you could help me out my uncle passed away many years ago and left his instruments to the family well not knowing that he had did this my dad put it in the cupboard and forgot I started to learn the drums and realised that I have a little talent for music and at 15 I put a band together and found that I love all instruments but always found my uncles ac10 jmi has a lovely clean sound and thats it it's got no guts no gain or drive could you please think of something that I could suggest if I bring it back to a shop thanks James
This was a fantastic video, not many out there for the AC10. I have one where the heat fuse F2 keeps blowing. I'm stumped on fixing. Resistors all seem ok but I didn't consider the amp running too hot, perhaps this is the reason..
Probably not, Dan. Check the 2 heater balance 220R resistors R18 and 19 are OK. Then pull out all the tubes. If the fuse doesn't blow, then start reintroducing the tubes one by one. When you pop the fuse, you have found your problem. As always, discharge the filter capacitors and unplug the amp before you do any work or you may suffer a small case of death and decide that Covid wasn't so bad after all. LOL!
This is a great video. Two questions. 1. Where are you clipping in to discharge those filter caps? I cant see in the video. 2. My 120 ceramic resistor seems to be glued in? How do you remove a glued in component on a pcb? Thanks
Thank you, Trail. There is no hard rule for draining the filter caps. If I can't access the B+ on the filter caps (eg because they are radial and soldered on the PCB), I will discharge at a number of places: The cathode of the rectifier diodes, pin 7 of a EL84 (or pin 3 of a 6L6/EL34) or pin 1 or 6 or a preamp tube. The latter will be a slower discharge of the first filter caps because of the dropping resistors, but that is not a bad thing. Good question! Thank you for asking.
Chris..looking over your vid again, the first thing you mention is how close the chassis is to the back of the speaker- the tubes are close too. I soon noticed on mine . That does 2 things (I'm sure you'll agree) there is a certain magnetic pull on the tube innards & worse , the constant heat goes into the magnet - then the back is closed ! what are your feelings about that? there are ways round this ..more mods !..it's all wrong though..
(Sorry I missed your comment originally). Like you, I would have preferred that there be more room and better ventilation for a tube amp. I don't know that the magnetism would be a problem. There has been much debate over the issue. If the magnetic fireld was a problem, the only suggestion I can make is to put a grounded metal shield between tube and speaker magnet.
Hi Chris, great insight into the workings of this Vox. I am also in Sydney and wonder how we go about contacting you for a quote on the same upgrade you have done on this amp? Cheers..👍🇦🇺
Hi Jimmy. I'd love to upgrade your AC10. I also have the Weber speaker in stock, if you want to go "full fat" as in my personal AC10. Have a listen when you come over. My number is 0412 420 7323 and at the end of every video.
Any chance of renovating/servicing an early 60s AC 30 ...please? Love your videos.! Cheers Rob from the UK......
Robert, they are absolute gems! I have been entrusted with some wonderful 60’s AC30s including top names in Aussie bands and a famous UK AC30 user. I promise the next time one comes across is my workbench, I will video it for you, Robert.
Great video. You mentioned owning two Chinese amps, what's the other?
I now own THREE! I own a Joyo JTA05 and of course, the Tweed Tone 12A I designed and sold through Artist Guitars. Check out that video if you haven't seen it yet.
Hi guys. If you want to see this AC10 in action, here is the (young lady owner) with the newly setup Vox in the video release of their new single Great Divide.
czcams.com/video/a1KInjLqk28/video.html&pp=wgIECgIIAQ%3D%3D
Thanks Chris 😊
Great video and explanations thank you! Didn’t finish it yet as it’s a lot of technical info for a clueless amateur, all the more non-native English speaker but a great way to educate on the topic! had a question regarding the master volume: can it be « changed » or modified so that it doesn’t go from silent to screaming loud with the slightest of a turn?! Mine can’t really stay low (and it rattles, with volume jumps) so not really playable at home without an attenuator, which a hassle to install every time and doesn’t sound the same to my ears. Could it be the taper (change for linear?) or the value? Thanks a lot for any suggestion you might have!
An excellent question, Flaneur. It sounds like you're having a taper issue, but it could also be a dirty pot so make sure you give it a "Clean & Lube" and rotate back'n'forth before you do anything else. You need a Log/Audio taper as Linear would worsen the effect you're hearing. The issue is pot manufacturers. Its hard to make a logarithmic pot, so manufacturers approximate a logarithmic curve with a series of linear tracks. I have found the best approximation to an Audio taper to be either CTS or Bourns. If the cleaning doesn't help, try one of the 2 I have recommended. Please let me know how you go.
@@theguitaramptech many thanks for your answer. I’ll try the CTS taper as they have some in store nearby! Do you know which value I should take? I wouldn’t mind losing some volume as the amp is mostly for home use. Is it complicated to change? I’ll bring the amp to a tech anyway as I don’t know how to do it myself… Thanks again very much for the help!
PS: and I’ll ask him to check before if it’s only a matter of cleaning the pot
@@flaneursong78 I have just corrected the stupid typos in my first response. Vox uses 500K so I would stick with that. It would be best to let your Tech order the correct pot. There are too many different shapes and sizes.
@@theguitaramptech yes this whole audio/log vs linear topic seems deep!
Ok I’ll give the tech the CTS and Bourns 2 reference and let him order. Thanks a lot again
Chris, found your video to be outstanding. I have an AC10 as well. I did the DT-10 speaker upgrade as well as a tube upgrade. But my biggest dislike of this amp is the hum. Do you think this could be that the speaker hasn't completely broken in? Or should I try this mod? Cheers mate
Hi Fred. I’m glad you did some of the mods. No, the speaker should have nothing to do with the hum. I would look at tubes, and ensure you have a good earth connections. If you are up on your safety standards, I would check the current flowing through the cathode resistor. You can do this by measuring the voltage across the resistor, then dividing by the resistance value that you have measured. Don’t trust the printed value on the resistor as being accurate. From here, and the plate voltage, you can figure out the static dissipation of the tube. I wouldn’t be surprised if it was just running way too hot. You’ll be measuring extremely dangerous voltages, so if you have any doubts about your safety, please stop. Good luck
Ordered a handful of 150r 5w this week. Should be with me soon. Amp been off for a month now. That's how much the hum, even at low volume annoys me. I'll swap the resistor, even change the tube as indicated from your mod. Keep you posted.
@@fredvanbeuren6507 let me know how you get on.
Chris...hi.. Just been watching your interesting little video again..can you just remind me, why did you replace the 2 large capacitors you pointed to for higher value ones on your own AC10 ? what's the advantage of that? and how's it sounding now?
That’s an excellent question Vincent. The AC10 has solid state rectification. It’s solid as a rock. There will be no tube rectifier sag. Similarly, solid-state rectifiers don’t have the limitation of capacitor size for the first stage of filtering. In a high current draw scenario, the only thing that will sag is the capacitor charge revealing the rectified ripple. That’s not pretty. That’s why whenever there’s a solid-state Rectifier I will always bump up the capacitance. Other benefits are a larger storage energy, noticeable at high volumes and big demands on current, for example a big powerful cord, or digging deep in a solo. This is why Marshalls with solid-state rectifiers use 100 µF capacitance for the first filter. Excellent question
@@theguitaramptech OK thanks..rectified ripple ? not heard that before ! after you switch off how long do you think caps take to discharge? you mentioned the lethal voltages. The gain on these it seems is by transistor..you mentioned the MOSFET...
@@vincentl.9469 When the main AC is rectified it becomes 100 Hz (or 120Hz). This is the ripple that your filter caps smooth out.
Not all amps have discharge resistors. ALWAYS measure the voltage.
The first gain stage Q4 is a MOSFET LND150N3. Look up the schematic and it will make more sense.
Hi Chris, I know nuthin' about electronics but loved watching the AC10 vid all the way to the end. All your mods are what I been wondering how to go about getting done. You mentioned Weber speakers being custom made for the Vox 10 and the narrow space. I've looked into some other boutique upgrade speakers but if space is a tight issue and Webber have made the DT10P specifically then maybe thats just sealed the deal. I just dont want an Over the Top high end, glassy speaker. Assume the Webber is good for mid range strength? Thanks greatly, Adam, Fremantle WA.
Its a shame you're on the other side of the country, Adam, or I would suggest you listen to my AC10 with the Weber.
We are all so individual when it comes to taste in sound. I love the Weber sound and of all the Webers I have installed only one customer said it was too bright. Even I thought the Weber California was bright! LOL!
@@theguitaramptech Hey thanks Chris, I reckon I will go for the Webber, they keep coming up in other references. I was driving back to Perth today from an overnight work trip. I listen to The Truth About Vintage Amps podcast with Skipp Simons - what a dude. And I heard this guy from Australia come on the voicemail qn and start singing Helen Reddy. I go "I think I know that voice!" It was you Chris The Guitar Amp tech guy! asking about Fender Silver face gunk. I had a good laugh in the car. Anyhow, my pair of JJ el84's were waiting for me when I returned home as well as your reply to my comment so that was great. The 12Ay7 is the only change i have made so far and i can notice the higher volume range for sure - just what I was after. The JJ's go into night and then the Webber will come. As I said earlier I also order a 5751 by mistake, can I put that into V2 and replace the chinese stock tube? Hey thanks for the info and snd another voice mail qn to Skip on the Vintage Amp podcast. Also, I have a hairy crackling sound on startup of the vox 10 seems to go away when tubes are warm🤷♂? I can live with it which Skip so often says is the next best solution. thanks again . Ad.
@@adamcherry434 I love SKip. He may not have the greatest in-depth knowledge of electronics, but what he doesn't know about the practical side of repairing isn't worth knowing. I was restoring a Brownface Concert and couldn't find anywhere the time when Fender changed from cloth power lead wire to PVC. Skip told me right away that it would have been PVC by then. Love it!!
@@theguitaramptech hi Chris. hope all is well. I bought my 10 used, the guy felt no need for it. He did however put in a Greenback 10 from the start but the stock speaker came with it too. After more than 3 years, I put the Vox VX10 factory speaker back in and I feel there was never any difference ! If anything the bass is tighter on the stock speaker. I would suggest you really need a good reason to upgrade. I remember the Webber cost you an arm & leg to import !
@@vincentl.9469 that’s very interesting feedback. Thank you Vincent. I haven’t heard the green back 10” before. The Weber was definitely a big improvement but it did cost a lot.
What kind of doping would be most appropriate for the Weber DT10-P speaker? I like gain, overdrive, and bending strings in the high register; however, I'm looking for high fidelity, as well.
Good question, Andrew. I prefer light doping on all my Webers, but I play much cleaner than you. You don't want speakers to lose control. Tube distortion GOOD. Controllable. Speaker distortion BAD. Uncontrollable. I would suggest Medium, perhaps heavy doping, but don't believe the ramblings of a madman across the water (great album title). Give CJ Sutton at Weber a call and believe whatever CJ tells you. Tell him I said "HI".
Thanks so much! I love the content
I want to order the DT10P for my amp. but it wants me to choose power, impedance and doping. I don’t don’t know what these are and what you choose.
Hi Sparticus. Thanks for your message. You don't have a choice for power. It will be 30w. I prefer light doping. Your Vox will need 16ohm speaker. You can call CJ Sutton at Weber. He will tell you the differences between doping better than me. Say HI to CJ from me.
On your personal amp, would it make more sense to relocate the power tube board if you intend to keep the amp to stay away from the speaker movement and expand speaker options?
Oh, if only I had such spare time! I have several far more exciting projects, waiting for that glorious day. But, yes your suggestion would certainly open more options.
@@theguitaramptech Awesome video. Thanks for the reply. I am always looking to learn new things.
Thank you. I am proud to contribute to your knowledge. Thank you for including me in your journey.
Hi Chris...just a quick query...if I may. I still have my '10 and as you've mentioned a few times, the first stage gain on these is via a transistor-but many of us will still plug in a distortion or 'drive pedal. Have you ever known any of these to damage a MOSFET ? Most will say they are harmless, some pedals are built round a 12AX7 tube, still running off 9V - but a small number need 12volts
I have never heard of a pedal destroying a mosfet. If the input goes too hot it will drive it into clipping or cut-off but not damage it. You should be ok Vincent
@@theguitaramptech OK
I own two of these. I split the signal with a multi effects pedal and run them stereo. A Brian May signature Jubilee gold top is my main guitar. It’s a good system. I have a question for you, will a green back celestial 10 inch fit in there?
Wow! That guitar sounds amazing, Rick.
My thoughts on speakers: The Weber DT-10P sounds amazing. It is shallow enough to fit inside that space. I haven't hear the Greenback 10. I had a UK Greenback in my Marshall 18w 1974 which sounded excellent, but I swapped it out for an excellenter (yeah, I know) Weber Blue Dog (video coming soon). I previously had a Chinese Greenback which I couldn't sell fast enough It was not good. If your 10" is Chinese, I recommend passing on it.
Check the dimensions of the DT-10P on the Weber site. It only has about a 1/4" clearance, so don't get the Celestion if its any deeper. If you're in the USA. Rich, the Weber is a no-brainer. (Hi hi to CJ for me.) I stumped up for the Weber in my personal AC10, even though the shipping was the same cost as the speaker. Love it. Let me know what you decide on and if you're happy with it.
Hi, thank you for the tips excellent video. By the way I did put a 10 inch Celestion greenback 16 ohm in my AC 10. It fits fine.
nice channel.
Thanks Jo
Curious if you know about how much it would cost to have an amp tech make these adjustments? I have this amp and I never realized it was set like this. At this point it’s about 5 years old, should I consider any replacement parts potentially since it’s been running that high?
Hi Andrew. Of course it will vary on your location, availability of parts in these turbulent times, and how much you want to do. If you were in Australia, I would say you'd be looking at $200 to $400 depending on parts and if you were upgrading the speaker or not.
You didn't mention it so I thought I would. As I'm sure you know, the current you measured through the bias resistor also includes the screen current. Therefore the plate dissipation is actually a bit less than your calculations indicate.
You are quite correct Clark. The screen component will add 8-10% to the Cathode readings. I don't mind this as it adds a built-in safety margin to plate dissipation. Thank you for bringing it up. It's always a challenge to decide how much detail to go into on a video.
Also , unless i turn the amp volume dwn when its turned off it thumps thru the speaker . Is tht harmful ? I Dont like it .
Do you know if there is a way to bias a marshall origin 20C for "more headroom"? I know marshalls are "supposed" to distort, or thats the status quote. I really like the clean marshall tones and my origin 20C distorts too quickly. Is the procedure alike in this video?
Hi Olle, I haven't seen a schematic for the Origins yet, so I am not sure how I can accurately advise. An educated guess is to reduce the gain of the preamp tubes closest to the input jack. It would be a 12AX7. Replace it with a 12AY7, then try the next tube along. It won't hurt the amp and may get you closer to the sound you want. I am not a huge fan of the 12AX7s at the best of times. You could also try a 12AT7 for your phase inverter (the one closest to the power tubes. Maximum volume will now be at 6 instead of 3 (for example) but with more cleans and less fizz.
@@theguitaramptech wow! Thank you for responding. Will def try swapping for lower gain tubes. Heard fender uses the 12AY7
@@olleheed That's right Olle. The Tweed era used 12AY7 for V1. I much prefer the tone of a Y7 to an X7. I think you will enjoy the new sound. If its too clean for you, try a 5751 in V1.
HI Chris, I bought the tubes for the upgrade you described in the vid. 12ay7 into the first position. I also got the 5751 thinking that was going in to but you only tried that one. Can I put the 5751 into the next tube position to replace the stock chinese tube in the second preamp position? Or will that just undo all the good results you set out to achieve. Thanks you , Adam
How was your out with the tubes you got I want to do the same thing any input is much appreciated
@@A.PGUITARS gday, the vox 10 is a sounding great but I think the biggest difference has been the Weber speaker Chris also recommended . And now it has had time to wear in a bit . It’s still voxy, chime, bright but it’s v nice . I’m preferring if over my modified BJ4
Just curious, why do you subtract the cathode voltage for power dissipation? If you’re calculating plate dissipation, you simply multiply the voltage of the plates by the current going through the plate P=E*I. If the current going through the plate is identical to the current going through the cathode. Then just multiply those two values.
That is an excellent question and I wish I had addressed it in the video. What you are saying is CORRECT for a fixed bias arrangement where the cathodes are grounded. However, when the amp is Cathode biased, there will be a positive voltage at the cathode which needs to be taken into account by subtracting that value from the plate voltage.
If we wanted to be even more accurate, we would take into account the current contributed by the screen. I allow 8%. So to be more accurate, I would say: P= (Vp - Vk)*I x0.92. But as my wife says, I am a pedantic pain-in-the-arse so P= (Vp-Vk)*I is good enough.
@@theguitaramptech thanks! I ran your numbers through Rob Robinette’s online calculator and he allows 5.5% screen current. And it came to 14 watts plate dissipation. Very interesting researching all this. It seems from doing a load line analysis, at the value Vox biased this model - it stays in class A operation for the first 10 watts of power. Which ironically seems very peculiar, considering it’s name. If you lower the bias to 12 watts plate dissipation, you only get about 2-1/2 watts of class A operation before it transitions into class B. Would Vox bias their amps too hot for the namesake of calling their amps Class A in their literature?
@@voxpathfinder15r What's in a name? That which we call a Class A amp by any other name would smell just as sweet. In truth, its too inefficient to run an amp in true Class A for anything other than HiFi purists. But they are beautiful sounding cathode biased amps with no negative feedback. Good research Pathfinder. Rob Robinette sure knows his electronics.
I have a vox vt30, but now there is a problem. if I play for 30 minutes, the sound suddenly decreases and hoarse. what's the problem?? Please help me. Thank you
Sorry Papah. That's too complex an amp to diagnose remotely. Good luck with it.
I have a ac10 a few weeks ago it blew the 500 ma fuse . Replaced the pwr tubes ,all is well for a month n blew again , i play everyday . New pwr tubes . What wld wait 4 wks to blow it again ? Thks
That’s just not right. If you’re not comfortable working with high voltages, do not do what I suggest.
Before you put in another set of tubes, test the voltages. Importantly, measure the voltage on the control grids. They should be zero. Even half a volt means you have leaky coupling caps.
The other possibility is the bias is simply too hot. I think I covered that in the video, but I don’t remember.
It cld be the coupling caps , this amp is several yrs old n they've not been changed . Ok thks again .
@@tomfoolery2082 unlike electrolytic, coupling caps don’t have a use-by date, but when they leak it can cause many lost hours. Good luck.
Excellent work, I will make the same mods to mine, I just bought. Could you please explain how to mod it, in order to add a FX Loop? Please, please, please!!! I will make this mod as well!!!
Thanks Joe. Making a good effects loop is not trivial. I would not recommend passive effects loop. You need to balance the impedances and levels and there's the risk of cooking your first pedal in the Efx loop if you get it wrong. I use the Mojotone Discrete Hi-Voltage Series Effects Loop For Vacuum Tube Amplifiers. Its pretty expensive at USD70 plus shipping to Australia, but it works very well. I would put it before your phase inverter tube.
Hi, would using a 5751 in v1 and v2 be beneficial? Or would a 5751 in v1 and 12ax7 in v2 be a better option, thanks.
Great question Jason. Its really a matter of taste. I like a 12AT7 for the phase inverter (labelled V3) and a 5751 in V2 for more cleans. If that's too clean, take it up to a 12AX7. The numbering of tubes in most Voxes is confusing. The schematic will clear things up for you.
@@theguitaramptech Thankyou for the reply, and thanks for the video, very interesting watch👍 will have a play, I have some 5751 tubes.
@@kbeacham100 Have fun experimenting. You can't hurt swapping out preamp tubes. Let me know what you settle on.
Is it possible to replace the 12ax7 with an au7 or something similar lower gain... would this work ok for someone like myself who isn't so electricly capable? Cheers
Edit: was being impatient, you got to the tubes
Definitely! I personally prefer a 12AY7, but experiment with all the tubes in that family. Have fun.
Hey Chris, great video. Just ordered an AC10C1 and keen to get more of that clean headroom as you mentioned. Would you change the bias resistor just off spec, or measure to make sure? Keen to get the biasing right, but not keen to get bitten taking the measurement in a live circuit. Already have a 5751 and a 12AT7 tube ready to go when the amp comes in.....
Great to hear, Simon. I know you will enjoy the improvements. Please don’t try to change the cathode bias resistor if you are not 100% sure how to make it safe. You could safely put in 150R, but even 180R would give better results than stock. Good luck. Let me know how you get on.
Please note that any mods you make will invalidate your warranty.
@@theguitaramptech Naturally....
Hey Chris, you mention changing out the capacitor in C4(?) to give better noise reduction on the input power signal. What value would you suggest changing to?
Just went back and rewatched the video, you suggest 100uF on C4...... :-)
But doesn't the 12AX7 have a better signal-to-noise-ratio? Plus I like overdrive/distortion myself. But if the customer is happy that is all that matters. More videos please.
The beauty of individual tastes, Zack. To my ear the 12AX7s sounds a bit "buzzy".
2 questions: 1. Does this lower the overall volume of the amp? (I love how loud these amps are) 2. Was that 12AY7 put in preamp V1 or V2?
Dear Impaler, the naming isn't that easy on this Vox amp. Vox calls V1 one of the EL84's.
I put in the 12AY7 in the first available valve gain stage. I say that, because in the AC10, the first gain stage is actually an N-channel MOSFET. The MOSFET charts show a non-linearity akin to tubes. At first I was shocked to see this, but it has huge headroom before a pedal can clip it, and it eliminates the dreaded V1 microphonic sensitivity. I put the 12AY7 in the first available position, called V2.
No, the volume stays the same, but there is less tendency for the 12AY7 to put the next stage into saturation or cut-off. To my taste, preamp tube cheapens the sound of a good amp, but I will always check in with the customer.
Full volume before clipping comes in around 6 instead of 2.
For my personal AC10, I also replaced the PI with a 12AT7 which I prefer as a PI.
@@theguitaramptech This is really good info to have. So would an even lower gain preamp tube in the "v2" position have gotten you closer or even within that 95-100% range you were looking for? The reason I ask, is because im not confident in my soldering abilities to change that component that you did, and trying different preamp tubes is a generally harmless attempt to find the tone i want. Also, what is the speaker mounting depth in the AC10?
@@lordimpaler3899 The preamp tubes won’t affect the bias of your EL84s. They will just drop the gain in the preamp stage. The only way to cool down that bias is to increase the value of the cathode resistor. It’s always a bit of a pain in the arse on a PCB amp. GOOD LUCK.
@@theguitaramptech Hi Chris, excellent video. Thank you for taking the time to make it. I am confused, above you say "preamp tube cheapens the sound", are you saying you prefer the mosfet as first gain stage? I think the AC10 sounds great but knowing it's not a full tube pre amp path, actually put me off. Curious to hear your thoughts further. Is he AC15 the same in terms of pre amp tubes? Why does the AC15 and AC10 sound different? Is the mosfet in the AC10 being the difference? Please do a similar video on a Mesa Boogie amp? And why not use a variable control for the bias with test points as opposed to the method shown here? Regards.
Very informative, would you say avoid the ac10 ? There’s another tech on CZcams who just about called it rubbish, although he doesn’t seem to like any amp.
No way, Tbone! I love the AC10...so much so that I bought one. Yes, I tweaked it and upgraded the speaker with a Weber (huge improvement). I was sceptical at first because it uses a LND150N3 MOSFET in V1 instead of a tube, but its still oozes Vox tone and I will never have to suffer a microphonic V1 again. Its only 10w, but I was told several times at the last rehearsal to turn it down! You just need to work on it a little to take from "OK" to "Vox chime".
@@theguitaramptech Thank you, appreciate your advice and response !
I'm about to replace the tubes and the speaker in my ac10. should I drain the power caps for that small a job?
thanks, Lee
Hi Lee. That’s an excellent question. Unplug the amp from the wall first. Anything you can reach from the outside without removing the back cover, you don’t have to worry about draining the filter caps. Once you remove that back cover and expose the circuitry, its best to drain them. it’s very simple to drain the caps. Of course you know not to short the positive directly ground or you’ll get a nice big Spark. Best just to make a coffee and load the dishwasher while the capacitors discharge. Double win. Wife will be happy. You’ll be alive. 😉
@@theguitaramptech thanks The Guitar Amp Tech. I've done a bit more research and I've made up a discharging jig with some aligator clamps and a resistor to be safe.
@@leekthomas737 excellent. I made a similar tool with a 100 ohm resistor. I also put in a series fast blow fuse for those times I power up with my draining tool still attached 😂
@@theguitaramptech oh boy, I had no idea
Hi. Have a Vox AC10 - all I get are clean sounds. With a Fender squire strat or a Tele w/85EMGs - the only time I hear crunch is when amp vol is turned very low almost off. Is my gain broken?
That is odd Brewer. I find it easy to get some breakup with my Tele. Forgive my silly question, but do you have the Gain set to maximum and control your level with the volume? I can't say you have an issue with your AC10 as I don't know anything about EMGs. Please try it with a guitar with "normal" pickups.
@@theguitaramptech - Found out Strat guitar volume must be max (or almost there) to get crunch. Gain 9:00am to 3:00pm. In my apartment studio, the Vox AC10 amp volume has to stay no more than 9:00 am. At low guitar volumes, with amp up high - the amp sounds clean.
@@BrewerShettles Yep. To get crunch, the guitar volume needs to be on max. Time to go out and buy a new overdrive pedal. Explain to the wife that some Aussie guy suggested you go and buy one. I really like my Wampler Tumnus Deluxe pedal.
@@theguitaramptech - Hi!! Update: Found video I made DAY 1 when I got the VOX AC10 - July / 2021. Now vs 2021, midrange presence is dull and breakup happens at max guitar vol. Changed strings today. Helped a bit. Neck strat is dark and boomy. Not sure new JJ tubes would add back the shimmer to the sound. Recently, sound was dropping out at low amp vol. Vox said I could bring the amp in for warranty repair. Amp Owner in a forum said Dodgy pots. Deoxit on the pots fixed dropouts. Amp is quiet if you turn guitar a certain way. Gain increases sound, but I remember there used to be breakup at 1pm. Service centers will do warranty repair and eval. New tubes on me.
@@BrewerShettles it’s a no-brainer. Take the warranty offer. They won’t cover new tubes, but you’d pay that anyway. Good result.
Hi would a celestion creamback 10-inch fit OK in the vox ac10
I don't know. Check that the depth is no more than 4".
@@theguitaramptech It took a while to find the information and here it is
Overall depth 4.0″, 102 mm; Unit weight 3.5lb, 1.6kg
Nominal diameter 10″, 254 mm
Power rating 45W
Nominal impedance 16Ω
Sensitivity 96dB
Chassis type Pressed steel
Voice coil diameter 1.75″, 44.5 mm
Voice coil material Round copper
Magnet type Ceramic
Magnet weight 14oz
Frequency range 95-5500Hz
Resonance frequency, Fs 98Hz
DC resistance, Re 12.05Ω
That sounds perfect. Please let me know how they fit and sound.
@@theguitaramptech
A perfect fit no trouble mounting the speaker what so ever the screw holes lined up just right with plenty of clearance regarding the magnet and the sound is a total improvement had to swap out the switch but that had nothing to do with the speaker while I was in there I put a set of jj valves and now it sounds even better
@@lakeb4561 That's an excellent result. Thanks for the followup.
sweet little amp, lovely reverb but too loud for home too weedy for gigs and a shit pedal platform
hated all my drive pedals!! sent it back and got a boss katana 100
bloody brilliant!! great for home and gigs, Ive got an artist Tweed tone much better than the vox good pedal platform on the clean but the drive channel is where things get weird, I looked inside, it has a channel select led! but for some strange reason they didn't mount it through the front (top) panel!
also noticed it had a tube screamer type circuit in the drive channel!!
I feel a little had, but it does sound
ok
Could you just put in a c older match set of tubes
Sure Mike, but where will you find a "cold set of matched EL84s"?
Seriously though, this channel is also about helping my subscribers understand and learn. (Please subscribe, Mike).
Chris..how do you think this amp would perform if you fed it via an external transformer ? say 20 volts below standard mains voltage...clearly a bit cooler..by the way I tried a 5751 in V2 of that amp and went back to 12AX7
Thanks for the question Vincent. It depends what sound in your head that you are trying to find in practice. Lowering the Mains voltage will lower all voltages inside your amp. Your B+ (plate voltage) will reduce as well as the other voltages inside the amp. This will reduce your clean headroom. But other things will be happening, that are not so good because they are not in your control. The filament voltage will drop by nearly 20% if you’re in the USA, or 10% if you’re in Australia. In a fixed bias amp, the bias voltage on your output tubes will also drop. In a Cathode biased amp, it will tend to self-correct to some extent.
Its great that you tried a 5751. Even if you went back to a 12AX7, you have explored the tonal colour palette open to you, and that’s a wonderful, never-ending journey.
@@theguitaramptech OK..it's UK voltage I'm thinking of (230-240 volt). sounds as if it wont actually harm the unit. I was thinking back to what you said about the AC10 C1 being biased hotter than it should. I found the 5751 to give a more sterile sound and wont react in the same way to overdrive pedal input...as I like overdrive. 12AX7/ECC83 seem standard on most amps...
@@vincentl.9469 I love the beauty of individual tastes, Vincent. The X7 is the most widely used tube for a number of reasons. Imagine the benefit to fender and Marshall in stocking only one tube! The accountants love it! To my taste, I find the X7 too fizzy. Viva la difference, mon ami!
@@theguitaramptech the other thing I wanted to add re: tubes is the heat that comes off them . I think in this respect Marshall & one or two others I've seen, are better in that regard in that the tubes sit on top of the chassis away from the other components. Maybe not all amps can be made that way ...
@@vincentl.9469 Well said, VIncent. Having downward facing tubes and transformers let Fender make use of that space in the speaker volume in a combo, at the cost of rising heat. But, who am I to suggest to Fender that they could have built the 410 Bassman or Super Reverb in a better way 60 years ago, when they are still running today. :-)
G'day, If I replace that 5w 150ohm resistor myself, does it need to be in a certain direction or either way? Any special precautions? : )
Hi SOJOURNER . A resistor can go in either way. I like to have the values of all components pointing up so the next Tech can see what I have done. As always, please discharge the capacitors and remove the power cord. If you are not 100% sure, then please put down your soldering iron, take 2 steps back, take a deep breath and say "thank God I am still alive".
@@theguitaramptech Thankyou for the message ... I'm confident I can do that resistor replacement. Another question: If I wanted to install a 12v case fan to cool the pcb attached to the tube bracket (cool the heat damage in general) where would be the best way to wire it to the amp circuit? ... The reason I wanted to do this as the tues are not ventilated at all. Or do you think that this is not necessary? I could always install a vent where the tubes are! . FYI : I installed a G12 10 Vintage speaker and it fits well. ;)
Hi SOJOURNER, tubes are designed to get hot and I would shy away from having a cold spot on the tubes. You are right that the ventilation in an AC10 is lacking. You may find 12.6vDC near the filament supply for the preamp tubes. I would tread very carefully though with the additional current draw of the fan and the potential to add noise in your filament supply.
@@theguitaramptech Cheers thanks, I'm just putting in a vent where the tubes sit to allow air & heat to escape as opposed to that metallic foil that's there and also the resistor you replaced on the your video. With the new G10 Vintage celestion speaker, the Sovtek tubes and the last mods it should be enough and I'll be a happy camper :)
@@thesojourner7722 That sounds like a great idea. If you're happy with the vent, can you send me a photo to amptech@teammania.com.au ? If it looks like shit, we will discretely pretend this conversation never took place. Deal? HAHAHA!
The last time I put in some Tubes without having them matched and not checking the bias, I opened a portal to another dimension a possible StarGate. 🤷🏻♂️
OMG! Thank God you controlled the Screen Voltage Hypergalaculator to make it back. Live long and prosper.
She gets cranky if u keep her tooth brush for too long . Thts the funniest thing iv heard for a long time .
hi just seen this...I spoke to my nearest Vox service centre and they said there are no bias adjustments to make on this amp..elsewhere I've read its not even possible, just fit decent quality valves...
Hi Vincent. Thank you for your email. Sadly, the person you spoke to doesn’t understand about setting up a Cathode biased power stage. Yes, once properly set up, a Cathode-Biased amp is self biasing. But, as they come from the factory, most of them are biased ridiculously hot. (It is definitely possible, but highly unrecommended for the average person not skilled in working with deadly voltages.)
Of course they’re going to say just replace them with decent valves, because that’s where they make money. If in doubt as to the knowledge of the person who gave you that advice, ask the simple question: “what would be the impact of increasing the cathode resistor on a class A amplifier? “If he doesn’t give you a direct answer, I would seriously think about looking elsewhere for your service. By the way, the correct answer is increasing the cathode resistor reduces the current by making the grid relatively more negative. Your tubes run cooler, last longer, and don’t breakup so early.
Thank you for your comments. The research you did is excellent. It’s exactly how people get to know more about their amplifiers. Well done, Vincent. I hope I’ve helped.
@@theguitaramptech OK thanks..the glow from the two power tubes (JJ) is white - not orange. I assume that's a sign they're not running too hot. Seems to be accepted EL84's run very hot . You mention the cathode resistor ..where is it? and what value would you put in ? I wonder how it would affect the rest of the circuit...but the real issue with this particular amp is the closed back- Not sure how or why this came about or why more people haven't flagged this up...I'll go into that another day....
@@vincentl.9469 Hi Vincent, I am not sure where you are getting your information from, but please broaden your knowledge through books, not forums. This is how wrong information spreads. For example, whoever said EL84s run hot doesn’t know what they’re talking about. Its Class A that runs any tube hot. Also the glow colour is no indication of “hot”, except when your tubes start glowing cherry red. That’s bad. I can see you are thirsty for knowledge, Vincent, but be careful of where you seek it. Wrong information can take you years to unlearn as you cling on to flawed sources.
@@vincentl.9469 I forgot to mention. Your Vox dealer is partly correct. If you correct the bias on your AC10, you will lose your Warranty, even though your amp will become more reliable. The BEST thing you can do to for the tone of your amp is remove the speaker, and install a Weber DT10-P. Talk to CJ at Weber and tell them Chris, the Guitar Amp Tech from Sydney recommended this upgrade. You will be sending me Christmas cards for years to come, Vincent.
@@theguitaramptech I stand corrected on the glow colour! however the amp is out of warranty so any mods are no issue. I guess Changing the cathode resistor as you did is a job for a tech..It has a Celestion greenback 10 already fitted....I think it could have been better with a taller case..there's not much room in there for some speakers..
Yeah, I don't mind the chinese tubes, once I swapped them for jj's, I didn't think there was any real difference in the sound
I agree Albert, I have used many Chinese tubes where I really enjoyed the sound. They get a bad rap because they are often biased or setup wrong and fail prematurely. They may not have as good quality control, but don't take my word on that. I have my preference, but I would have no hesitation in using Chinese tubes.
@@theguitaramptech just an update:I used the chines tubes I took from my AC10 and tried using them in my EHX blackfinger compressor pedal (uses two). And oh boy were they microphonic! I wouldn't have realized it if I hadn't swapped them. But, granted, I bought this Amp used, so who knows how long they'd been played.
@@albertplaysguitar like you, I have found microphonics to be the greatest problr
Isn't the dissipation for a push-pull amp max 70%?....Blencowe even allows only up to 85%
Good question Squizzy. It depends on the amplifier. Some amps sound great at 55% while others sound great at 95%. I usually bias an amp by monitoring for crossover distortion and clipping on my oscilloscope scope and then double check the plate current and voltage to make sure I’m in a safe range.
@@theguitaramptech Fair enough, how about a single 6V6 running at 15-18W?....ah la 70's Vibro Champ, dudes go anal abut the factory 470R cathode resistor being too small a valve yet those tubes last 40+ years.....by adding a higher value all it does is make the amp sound more senile....IMO
@@gingercat777 that sounds way too hot. Where are you set the bias will be influenced by the sound you’re looking for. Set it too cold and you’ll be cutting off the tube, set it too hot and you’ll be saturating too soon. It’s all about taste, but 18 W is just way too hot for 12 W tube.
@@theguitaramptech Try it some time and let your ears decide 😉
Got my tea and a doobie,let er rip
Excellent, Gary. Sit back and enjoy. Speaking of a doobie, I got to see the Doobie Brothers (with Michael McDonald!!) performing at the magnificent State Theatre in Sydney 2 weeks ago. They still are so fantatstic. Better than I remember them in 1977.
@The Guitar Amp Tech haven't seen then since my high school days if their ever near ne I'd see them again good memories from the 70s
Don't believe the "Class A" marketing of 2 x el84 amps. I have a 2 x el84 amp with nearly identical voltages and I confirmed via scoping both outputs that it operates nowhere near class A. I think the plate voltage would have to be below 200 to be able to bias into Class A on that setup.
jjsant3250, yes, you have brought up a good point regarding the old Class A discussion. Many say that the easiest example to understand of Class A is a single ended amp like a Champ. Thank you for your well-informed comment.
I saw an interesting interview with one of the Vox UK design guys. He said this class A idea with Vox amps is wrong, they were designed to be class A/B but they just aren't very good at it. The amp sounded so good though they stayed with the design they had.
Interesting that JJ tubes are seemingly slightly weaker than the Chinese el84 tubes
I haven't found that with my repairs yet. I find JJs to be very reliable. I work on many Chinese amps, and I am often replacing the Chinese EL84s. I should not judge them too harshley, as the bias on those amps is often set between "unwise" and "stupid".
@@theguitaramptech oh that’s right you’re taking a power reading from the new tubes along with a cooler bias. Have you ever taken a power reading of an AC15? I wonder what the difference in power is?
@@voxpathfinder15r I wish I had kept records, but sadly that great idea didn't last too long. From memory the AC15 were also biased way too hot. My prferred setting is 90-95%. I may go up to 100% depending on what I see and hear.
I must have missed something..the current thru the cathode resistor was 48ma...would this not be the total current for both tubes....?? each tube then flows 24ma.
Of course you are correct, J. However, I already took that into account at 12:47. The current through the cathode resistor is 11.27v /117R = 96mA. Divide the 96 by 2 and viola!...48mA. You made me check. Thank you for keeping me on my toes.
@@theguitaramptech I feel like a "dufus"
Not at all. I reckon I have an amp each day that makes me feel like a dufus. I prefer to Poirot's take on it as "exercise for the little grey cells".
Sweet home Alabama.............................!!! Is it home still?
Hahaha 🤣
Misophonia warning 17:01
Hahaha! For me that sound triggers the need to get baking!!
coffee and greek moussaka is better
HAHAHAHA! A delicious combo.
This amp is not class A.
You are correct Ben. It is Cathode Biased Class AB. But I’m using the common terminology for Vox amps and my method is still correct. Thanks for pointing that out. 🙏