Vox AC10 Reissue Repair and Demo
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- čas přidán 30. 05. 2023
- Repairing a Vox AC10. This is a reissue and not an original one from the 60s. It is of PC board construction. It came in with fault on the heater supply. We fix the problem. After checking the bias we discover that it is dissipating over 13 watts on the output tubes. With the transformer getting hot, this bias problem would need to be resolved for the amps longevity.
- Hudba
Great playing, awesome job at repairing it. Personally though, I’m glad I built my own using vox’s original schematic.
Hello Dennis, thank you. Yes, the original design is much better, and even better if you have built it yourself. Thanks for watching and take care.
The heat from the "diode" (actually a *bridge rectifier* with 4 diodes inside) has been baking the capacitor next to it from day one, and that cap really should be replaced with a high quality 105ﾟC rated cap (it's almost certainly the cap for the DC heater supply). If/when that capacitor becomes electrically leaky it will stress the bridge rectifier and cause it to run hotter, and eventually fail.
There's no real advantage in running the EL84 off of a DC supply in terms of noise or hum, because a push pull output stage will cancel hum, assuming that the tubes are reasonably well matched. The sensible thing to do would be to either add a second, larger bridge rectifier just for the EL84'S, or simply run the raw AC heater supply right to the output board, heating the output tubes directly off of AC so that the bridge rectifier is only responsible for the 600 milliamp needs of the two preamp tubes. Of course, reinventing the wheel and fixing the design flaws of this amp may not be practical for such an inexpensive and presumably Chinese made product.
Hello. That's a really good spot. I don't know what I was thinking there. If it's rectified then it's smoothed. I will open it up and change the cap. Thanks for spotting that one. A school boy error on my part.
So I have checked three of the caps next to bridge rec. They all test fine, like new. But I noticed the screen resistors had blackend. This was probably due to the problem with the cracked valve. Or was it due to the fact that they were 1/8 watt resistors? What a joke. They were 470 ohm. I changed them to 1watt, 1.2K metal oxide. It's on test now while I am having dinner.
Needs some Mullard NOS valves after the fix. 😊
Hello, yes you can't beat the Mullards. Thanks for watching and take care.
nice fixing and stellar picking, cheers
Thank you. Thanks for watching and take care.
Nice repair, these latching on/off/HT switches cause a lot of head scratching. If it was my amp I would have shoehorned in a bigger, higher rated rectifier possibly on leads to a small strip board to avoid constant resoldering on those pads and traces. Cooler bias means less current draw through the rectifier but it's way too small. Naughty old Vox. 😊
A separate bridge rectifier just for the heater supply of the EL84's would have made more sense, or perhaps just run them directly off of AC (output stage hum for a push-pull pair largely cancels out due to common-mode rejection), and use the DC supply only for the preamp tubes, where it can indeed lower hum.
Hello Rob. That did cross my mind. If it fails again then I think that is something to consider. Thanks for watching. Take care.
Modern electrical safety codes, especially for the EU, I expect, usually dictate that power transformers must have a thermal fuse inside of them (buried in the windings where it can't be accessed or replaced) which will blow and go open if the transformer runs too hot (to prevent the tranny from catching on fire and burning down your house). This is another reason why it makes sense to cool the biasing and lessen the overall current draw from that power transformer, to prevent the internal thermal fuse from failing.
Good point. I don't know why they bias them so high.
@@vintageguitaramp_guitarteacher Also worth noting that many modern devices with integral heating elements will also have thermal fuses in them, such as Keurig and similar pod-type coffee makers (and occasionally the thermal fuse will fail for no apparent reason other than continued exposure to heat). They're in older units as well; I once repaired a 1980's direct drive turntable that wouldnt spin because the large coil winding for the platter motor, which was covered in shrink-wrap tubing, had a blown thermal blown fuse, which I replaced (it was on the surface of the windings and fortunately not buried inside them). I subsequently discovered the platter to be spinning at about 100 rpms! Somebody had probably left it on overnight, and one of the motor-driver transistors shorted, turning the platter on full blast at high speed, and it ran that way for a few hours until the thermal fuse melted.
Nice work man, like the walk thru and remedy...Nice info..Thanks for sharing, learning myself..Ed..uk..😀
Hello Edward. Thank you. There is always something to learn in this game. Everyday I learn something new. There is some great guys on here offering advice. Thanks for watching and take care.
Nice job Martin
Hello Johnny thank you. Take care.
Sonds good in your hands. Good video frend. thank you.
Thanks Miguel
I think you could make a 12ax7 plugged in to a shoe box sound good. Nice playing!!
Hello Lee, lol. I have had similar things said to me many times. It's not a bad sounding amp though. Thanks for watching and take care.
I knew you would get it sorted out😎
Thank you. It was a bit of a tricky one. Thanks for watching and take care.
If the amp repair gig would happen to fizzle out . Pretty sure ur guitar playin would carry ya on . . Sounds good man .
Hello Tom, thank you. I do make money teaching and gigging as well as doing the amps. So I would probably be okay if the amp side fizzled. Thanks for watching and take care.
Great playing and a really interesting video. I was keen to hear what you had to say as I have recently purchased a AC15H1TV (50th anniversary). I am pretty new to electric guitar (played acoustic for decades), but it sounds quite good to me. I was wondering if the hand wiring was more marketing than performance given it still used a PCB. It does look like it is using better components and would be easier to fix than this one. Have you come across this amp?
Hello Gavin, thank you. This amp does sound okay. They are much better hand wired. But they are a lot more money. They are much more reliable when they are hand wired and will last years. Some of the more high end PCB amp do have better boards in them. Some of the early PC boards were great too. Thanks for watching and take care.
The rebranded "Bugera" EL84 might be a JJ as they are famously known for failure of the glass bottle. On larger tubes like EL34 and 6L6, the glass will fail near the base, and the "coin" or "wafer" base versions of those tubes are the worst offenders, especially if used in a Fender or other amp with "beartrap" tube-clamps/retainers. (PS, it seems obvious that Vox would not have used "Bugera"-branded tubes, so those are likely non-original).
I'm not so quick to blame any one maker for an issue. There is a 33.33% chance it came from one of three tube makers. JJ is plenty fine, amp makers keep abusing tubes, especially EL84's, by making amps with more voltage than the tube was ever rated for. There are going to be bad runs of tubes, but anything that is branded to an amp maker, has a 33.33% chance of getting that tube from any one of the three tube makers.
You have some interesting videos on you channel A/Tech. I will have a look at some of them. Thanks for watching. Take care. I have subscribed!!!!
@@atech9020 , both Psionic Audio and Brad's Guitar Garage have noted the prevalence of failure on JJ tubes, but it doesn't seem to be an issue on olderk JJ tubes that they had in stock, only on more recent production JJ'S. Granted, JJ doesn't make the glass and they might even buy the bulbs pre-formed, but there are special annealing processes for vacuum tube bulbs and it does seem as if somebody might be skipping a step here.
PS, I've watched a fair number of videos on vacuum tube manufacturing, but most of those films are at least 60 years old, and we don't get to see much of how it's being done in China and Russia these days.... I think it's safe to say that they're not working to the same quality standards that RCA, GE, Tungsol, Mullard, Amperex, Telefunken etc etc did back in the day.
Great bit of fault finding Martyn. Just another $1 spent on better components would make the purchase of this model very tempting. Once again your playing has made it sound better than it is in the hands of most of the rest of us, ha-ha..
Hello Pete, thank you. Yes I agree. A great looking amp and sounds good. But not made well inside. Thanks for watching and take care.
I have those switches on my truck! They operate a water pump and a lift.
Hello. Way better on your truck than on this amp. Thanks for watching and take care.
I was ready to trash this amp until you started playing -- it sounds fantastic in your hands, too bad they don't go the extra mile and make it to last more than 4 or 5 years, great picking!
Hello Gary. Yes, sound okay but as you say not built to last. Thanks for watching and take care.
@@vintageguitaramp_guitarteacher I've had mine 5 years. It was used when I bought it. Never a problem. However, I got someone to change the grid resistor above the power valves early on to a lower value, I also modified the back and more. The closed back was a dreadful idea. It seems the pre amp circuit on this is too 'hot' which causes it to clip earlier. The clean sound is good, it doesn't take pedals that well. The chassis layout is also not too clever. i've seen reviews saying it's a 'great' amp. It's good but not great ..
The DC heaters is probably only for the first valve and maybe the second as well, nonetheless that rectifier seems seriously underrated so maybe they went for broke.
Hello Steve. I think you are right. I am surprised it has lasted so long. It's not a new amp as you can see by the condition of the cabinet. Thanks for watching and take care.
Could you not double up that rectifier? Literally solder a second rectifier on top of it?
Hello. Never thought of that. Maybe could have soldered another on the underside of the board. That's a good idea. I will have a think about that one if I get another in for repair. Thanks for watching and take care.
@@vintageguitaramp_guitarteacher My rectifier is blown like this one. Would you suggest replacing with same value or as others have suggested go larger? and if so by how much? 🤔 I bought them amp used, the screen resistors had already been upgraded. I added 2 new filter caps, I went with higher value caps since there is no sag on solid state rectification( I had been told)
I find the input stage a little too hot and the reverb a little too wet, otherwise a great little amp. The ECO switch is a EU required thing, to shut it off after a while without playing.
Hello Christian. that explains the ECO switch. I might have known it would have something to do with the EU. Thanks for watching and take care.
Does the relay only latch on and stay on for a preset amount of time without signal going through the amplifier before the relay opens and shuts off the high voltage? It would be interesting to see from a schematic how they were doing that.....
You could give Brad Paisley a run for his money with your country picking!
Hello Michael, thank you. He's a great player. I would have my hands full taking him on. Thanks for watching. Take care.
You practically have to be a cellphone repairman to fix these things now. This isn’t a “tube amp”, it’s a piece of consumer electronics.
If those EL84’s were old Mullards or something, sure, go ahead an bias it like an old JMI AC10. New tubes don’t survive that abuse and yet, every new amp manufacturer does that to EL84’s. It’s a conspiracy to sell more tubes 😆.
To true Matt. not the nicest things to work on. If they are doing biasing to sell more tubes it is certainly working for them. Thanks for watching. Take care.
If you’re thinking about getting an AC10,just save another $200 and get an AC15 C1. Way better purchase.
Hello William. Agreed. Thanks for watching and take care.
Geez, the insides of that Vox AC10 Reissue looks nothing like a real Vox AC10, it's more like a modern equivalent.
Hello Simon. Like the inside of a laptop lol
the chassis layout on these is not too clever..bringing the power tubes right behind the speaker and the closed back is also wrong. I keep hearing people describe this amp as 'great'. It's good, but not great
Hello Vincent. Yep, not a great build. They don't sound bad though. Just like most modern amps of today, not built to last. Thanks for watching and take care.
@@vintageguitaramp_guitarteacher the valve amps that are built right , with good parts, usually from the USA, cost a lot more. I think with Vox amps speakers with Alnico magnets give a less harsh sound ..
@@vintageguitaramp_guitarteacher I just remembered this post. Looking at the closed back on this amp, in hot weather it can get quite warm inside , but also that small vent at the bottom can get blocked . you might put it down somewhere and if the airflow is restricted, will get even hotter. Only one vent on top too ..