Budget Vintage - Knight KM15 Guitar Amp Conversion (Dr. Z, Marshall, Vox)
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- čas přidán 11. 07. 2024
- In this video we will follow the conversion of a cool, vintage EL-84 powered Knight Allied Radio hi-fi mono amplifier head into a kickass 15-18 watt tube guitar amplifier. We will end up with something comparable in tone and power to an 18w Marshall, a Vox AC-15, or a Dr. Z Carmen Ghia.
Tone stack information: amps.zugster.net/articles/tone... - Věda a technologie
I am always impressed by anyone who can play a guitar well.
I was never a good guitar player, but I am an excellent design engineer who loves working with vacuum tubes.
You created your biggest problems yourself when you modified the circuitry around the first 12AX7 tube.
You were basically running them, (both halves of the 12AX7), in saturation because they had no bias on them.
Those very large value resistors, (multi megohms), from grid to ground were there to use a self biasing system called grid-leak bias.
Without bias the tubes operated more like rectifiers than linear amplifiers.
That, coupled with the extremely high gain involved and the fact that you never removed the bass boost compensation that is required for all phono inputs, was the most likely cause of your motorboating.
You have to go back to those multi megohm resistors to regain some bias and get rid of the RIAA compensation (bass boost, treble cut) network between the two halves of the tube to use it with a guitar.
You could also switch to more conventional biasing with resistors in the cathode circuits of each half of that 12AX7 tube.
Look at the 1st stages of other amplifiers that do not use grid leak bias for examples of what those circuits should look like if you decide to switch to cathode biasing.
I have one of those amplifiers, with no parts changed in it, still working perfectly with zero hum and no motorboating. I know I should replace the stand up multi section filter cap but leaving it in to see how long it will last.
Allied radio was in the electronic parts business, (they still sell parts), and they used high quality components in their kits, which is probably why it has worked so well for so long!
Way back in the 50's when I was a kid it used to take me 2 buses and a train to go shopping at Allied Radio... this video was a kick for me, brought back lots of memories of building all sorts of Knight kits. Wish I still had some of them. Thanks for sharing.
Me too!
I lived in the 'Back of the Yards' neighborhood in Chicago.
I also spent a lot of time at the Olson radio store located right across the street from Allied.
Still have any you could let go of?
These were kits that you built at the time that Stereophonic sound was starting to take off.
I built my first one in '63 when I was in high school and just starting to play guitar.
Eventually built a second one with some 12" speakers for a parent annoying stereo.
Allied Radio was an electronics supply company back then and had the Knight Kits as a competitor to the Heathkits that were a little more sophisticated and more expensive.
The Tandy Corp, parent of Radio Shack bought Allied in 1970. After 85 years, Allied Electronics is still in business selling electronic parts.
Thanks for the info! I didn't know that.
From one Louisvillian to another: Your videos are as interesting and informative as one could possibly hope for, especially for the 'hands-on' tech and guitarist. I've picked up some great tips/ideas from your stuff, most recently during my rebuild of a 6bq5-driven [RCA Victor] stereo console for guitar use which I had converted some time ago but was never pleased with the sound of. Lookin' forward to your next one.
Awesome Scott. Thanks for that. Maybe we can hang out some time.
Holy smokes man, that thing sounds great! Good job!
Excellent video. Picked up some great tips. Thanks for sharing.
Very cool! Thanks for sharing your knowledge with us!
Sweet man I love it. I'd love to take courses on building and moving amps. Great job!
Hi Guitologist, Cool little amp, I can see where it would make a great conversion project. I watched an Uncle Doug video on his home made Fender 5F2-A and the volume and tone circuit used is really good and can be applied to other amp circuits. Doug has a video on some PA amps he converted and one was motor boating, turned out it was the transformer, a wire was broken, causing the problem. Take it easy, C.
Damn! You are bad dude on that tele!
Great sound and great licks.
Thanks!
Hey, thanks for the video, you are not the only one that likes to mod the stuff that aren't meant for guitar amps, actually my favourite amp is a Biennophone Sottens old EL86 radio reciever! Much respect here!
Hi, this is Don in Ohio. I found your Knight KM15 vid on conversion, I have one and would so like to do this. I printed the original schematic and watched over and over, read every question and answer, but can't fill in the holes on my schematic. I know it's probably a lot to ask because you told someone you lost your schematic, but could you revisit this build and make a final one to show? I just don't have the knowledge to fill in the things I don't know by myself, but I can read and follow a schematic. I think a final schematic would give your video a happy ending for a lot of folks like me.
I built this amp back in 1968 and it motor boated too. I loved it.
Hi G, you can use a metal shield to cover the input jack that may help with power supply interference. Nice conversion. You do pretty darn good on the amp conversions. Be good, C.
A friend of mine back in 1971 had Knight amp very similar to this one. I think it was mono and had a 1/4 inch jack. we hooked it up to my dads stereo speaker cabinet ( 12" 4" and tweeter). I plugged my Ric into it and it sounded awesome! My friends Less Paul Jr. sounded impressive also.
Sounds like a good combination. I bet the P90 rocked.
I've seen that "Zobel Network" in circuits before, but dint know it had a name! Cool project (and playing). Thanks for posting :-)
The zobel network in the Hammond ao35 was most likely the inspiration for dr z including one in his Carmen Giha amps
Just as a point of interest: ALLIED RADIO made Knight (and Knight Kit) equipment. I ran in to them when trying to get into amateur radio-they made kits that some guys liked to build. Apparently, they tried to make it in audio too.
Sounds great
you make it look easy. did you change out the bass and treble pots? where are you taking the in from for the tone stack and where does the out go to
Great video! The Hammond conversion looks good too.
+Mike Choffin Thanks Mike! Yeah, it occurs to me I never did a video tour of that finished project in the cab. It's my little Amway pyramid scheme amp. :P
+The Guitologist I was hoping you had some video or still shots of the finished product. Looks cool
You mean the Hammond? Or the Knight?
+The Guitologist Yes the Hammond
As soon as I'm done with this Teisco bass restoration, I may make a quick video of it. :)
Thanks Brad. Most of the hum was from poor cap grounding. I grounded the negative side of the rail to the chassis instead of the buss and it's much quieter. I used 2x 33uf and one 22 uf. I temporarily added a 70uf in front of the first 33uf cap with jumpers and it's dead quiet. The 70uf was an older Sprauge that's just way too big (C cell battery size) so I ordered a much smaller 60uf which should work fine. I used the standard 2.2k 2W dropping resistor and the voltage was perfect on that node but I had to up the 82k dropper to 180k to get the last 22uf down to spec.
Now to attack the tone stack. I'm fairly new to building amps (Ive built 5 kit amps from Tweed Deluxe's, an 18W Marshall and a Deluxe Reverb) but Ive never built a tone stack but I'd like to try try some type of blackface Fender tone stack. like a Champ or similar. Any comments???
Thanks so much for the help. I really appreciate it and your YT channel as well.
Next up for modding is my Bogen CHB 50 PA. ;).
Jim.
Great to hear, Jim. Sounds like you did some effective troubleshooting there. Nice catch on the ground issue. Those are hard to track down! As for tone stacks, I liked the range on the Bandaxall stack I used in this amp. I've also used the Brownface stack and the Marshall stacks on that page. All are good for different things. The tone stack is the most important choice in voicing the amp. Far more effectual to the overall tone of the amp than anything else, including tube types. This is a good site to start looking at stacks: monster.partyhat.co/article/amplifier-tone-stacks/#Baxandall
I think I'll try the Bandaxall since it looks to be one of the simpler mods. I have about 14 tube amps so it's not like I need this one right away. My first build was a Deluxe Reverb kit because It was the only amp I was going to build. A few 5E3 Tweeds, an 18 watt Marshall and a couple of mono block/Hi Fi conversions later and I'm starting to think I have an addiction. I couldn't even read a schematic 3 months ago. I think the Bogen CHB 50 is going to be the toughest one so far. I've watched your video a few times so that should be very helpful.
Thanks again.
Jim
Hi Brad, I know this is an older post, but I just noticed that the input jack isn't grounded and doesn't have a grid leak resistor.. Isn't there typically a one million ohm resistor to ground to establish the proper impedance? I'm just curious as I thought those reduced hum.. Does it not matter? Thanks, Tom
Nice job. Lots of gain.
Where do you get your caps from and what brands do you prefer ? Sorry if that's been asked before in a different video.
No prob. Ask all the questions you like. I get mine from Antique Electronic Supply. They are the inexpensive variety. I have had good luck with them and they are way smaller on average than F&T and Sprague caps, so they fit in more applications. This way I can keep just one brand in stock that I know will work well with everything I might encounter.
what were your settings on the amp and guitar at the end of the video?
that thing sounds pretty dang good. I wonder how cheap\easy these things can be had?
That initial minor chord is pure Pretender / Foo Fighters
I love that tonestack! I make them with a potentiometer for R2 and R3/4 connected to an 1M resistor parralel with a switch to ground for a little boost.
Got a schematic for that? IF you don't mind sharing here, please do. Others may be interested in that. Thanks for the comment!
I made a schematic in multisim with some bode diagrams to see the effect of the switch and shift function:
I forgot to edit the schematic: the 1M resistor (R6) should be a 39K resistor!
i1149.photobucket.com/albums/o591/Jarno_Verheesen/tonescack%20image_zpsehttq66i.png
i1149.photobucket.com/albums/o591/Jarno_Verheesen/tonestack%20bode%20plots_zpsvm5bjdoe.png
It works great and sounds very nice :)
Thanks for the comment. I'm sure some of my viewers will find that information very useful.
I'd be interested in seeing that schematic myself
I have one of these amps that was handed down to me, can you recommend someone to convert as you did? tks
safe to assume you just made a modded Dr. Z Carmen Ghia?
The sleeper is alive! Nice job Brad. Out of my own curiosity, did you pin point what the cause of the motor boating was? Apologies in advance if I missed an explanation in the video.
+Dan79istheman I don't think I adequately explained. I got rid of the NFB entirely and eliminated the big ground connection between the input jack and the output common. That cured it.
+The Guitologist ah ha! Them damned Eddy currents... Nice detective work!
YeahI could have sworn I did a better job explaining that in a clip, but I guess not. This was filmed over the course of several days.
For those interested, I have a Fanon Masco 3320 pa amplifier that I can not seem to find the schematics for, The Knight KM15 is almost the same tube line up and configuration.
Still have that masco?
How does yours break up so much? Even at full volume mine stays clean
ok, so i picked up one of these at a swap meet for literally $6. it had old stock Mullard tubes in it as well. anyhow, what I'm wanting to know is how much clean headroom can i get out of this little guy? theoretically speaking, if i were to use 12at7's on the Phase inverter as well as the preamp (phono), would that get me enough clean headroom to run a Klon type overdrive pedal like a soul-food or an archer? OR would i be better of keeping the 12ax7 in the phase inverter and using something like a 12 at7 in the preamp stage to mimic the power output of a EF86 (and perhaps get a little closer to a Vox ac15 kinda output).
If try a 12au7
where can i buy these amps??
Nice job Brad. I have the same amp and am trying to follow your conversion. Can you please tell me if you ditched the entire cap can or did you use any of the internal caps?
I cant see in the video how you have the new power rail wired. Is the 22uf in series with the 33uf's or are the 33uf's in parallel etc. .
Thanks very much.
Jim.
I ditched the entire cap can and wired in new caps exactly as they were wired originally.
Thanks so much for the reply Brad. Was the last cap that was mounted with the lettering facing down a 33uf as well?
Thanks.
Jim.
Hi Brad, can you PLEASE tell me what the value i of the third filter cap is. I can read the 22uf, the 33uf but the last one is installed so the lettering is not showing. Also did you use the stock value dropping resistors? I cant see the colors well enough. I'm stuck on my conversion and need to know what I did wrong. PLEASE....... Thanks.
Jim
Did you google the schematic? That cap should be either a 33uf or 47uf.
Thanks for the reply Brad. I have the schematic as well as the entire assembly manual that came from Knight when they sold these kits. The schematic shows 60uf, 40uf, 20uf and a 20uf/25v for the cathode bias. I used 33uf/ 33uf and one 22uf as thought you did. I used the stock spec dropping resistors. I have a very loud hum that I cant track down. I already did the artificial center tap on the 6.3v so it's not that and my high voltage tap didn't have caps like the schematic shows. It has a normal CT
My power rail and most other voltages are high everywhere except the plates which are only around 280v .
I wish I knew what size caps and dropper resistors you used so I could narrow down this noise.
Thanks for the reply, I really appreciate iy.
Jim
I just bought a KM-15 to convert to guitar as a present to a good friend of mine. I am having a difficult time using the video as a schematic. I would love to replicate your exact build. Could I find your final schematic somewhere online? Thanks!
Hi Nick. Thanks for the question and for watching... I'll dig around when I get a chance and see if I still have that schematic, but assume I do not. I think I showed it in this video, didn't I? Haven't watched this one in a while...
Thank you for looking! And yes you did show it a few times but I think there were changes made that were marked and some unmarked. Thanks and let me know if you find it!
Man I played that same guitar last week, exactly the same. Hey can you barely see the wood grain through the paint?
Yes. You can see grain through the finish on these.
I have the same amp set up similar to yours. my transformer gets burning hot. is that normal for this amp?
Mine didn't get hot.
Brad if I could Extract the talents you have for building and playing out of your little finger bra I'd be good for life!!! Excellent Sounding amp! WHAT a great little Cooker for rock and blues!! Just Wow!!!!
I did the same thing with a radson 305d, but I left the speaker outs as they were and just cup one end of a shitty speaker cable.
That works too. I just like having the jack so you don't have cables hanging all the time.
Yeah, I considered it, just didn't seem worth the hassle at the time. Btw I love your channel, I'm now planning to replicate a couple of your projects.
interesting seeing a guy messing around with electronics apparently without much theoretical knowledge : calls it "weard" if he doesn't understand neg. feedback in a hifi amplifier to improve distorsion for instance, and two caps on the filament supply to deminish hum by counter-phasing to the ground (less powerwasting then resistors). But he gets along and I enjoy!
Thanks for the comment. I'm a guitar amp guy, and the features you mentioned are not usually seen in guitar amps. I understand NFB perfectly well, but it's "weird" seeing it tied into the tonestack like this one was in guitar amps. In fact, it's weird even for hifis. The cathode of one of the preamp tubes is the usual place. Also, I do understand the theory behind a false center tap to reduce noise, but I'm used to seeing resistors rather than caps in that role. And I believe I was thinking at the time if one of the caps had failed or drifted in value, maybe the virtual centre tap was unbalanced. That was a new one on me! Learn something every day. Glad you enjoy the videos, man. Keep commenting!
Quite ok! Thanks for correcting my grammar, slip of my "frenchie" pen! Just built a 15W guitaramp 2x ecc83/el84 from scratch, as a good radio-amateur (ham F5FLM / PAoGDS) made a chassis from double sided printed circuit board and included some recovered stuf from several valve radios I repair for nostalgic friends. I've taken some pictures of it and will publish them shortly. Kind regards.
Your jam sounds kind of like Big Star
Allied-Knight was bought back in 1970 or '71 by Tandy when they bought Radio Shack.Around a year or so later,the courts stepped in and forced Tandy to sell off Allied.They never got back into the business of selling audio,ham radio or anything else other than parts.The courts told Tandy they had 60% of the market.
Man, I`m sorry but this takes too long... love K