A friend brought an old amp for repairs. He said someone modified it long time ago and it was a good guitar amp. Originally it seemed to be a 50-60's hi-fi monoblock. I could not find any label or whatever indication on who made this amp, I'll post some pictures of it here - maybe someone recognizes it and there's the original scheme. Now it's not working at all, so the aim is to get it back to guitar - amp shape.
When I opened it it was quite dirty, but the original wiring was neat, except for several spots where the unnamed genius added his strange mods. The tube lineup was very, very strange: a pair of EL34 (Mullards!), 12AU7, 12AX7 and 6CA4. I'm pretty sure I haven't seen 6CA4 powering any PP EL34 amplifier before.
The fuse was rusty and that was the first reason it was dead, so after some basic cleaning it was "ready" to work. I fired it up with a variac on low voltage too see how bad it is, and pretty soon it started humming and even some remaints of the input signal were sometimes heard through the speaker, but it was farting and humming and the wall of noise was terrible. Of course, the filter cap was dead, but I feel the signal path was messed up badly because it sounds like a zombie raising from a grave. I raised the voltage a bit and it immediately started smelling real bad, like a burned transformer I believe. So I've turned it off.
I've drawn a scheme of the thing and it's crazy (or maybe I'm not too clever to understand). One section of 12AX7 is disconnected (who needs second gain stage in a guitar amp really?), the output tubes' screens are disconnected from cathodes and wired to the output (!!) and to the power supply (!!!) via a large electrolytic cap. There's some sort of a feedback pot which is also a mod, and it is wired to a jack (speaker output?) on the front panel, next to the input - I forgot to draw it in the scheme.
What is original?
I guess the PI, the output stage wiring and all the sockets are stock. It is possible someone swapped the tubes. I don't think the rectifier is a mod, but EL34 were probably 6L6's or 6V6's (200 ohm cathode resistor), and I'm not sure about 12AU7, although I loved the way someone wrapped it in a piece of yellow cloth (keeps it clean!).
Before anything I need to understand what it was originally. The PT doesn't seem to handle two EL34's and I don't think 6CA4 is capable of it. But I may be wrong. Any ideas on the original tube lineup? Or maybe it IS original?
When I opened it it was quite dirty, but the original wiring was neat, except for several spots where the unnamed genius added his strange mods. The tube lineup was very, very strange: a pair of EL34 (Mullards!), 12AU7, 12AX7 and 6CA4. I'm pretty sure I haven't seen 6CA4 powering any PP EL34 amplifier before.
The fuse was rusty and that was the first reason it was dead, so after some basic cleaning it was "ready" to work. I fired it up with a variac on low voltage too see how bad it is, and pretty soon it started humming and even some remaints of the input signal were sometimes heard through the speaker, but it was farting and humming and the wall of noise was terrible. Of course, the filter cap was dead, but I feel the signal path was messed up badly because it sounds like a zombie raising from a grave. I raised the voltage a bit and it immediately started smelling real bad, like a burned transformer I believe. So I've turned it off.
I've drawn a scheme of the thing and it's crazy (or maybe I'm not too clever to understand). One section of 12AX7 is disconnected (who needs second gain stage in a guitar amp really?), the output tubes' screens are disconnected from cathodes and wired to the output (!!) and to the power supply (!!!) via a large electrolytic cap. There's some sort of a feedback pot which is also a mod, and it is wired to a jack (speaker output?) on the front panel, next to the input - I forgot to draw it in the scheme.
What is original?
I guess the PI, the output stage wiring and all the sockets are stock. It is possible someone swapped the tubes. I don't think the rectifier is a mod, but EL34 were probably 6L6's or 6V6's (200 ohm cathode resistor), and I'm not sure about 12AU7, although I loved the way someone wrapped it in a piece of yellow cloth (keeps it clean!).
Before anything I need to understand what it was originally. The PT doesn't seem to handle two EL34's and I don't think 6CA4 is capable of it. But I may be wrong. Any ideas on the original tube lineup? Or maybe it IS original?
OK, first steps: replace the electrolytic caps, then change the rectifier to something a bit more reasonable (5AR4). Output stage suppressor grids (or beam plates) get connected to their respective cathodes. This will at least get you in the ballpark, at which point you can start peeling back the next layer of the onion.
Original tubes were probably 6L6.
Original tubes were probably 6L6.
I've heard of guitar players asking for additional gain stages in the front end before the volume pot so they can overdrive the input without overdriving the output. With the input pot as the volume that is not likely unless the volume pot was later moved.
I'm working on a JCM800 that the owner has specifically requested this. I convinced him that I should restore the amp to original condition and fix all the problems, then let him play it a couple of weeks before adding any mods.
I'm working on a JCM800 that the owner has specifically requested this. I convinced him that I should restore the amp to original condition and fix all the problems, then let him play it a couple of weeks before adding any mods.
Thanks for the advice. I hope you forgot that 6CA4 is a 9-pin indirectly heated rectifier, and 5AR4 is an octal directly heated one. I'm not sure a more reasonable 9-pin 6v rectifier exists, so I've thought it is original. And still I'm not sure it can power a pair of 6L6's under 350V plates.
At this stage I don't need an advice about guitar amp wiring, this is not a problem here. Of course, no guitar amp features volume control before the first gain stage, that's plain ridiculous.
Nope, didn't forget about the envelope, I assumed you'd change the socket. I can't think of any 9 pins that would be reasonable to use in this service.
5AR4 is indirectly heated.
350-0-350 trafo would give 430..450V dc, depending on the trafo quality.
This is too much for an ordinary 6V6. Then again, 7W 2700 Ohm resistor in the screen supply is 10x too big for EL34, though.
Whatever the original valves were, if you want guitar amp duty, there is an ideal valve to use here: the JJ 6V6S. This splendid tube can take 500V on the anode, and your 2700 Ohm resistor will be just right, if you like a sweet early breakup. And, you can keep the 6CA4/EZ81.
Another serious consideration is that the JJ takes the standard 450mA heater current, as opposed to 1.5A EL34, or 900mA 6L6. This helps keep the trafo cool, rather than on fire. If the amp used lighter heat current by design, using 2x EL34 could destroy the trafo. Similarly, I can't recommend using 5AR4/GZ34 in the rectifier position, since you'll double the heater current draw on the rectifier heat winding, and looking at that old trafo, overstressing is not what he needs at his time of life.
Old elkos in the power supply can destroy the trafo too, but choose the new one carefully. Too high a value brings high peak currents, and increased rms current = more trafo heat. The 6V6S works perfectly on 2x series-connected 68uF/400V Samwha HJ series (220K balance resistors). Avoid ordinary 33uF/450 or 500V, as the ripple current handling is too low.
Looks like Dr Mod was experimenting with squeezing more power from EL34s by raised VG3. Might worry about what has happened to the trafos with that kind of abuse going on.
This is too much for an ordinary 6V6. Then again, 7W 2700 Ohm resistor in the screen supply is 10x too big for EL34, though.
Whatever the original valves were, if you want guitar amp duty, there is an ideal valve to use here: the JJ 6V6S. This splendid tube can take 500V on the anode, and your 2700 Ohm resistor will be just right, if you like a sweet early breakup. And, you can keep the 6CA4/EZ81.
Another serious consideration is that the JJ takes the standard 450mA heater current, as opposed to 1.5A EL34, or 900mA 6L6. This helps keep the trafo cool, rather than on fire. If the amp used lighter heat current by design, using 2x EL34 could destroy the trafo. Similarly, I can't recommend using 5AR4/GZ34 in the rectifier position, since you'll double the heater current draw on the rectifier heat winding, and looking at that old trafo, overstressing is not what he needs at his time of life.
Old elkos in the power supply can destroy the trafo too, but choose the new one carefully. Too high a value brings high peak currents, and increased rms current = more trafo heat. The 6V6S works perfectly on 2x series-connected 68uF/400V Samwha HJ series (220K balance resistors). Avoid ordinary 33uF/450 or 500V, as the ripple current handling is too low.
Looks like Dr Mod was experimenting with squeezing more power from EL34s by raised VG3. Might worry about what has happened to the trafos with that kind of abuse going on.
At first I thought it was a Philips amplifier but the underside tells perhaps a different story.
It has all the construction hallmarks of a Linear Concorde amplifier and your example is probably one of its many clones. In the 60's the hobby radio magazines carried many advertisements for amplifier kits like this one.There are certain components YOU MUST check before switching on vintage amps like this. Apart from having to reform or replace the electrolytic smoothing capacitors, the components that are a MUST are the coupling capacitors feeding the grids of the output valves. On the circuit diagram they appear as 0.1uF 400V and looking at the photographs I suspect them as being the dark brown shiny ones. Check that these are not passing HT onto the grids of the output valves.
Normally if I am changing the electrolytic capacitors I also change these couplers especially if their outer shells have cracks as this allows dampness to get in.
I hope the output transformer is still OK, many amplifiers of this design seem to suffer from one half of the primary going open circuit.
Les
It has all the construction hallmarks of a Linear Concorde amplifier and your example is probably one of its many clones. In the 60's the hobby radio magazines carried many advertisements for amplifier kits like this one.There are certain components YOU MUST check before switching on vintage amps like this. Apart from having to reform or replace the electrolytic smoothing capacitors, the components that are a MUST are the coupling capacitors feeding the grids of the output valves. On the circuit diagram they appear as 0.1uF 400V and looking at the photographs I suspect them as being the dark brown shiny ones. Check that these are not passing HT onto the grids of the output valves.
Normally if I am changing the electrolytic capacitors I also change these couplers especially if their outer shells have cracks as this allows dampness to get in.
I hope the output transformer is still OK, many amplifiers of this design seem to suffer from one half of the primary going open circuit.
Les
Rod may be right about the sterling electrical qualities of JJ's 6V6S. Unfortunately, the quality of JJ's Octal production stinks on dry ice! 
The 6CA4/EZ81 stands very tall, for a Noval rectifier. It's rated for up to 150 mA. of B+. That 40 μF. 1st filter cap. is within published limits.
I too think that the original O/P tubes were not EL34s, because of their 1.5 A. heater current draw. 6L6s (not GCs) are a possibility. The sockets could have been rewired to take EL34s, while the original tubes may have been 7591s. Factoring in tone control losses, the gain structure present favors "12" W. tubes and 7591s. That 350-0-350 rectifier winding argues against 6V6s.
Let's focus on the 7591, whose data sheet is here. Heater draw is a comparatively modest 800 mA. Look at the cathode bias AB1 full pentode mode column. Notice the 200 Ω shared cathode resistor. Notice that a pair of 7591s, along with the small signal complement, do not exceed the 6CA4's capability. The case for 7591s being original is (IMO) pretty strong. EH is currently producing decent 7591s.
Engels, please measure the O/P trafo's primary. That number might be revealing.
BTW, a way to pick up some additional heater current capability is to replace the incandescent pilot lamp with a LED/PN diode/current limiting resistor combo.
The 6CA4/EZ81 stands very tall, for a Noval rectifier. It's rated for up to 150 mA. of B+. That 40 μF. 1st filter cap. is within published limits.
I too think that the original O/P tubes were not EL34s, because of their 1.5 A. heater current draw. 6L6s (not GCs) are a possibility. The sockets could have been rewired to take EL34s, while the original tubes may have been 7591s. Factoring in tone control losses, the gain structure present favors "12" W. tubes and 7591s. That 350-0-350 rectifier winding argues against 6V6s.
Let's focus on the 7591, whose data sheet is here. Heater draw is a comparatively modest 800 mA. Look at the cathode bias AB1 full pentode mode column. Notice the 200 Ω shared cathode resistor. Notice that a pair of 7591s, along with the small signal complement, do not exceed the 6CA4's capability. The case for 7591s being original is (IMO) pretty strong. EH is currently producing decent 7591s.
Engels, please measure the O/P trafo's primary. That number might be revealing.
BTW, a way to pick up some additional heater current capability is to replace the incandescent pilot lamp with a LED/PN diode/current limiting resistor combo.
Eli, I think the 7591 is right. The other correlator is the 2700 Ohm screen supply resistor. In the same AB1 pentode data column of the TungSol sheet, the Va/Vg2 spec is 450/400V, and screen current at idle 11.5mA. The Screen drop is much more than would be necessary for EL34.
I have read many accounts of JJ Octal quality, but have never suffered it. I switched to ElectroHarmonix EL34 for my customers' guitar amps because of the scare, only to find 10% failure at switch-on for the first couple of dozen, and some early field failures, too. I won't be buying any more of those. IME, the JJ EL34 is tougher in guitar amps, and I have had zero failures in these. I buy only from a vendor I trust, and so it's possible that my supplier (AC/DC Valve) is doing a good job of weeding out bad samples - but there is no indication in the price that this is happening.
The reports of undersized JJ octal pins may be true. But the McMurdo style base found in old amps in the UK grip them perfectly well without change, and with Fenders, it's a simple job to retension the sockets, which should be attended to in a major service anyway.
Eli, didn't want to go arguing with you about vendor relative quality, but I think the picture widely presented on DIYA is much worse than the reality. In the UK there are at least as many vendors offering JJ EL34 as EH, & I think that really bad quality would have made them give up - it is quite a while since these claims were first circulated. Surely, the dealers will give them hell if the returns rate is too high. I hope the factory is listening, and will fix the problems, but for many sockets (EL84, EL34, 6V6 - at least) the JJ is the best-sounding of all new production, and very tough, too. The worst outcome is that their sales collapse from overstated quality problems, and we lose the option of this factory's output.
The 6V6S is a unique valve for installing in a 6L6Gx socket (use a larger screen resistor!) and gives many guitarists just what they need - proper output-stage breakup at a much quieter level. I have had zero reliability problems with them, even among those run at above rated anode dissipation.
For the amp in question, the 7591 is likely to be right for the original hifi purpose. But for guitar, the 6V6S will give a sweeter sound, and make life much more comfortable for the old trafo.
I have read many accounts of JJ Octal quality, but have never suffered it. I switched to ElectroHarmonix EL34 for my customers' guitar amps because of the scare, only to find 10% failure at switch-on for the first couple of dozen, and some early field failures, too. I won't be buying any more of those. IME, the JJ EL34 is tougher in guitar amps, and I have had zero failures in these. I buy only from a vendor I trust, and so it's possible that my supplier (AC/DC Valve) is doing a good job of weeding out bad samples - but there is no indication in the price that this is happening.
The reports of undersized JJ octal pins may be true. But the McMurdo style base found in old amps in the UK grip them perfectly well without change, and with Fenders, it's a simple job to retension the sockets, which should be attended to in a major service anyway.
Eli, didn't want to go arguing with you about vendor relative quality, but I think the picture widely presented on DIYA is much worse than the reality. In the UK there are at least as many vendors offering JJ EL34 as EH, & I think that really bad quality would have made them give up - it is quite a while since these claims were first circulated. Surely, the dealers will give them hell if the returns rate is too high. I hope the factory is listening, and will fix the problems, but for many sockets (EL84, EL34, 6V6 - at least) the JJ is the best-sounding of all new production, and very tough, too. The worst outcome is that their sales collapse from overstated quality problems, and we lose the option of this factory's output.
The 6V6S is a unique valve for installing in a 6L6Gx socket (use a larger screen resistor!) and gives many guitarists just what they need - proper output-stage breakup at a much quieter level. I have had zero reliability problems with them, even among those run at above rated anode dissipation.
For the amp in question, the 7591 is likely to be right for the original hifi purpose. But for guitar, the 6V6S will give a sweeter sound, and make life much more comfortable for the old trafo.
The only vendor I know of that has given up on JJ octals is Jim McShane. I also had a bad experience with a quad of their KT77s, but the ones that replaced them have been working great and don't suffer from the sliding cathode issue of the first. No issues with the GZ34s or 6L6GCs I have, though mine don't see guitar amp service.
Like you said, all the major vendors that appear to cater to mainly guitar amp owners all carry JJ octal tubes.
Like you said, all the major vendors that appear to cater to mainly guitar amp owners all carry JJ octal tubes.
Hi, I'm not a guitar amp expert but...
If you haven't done so yet, I'd recommend checking all the resistors. Maybe even replace all the old ones on principle. If they haven't died or drifted yet, they're probably thinking about it.
Having the EL34 screens (G3/suppressor/whatever) connected to the 4ohm tap on the OPT doesn't look too bad to me. DC-wise they're at ground i.e. a little lower than the cathode voltage, which shouldn't hurt the tubes.
OTOH, modulating the screen voltages like that should cause some distortion, which was probably the intention. (About the only thing I've learned about guitar amps is that distortion is often desirable)
The 50K log pot connected to the 2'nd stage cathode looks like it's intended as variable feedback, so the other end of the pot should perhaps be connected to one of the output taps too? Being a guitar amp, I wouldn't put money on whether positive or negative feedback was intended though.
Cheers - Godfrey
If you haven't done so yet, I'd recommend checking all the resistors. Maybe even replace all the old ones on principle. If they haven't died or drifted yet, they're probably thinking about it.
Having the EL34 screens (G3/suppressor/whatever) connected to the 4ohm tap on the OPT doesn't look too bad to me. DC-wise they're at ground i.e. a little lower than the cathode voltage, which shouldn't hurt the tubes.
OTOH, modulating the screen voltages like that should cause some distortion, which was probably the intention. (About the only thing I've learned about guitar amps is that distortion is often desirable)
The 50K log pot connected to the 2'nd stage cathode looks like it's intended as variable feedback, so the other end of the pot should perhaps be connected to one of the output taps too? Being a guitar amp, I wouldn't put money on whether positive or negative feedback was intended though.
Cheers - Godfrey
6l6
Okay, I've unsoldered all the idiotic mods - the feedback loop, the screens etc., connected the unactive half of 12AX7 to the input, then the volume pot, then the second stage (which was the first before). And a pair of 6L6 instead of EL34. It took me about 15 minutes. Turned it on - works fine! The transformers doesn't seem to overheat and EZ81 handles two 6L6's, although I couldn't play it at the full volume - too damn loud. The sound is pretty clean so I think 6L6 is the original output tube.
I guess it is some DIY-thing from a kit because the circuitry is a bit too simple to be a production model. The sound is NOT cool, very dull and dark, the preamp is noisy, sounds like something is cooking in there.
I will probably have to change the whole preamp to something brighter (and less noisy). One tone knob is no good because one pot tone control is usually hi-cut and I need hi-boost, sort of VOX top-boost (which is unfortunately impossible since it needs another stage as a cathode follower and two knobs instead of one). Whatever...
Additional question:
I was surprised to find out I have only one pair of 6L6 left and they're pretty expensive Brimar 6L6G's I would'nt sacrifice for a guitar amp. So I need to buy me some 6L6's. Should I still give a try to 7591's? Actually, I was thinking about trying a pair of coke bottle chinese 6L6G's because they're the cheapest and look cool (ST is the best!), plus some folks reported they're good in guitar amps.
Okay, I've unsoldered all the idiotic mods - the feedback loop, the screens etc., connected the unactive half of 12AX7 to the input, then the volume pot, then the second stage (which was the first before). And a pair of 6L6 instead of EL34. It took me about 15 minutes. Turned it on - works fine! The transformers doesn't seem to overheat and EZ81 handles two 6L6's, although I couldn't play it at the full volume - too damn loud. The sound is pretty clean so I think 6L6 is the original output tube.
I guess it is some DIY-thing from a kit because the circuitry is a bit too simple to be a production model. The sound is NOT cool, very dull and dark, the preamp is noisy, sounds like something is cooking in there.
I will probably have to change the whole preamp to something brighter (and less noisy). One tone knob is no good because one pot tone control is usually hi-cut and I need hi-boost, sort of VOX top-boost (which is unfortunately impossible since it needs another stage as a cathode follower and two knobs instead of one). Whatever...
Additional question:
I was surprised to find out I have only one pair of 6L6 left and they're pretty expensive Brimar 6L6G's I would'nt sacrifice for a guitar amp. So I need to buy me some 6L6's. Should I still give a try to 7591's? Actually, I was thinking about trying a pair of coke bottle chinese 6L6G's because they're the cheapest and look cool (ST is the best!), plus some folks reported they're good in guitar amps.
After you added an extra gain stage, the 6L6s worked. 6L6s and 7591s use the same O/P "iron". However, the 7591 is as easy to drive as an EL84.
You have a lot to work with in turning out a nice guitar amp. A 12AX7 preamp will take care of increasing the pickup's O/P and driving a proper tone stack. A 12DW7 (1/2 'X7 & 1/2 'U7) into EH 7591s, using Fisher style circuitry, will give you 30 very honest W. JJs 12DW7 is decent and a "reissue" TungSol 12AX7 in the preamp will give you some "chime".
The Russian 6П3С-Е (6p3s-e) is good deal in a 6L6 family tube. If you are equipped properly and have the time, buy in bulk and cull out pairs. Otherwise, buy properly culled tubes from Jim McShane.
schem
I've drawn the scheme of the current state of the amp -
While I'm waiting for 6L6's and 12DW7 to arrive I need to think of a brighter preamp for this thing. There should be one tone and one volume knob, but I don't like the classic one-knob tone circuit with 0.00... cap to the ground, it's usually useless. I will keep the funky PI following Eli Duttman's advise.
One thing I don't need is distortion, although everyone who hears "guitar amp" thinks it must distort. What I need is a sweet and clean amp, rather VOX than Fender. Not Marshall or Orange.
These days only genuine american blues dinos play one-channel amps with no clean headroom. The rest of the pack goes into clean channel with distortion pedal.
I've drawn the scheme of the current state of the amp -
While I'm waiting for 6L6's and 12DW7 to arrive I need to think of a brighter preamp for this thing. There should be one tone and one volume knob, but I don't like the classic one-knob tone circuit with 0.00... cap to the ground, it's usually useless. I will keep the funky PI following Eli Duttman's advise.
One thing I don't need is distortion, although everyone who hears "guitar amp" thinks it must distort. What I need is a sweet and clean amp, rather VOX than Fender. Not Marshall or Orange.
These days only genuine american blues dinos play one-channel amps with no clean headroom. The rest of the pack goes into clean channel with distortion pedal.
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