Help needed with messed-up-amp repairs.

[engels]

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?

[SY]

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.

[TheGimp]

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.

[engels]

Originally Posted by SY

change the rectifier to something a bit more reasonable (5AR4). Original tubes were probably 6L6.

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.

[DigitalJunkie]

Perhaps it originally used 6V6's,those might be a little more reasonable with a 6CA4.

[SY]

Originally Posted by engels

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.

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.

[engels]

Here are some pictures of the amp. Anyone recognizes it?

[Rod Coleman]

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.

[Hi-Q]

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

[Eli Duttman]

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.