1982 JCM 800 2204 problems

[frankeg]

Has anyone seen/did/heard of modding a jcm 800 (50 Watt) for chan switching? I have one that was modded using a 6 contact (3 n.o & 3 n.c.) 24 vac Potter & Brumfield relay. The relay coil circuit includes a 120/24 vac transformer, 2 lights (normal on and switched on), and a footswitch jack. It uses 2 of it's own contacts (n.o. & n.c.) to power the lights. This all works. The other contacts are used to switch in a 2nd pre-amp and master vol pot. The 2nd master vol works as well. It's the 2nd pre-amp vol that seems to be the problem which is low note buzz and sometimes a high freq sound not unlike tuning in a ham radio... I had to draw the circuit myself (no known history).
Question is: has this been done successfully before and is there something I can compare it to? Thanks in advance for any info...

[Chuck H]

The 2nd master vol works as well. It's the 2nd pre-amp vol that seems to be the problem which is low note buzz and sometimes a high freq sound not unlike tuning in a ham radio...

Sounds like parasitic oscillations due to the circuit mods. Increasing the gain and then running all the wires to a big relay is probably a layout problem for stability.

Question is: has this been done successfully before and is there something I can compare it to?

Sure it's been done. But there's no "standard" I know of for such mods so any comparison isn't likely to be useful. Your stuck modding or refining the circuit as a one of a kind project with no known working example.

Do you know the previous owner, or is there any way to find out if the amp always misbehaved with this mod? Do you have a scope? can you post the schem you drew and possibly some pics?

Chuck

[frankeg]

Here's a scanned image of the drawing I made. I don't know if this ever worked but I'm assuming it did for now. I ordered another relay although this one appears to work. I know from work that older relays can develope "wiskers" and act strangely. I really appreciate your response! This is getting frustrating.

[frankeg]

Some pics of the jcm 800.

[Chuck H]

Well I looked over your schematic a little but because it references the circuit board (so I'd have to fill in the blanks) and doesn't show any part values it's hard to asses much from it.

The layout due to the relay, while nicely bundled and kept on the preamp side where it should be, has a lot of what should be short leads doubled and lenthened. This is usually a problem with high gain preamps. The amp could probably be made stabil with top end bleeders and smaller coupling caps but the tone would suffer for it. Hardly matters since the amp doesn't work as it is.

If you fill in the blanks and add circuit values to the schem you'll probably get some responses.

Chuck

[Robert M. Martinelli]

Hi frankeg, Hi Chuck!
I think Chuck's right about the problem being due to some form of parasitic oscillation, only, I was thinking about a different reason for it. My opinion is that, by mounting the relay on the valves' side of the chassis you have created an unwanted coupling between the the relay contact blades (which, remember, carry the signals to be switched) and the preamp valve(s) grids. This results in a positive feedback which, most likely, is the reason for the parasitic oscillation you're dealing with. You should try to shield the relay either by mounting the relay on the other (components/PCB) side or by trying to shield it from the grids with a metal foil electrically and mechanically connected to the chassis.

BTW, where are the preamp valves' shielding covers? Looks like they're missing....

JM2CW

Hope this helps

Best regards

Bob

Edit: a couple of things I didn't mention, as they have nothing to do with the problem, but could be useful nonetheless.....
- 1- the relay you used seems way too big for the purpose. It looks like a medium power relay, primarily intended for switching currents in the 10 Amps range. You'd better use a smaller (signal) relay instead IMHO (being the relay smaller it would also be easier to install it on the other (PCB/components) side of the chassis.
-2 - It's a good design practice to install a reverse-biased diode (a 1N4007 will do just fine) in parallel with the relay coil. Its purpose is to "absorb" (shunt) the back EMF generated by the relay coil when you de-energize it.

[Gtr_tech]

Some pics of the jcm 800.

Boy...that sucker's sure hacked up ainnit....

[frankeg]

Thanks everyone for the replies. I got this amp as shown with no known history of why it was modded or if it ever worked right. I have managed to take it back to stock and it works fine that way. I "thought" it may never of worked right and your replies all seem to confirm that. It was a puzzle that interested me but I'm convinced it won't work now so I'll keep her stock. I would of thought a simple external a/b box connected to the high and low jacks would do the trick but who knows what the modder was aiming for. Nevertheless, thanks for the help...........

[Gtr_tech]

CAn't do the a/b box trick on that. The low gain jack is a normally closed switch that passes signal between the first and second stages when theres nothing plugged in. Plug something into that and the path is interrupted....you're putting signal directly into the second stage.....which sounds like crap btw.

Here's what I do to play clean on non ch switching amps: Put a .001u bypass cap across your gtr's in/out tabs on the vol ctrl(s), use a single coil, roll the gtr vol down to about 3. With hotter humbuckers it won't really get clean-clean, but you'll get a nice range of useable timbres across the range of the vol knob. Thats how they did it in the "old days" and it still works fine today. Less is more sometimes....

Know what I use the clean ch on my gigging amp for? A mute when I change guitars

[frankeg]

Well I got it restored to stock and it sounds good on both inputs. I read what Robert said about the relay and I agree that was the problem. Keep in mind that the unknown modder installed a heavy duty (10 amp) ac relay. That 24 vac circulating around along with poor lead dress was my problem. This amp came with 6550 power tubes which I understand Marshall did use for a while. Can I change them to EL-34's without any modifications except re-biasing? Speaking of bias, I measured my bias supply which was -55.5 vdc. Is that a little too low? I tried to measure current through the output winding but my meter isn't sensitive enough. I'll borrow a fluke from work tomorrow for that. Thanks all for your help and comments.....

[frankeg]

Me again. I found a very nice schematic drawn up by Mark Huss and he was kind enough to add a note about changes for EL-34 tubes. Different grid resistors and bias supply resistors are required.... Many thanks to Mark!!!

[jakehop]

IMHO the AC-driven relay doesn't help the situation. Can it be run off DC?

[frankeg]

I tried the transformer shunt method with a good fluke 189 and at one point got a really low 2.14 ma reading and another I blew my fluke fuse. I've done this many times on Fenders and never had any issues. Is there something I'm missing here? I'm going to pin 3 of the power tube to the B+ center tap of the OT. My schematic shows the B+ tap tied to a .5 amp fuse, one side of the choke and a parallel connected power cap can. Going to the fuse gives the low number and the cap junction blew the fuse - It actually arched pretty good - scared me a little. Since I'm working on this thing I am going to install the 1 ohm percision resistors on the cathodes as a lot of folks recommend but I am very curious as to what I'm doing wrong here. Thanks in advance...

[Ronsonic]

I'm not sure what problem you're having, no experience with tranny shunt bias measurement. What I do have experience with is old Marshalls being about as ornery and contrary as an amp can be using any available excuse to give you trouble like oscillating whenever you mess with, measure or interrogate the output section. I know they don't mind having a 1R resistor on the cathode if it is close to the tube socket.

[frankeg]

Yea, I agree with "ornery and contrary" it looks simple but it hasn't been fender friendly at all for me. Funny cause I built a great sounding 18 watt clone with no problems - I guess that goes to show how much R&D those smart guys over at 18watt.com must have done. I'm gonna do the 1 ohm trick tomorrow. Thanks...