Marshall JCM 800 (2203) Microphonic O/P Valve problem.

[Simonsaysbang]

Working on a JCM 800 (2203) 4 x EL34 at the mo. It had a multitude of problems and the amp was filthy, splattered with mud. It's been in regular use but living in a barn. It even had hay dust in the valve seats. Anyhow, the amp had bad arcing joints on reservoir caps, bad bias, noisy pre-amp valves, PI valve dead. One output tube dead and one that was arcing. Broken lead on input resistor, dirty jacks and bad coupling caps.

Anyhow, pulled the board and changed all 0.022uf caps plus the 0.1uf's and whilst I was at it replaced anode resistors for pre's. Replaced 27k resistor plus 1n4007 leading to bias plus bias caps. Lastly, I cleaned the valve holders but didn't re-tension them as they looked ok. Fired the beast up with a new matched set of JJ El34 and JJ pre's and amp powers up and plays nicely. After 15 seconds though, there was a crackling and occasional light pop and then a slow long progressive crackle that turned into like a sound of a long fart (almost a very slow motor=boating). As things get warmer, after say 25 seconds the bad noise goes and all is well. I pulled all the pre-amp valves and tried again. Exact same problem. So I very meticulously cleaned the output valve seats and re-tensioned them. Powered up again and same problem. This time I disconnected all the pre-amp areas from the output section plus presence, leaving just the biasing and fired up again. Same problem. Although filter caps test fine I thought they might be failing so I had a spare 50/50uf 500v (Ruby) and wired it in place of the others one at a time. Same problem.

This was becoming a real head-scratcher so I replaced the output tubes for another matched set of JJ EL34 and bingo bad batch of JJ's and all the noise is gone but all valves are micro-phonic and ringing like church bells when you tap them quite unlike what I am used to with output valves going micro-phonic and sounding like pinging rubber-bands. I remember being told that Marshalls sometimes don't like JJ's for a variety of reasons. I have another Marshall amp in, a TSL122 that I replaced the leaky motherboard only to send it back to Marshall as it burnt up a pair of JJ EL34. The motherboard was badly made so another motherboard was sent and it was fine plus I was given a matched set of Tung-sol EL34 which was kind of them to make up for the red-plated JJ's. I thought ok, let's try the Tung Sols and if they work well I'm packing up JJ EL34 with Marshalls. Swapped the Tung-sols in, powered up and they all ring like church bells too!

Grrrr!

Now I've only been repairing Valve Amps for two years or so as I've mainly done up to date hi-fi, projectors, car ecu's and the only valve gear I've repaired is vintage valve radios but most don't approach the operating 500-600v.

This is the first JCM800 (2203) I have repaired. I'm beginning to wonder now that as Marshalls drive their EL34's very hard, is this church bell ringing noise inherent with this particular amp? I've repaired many other 100w Marshalls and not come across this micro-phonic ringing noise before. Could it be that as I have all the pre-amp disconnected, this is causing the bell-ringing problem?

Just to clarify, I didn't tap the first replacement set of EL34 because of the crackle and pop I was experiencing. I did move them around in the valve seats though checking for arcing and didn't notice ringing.

Any pointers with this problem I would be most grateful. Thanks.

(Sorry this has been a long-winded write-up for the ringing problem but thought it would be a good idea to write about the repair so far).

[Simonsaysbang]

Sorry. I forgot to add that I have dismantled and cleaned all groundings and re-soldered I think 8 cold looking joints on output valves..... Valves, tubes..... I must pick one and stick to it!!

[Simonsaysbang]

Also replaced input and screen grid Resistors on EL34's...

[guitician]

An open input on the power stage may cause the tubes to be more sensitive to microphonics. How does it sound with just the V3 in?

[drewl]

Had amp amp with a bad volume pot ground that caused all tubes to behave like they were microphonic.

[g1]

It's also very difficult to tap on an output tube and guarantee you are not also vibrating the chassis to trigger some other microphonic component. So I wouldn't rule out the possibility it is something other than the power tubes. If you tap something else, like one of the big blue caps, do you also hear the same sound?

[Simonsaysbang]

Hi Drewl, g1 and guitician and thanks for your help.

Ok, just for good measure, I'm going to swap in the first replacement set of JJ El34 and see if they ring like bells first as, apart from the crackle and popI can't remember if they were doing "church bell" impressions.

Drewl, I will reflow all the pot joints and clean chassis grounds I think just for good measure although all those were turned down to minimum. Worth doing anyway.

g1, the microphony exists on the power tubes. I'm almost certain that's the case. I have a bare 8 ohm loudspeaker connected that when the amp is warming up from cold and I hold the loudspeaker to my ear, you can hear the internals of the o/p tubes ringing as they get hotter. When the amp is running, very gently touch or tap the o/p tubes and you get the ringing, tap anything else in the amp and you get the ringing but it sounds a bit like it's coming from the room next door as it were, so it is very localised to the o/p tubes themselves and all other physical tapping of the amp / components is vibration being transmitted physically through the amp.

guitician, well I had all inputs open from the first set of replacement o/p tubes, just running the output stage with bias. I need to check that first replacement set of o/p tubes first for ringing.

Anyhow, thanks to all of you for your replies and I will work on with this amplifier now and report back later ok.

[Simonsaysbang]

Well chaps, odd one this one but it's quiet as a Church mouse now and screams like a screaming thing when you want it to. The first replacement matched set of JJ EL34 were definitely trash. As I gradually reconnected the pre-amp from the PI going backwards to front-end, the "church bell" sound got less and less to next to nothing to worry about. I wonder if having the preamp disconnected sailed the voltages up higher to the point where the amp played up? I'm also wondering as the amp was in store for a long time whether the unusual higher voltages influenced the filter caps in some way and they didn't like it until they were properly reformed? They tested out fine originally. The only other thing I wondered about is whether the o/p TX was a little damp by being stored in a barn for so long and whether perhaps that might have influenced things?? Fact is the amp sounds mighty fine now and had it running for two hours, the bias hasn't shifted one jot, the anode currents on o/p tubes are healthy, the noise floor is very low and my, my this amp is loud when you want it to be!! So, I'm going to let this one go. Cheers for your help g1, drewl and guitician....

[guitician]

That was probably due to reflected impedance back through the actual conducting valves being connected. Glad to hear that it's sounding fine again. Cheers

[Simonsaysbang]

:-)