plexi 50 watt clone with very strange issue - Non Tremolo making pi trem sounds ?...

[spinfresh]

Hello Everyone,

I have a ceratone plexi that I am trying to make work. it is a 50 watter with a ppimv and all the typical controls.

here is the problem... it was working perfectly for a few years... then about a week ago, the sound got super distorted, and the high frequencies had a different amount of distortion than the low frequencies also the amp sounded like it had a tremolo - it oscillated in and out and when you turned up and down the ppimv knob the the speed of the oscillation changed - very strange.

so far I have done the following to troubleshoot.

1. all new tubes, I swapped them with a known good set - No change
2. removed the ppimv and tacked in a new pot, (thought that 1/2 of the pot failed sent PI out of phase?)
3. tacked in a new OT - just in case -
4. used a signal tracer with power tubes removed and all is well up to output tubes
5. used a signal injector and output tubes seem to work.

could a coupling cap between the PI and the Output tubes cause this? they measure correctly when the amp is off but under voltage they might leak DC?

any help would be appreciated

thanks in advance

Paul

[pdf64]

Motorboating? ie oscillation via the B+.
If so may be a B+ cap gone bad (excessive ESR).
Try tacking a good cap, even just 15uF, across each in turn to identify it.
If it is a bad electrolytic, consider replacing them all.

[spinfresh]

not really motorboating ... but worth a try, it sounds like tremolo and when I turn the mv control it speeds up and slows down like a tremolo...

[pdf64]

Do any of the other controls have any affect on it?
It may be worth verifying that the bias supply caps are ok too (in the same way as described previously).

[spinfresh]

i replaced bias supply caps and I put my scope on the power supply caps, and the last stage on the power supply is oscillating, at about 5hz (the tremolo effect is showing up on the scope!) at the last filter stage - so I de-soldered the cap (1/2 of the dual cap) and put in a new cap no change, I am thinking that the phase splitter circuit or maybe a coupling cap? very strange.

thanks again for your help...

[loudthud]

See if removing any of the preamp tubes makes it stop oscillating. Start at the input.

[Tom Phillips]

...the last stage on the power supply is oscillating, at about 5hz...

To me the "last filter stage" is the pre-amp is that what you mean?
Since you already replaced the cap an additional idea is to check the integrity of the ground connections of all the filter stages.

[spinfresh]

it was at the last filter cap in the output power amp stage, I have not done the tube removal trick to see, but I did use a signal tracer to the power tubes and no oscillation --- I put my scope on the second half of the last filter cap in the power section and it has a perfect sine wave that would defiantly cause a tremolo effect, I have not followed it back to see what might be feeding back into that cap yet... doing that tonight.

thanks all for the help...

[g1]

Can you post the schematic? If it is the main filter we are talking about, is it a totem pole arrangement? If so, you would have to replace both caps, not just one.

[pdf64]

Is this http://schems.com/manu/marshall/jmp_lead_50w_100w.pdf your schematic?
If so, it may be helpful to refer to part numbers, eg was it C105 or C106?

[spinfresh]

that is the schematic and 1/2 of C106 has a nice sign wave on my scope -- alll other caps are normal 60cycle ac wave... when I play a note the sine wave changes...

[g1]

The 2 halves of C106 are in parallel. The only way the 2 halves can show different on the scope is if they are not getting proper connections.
If they are in parallel with good connections, what you see on one half you should see on the other.

[The Dude]

that is the schematic and 1/2 of C106 has a nice sign wave on my scope -- alll other caps are normal 60cycle ac wave... when I play a note the sine wave changes...

What is the level of the AC on your filters? A "nice sign wave" or "normal 60cycle ac wave" on your filters is not something good. There will, of course be some AC there, but by comparison to the DC it should be minimal.

[pdf64]

A normal ac waveform on the B+ would surely be a 120Hz sawtooth?

[Chuck H]

I don't know about the longevity of the filters Ceriatone provides, but it sure sounds like faulty decoupling to me. Bad filter cap/s or bad filter ground/s. That it's a new problem for the amp is telling.

You mention that the MV seems to instigate the problem. They can do that. Shielded leads are always recommended for post PI MV's.

Swapping a known good preamp tube for each socket is also a good idea.

I don't expect a leaky coupling cap. That usually manifests differently.