subharmonics in Marshall 4210 (2205)

[11 Gauge]

This is a somewhat moot point, as I will be converting this amp over to point-to-point some time in the next month, but I'd really like to know what the culprit is of these subharmonic notes.

I know that ghost notes can be caused by filtering issues, but that's not the case here - the notes are an octave under, most noticable with the neck pickup on, at the 10th fret and above, on the first string.

I've read where this is typically caused by 'oversized' coupling caps after the phase inverter. I tried modifying these caps (actually bypassing the PPIMV in the process) - no dice, still getting the subharmonics.

I did notice in the schematic that there is a huge honkin' coupling cap right after the fx loop (.22), and it sits atop a rather big load resistor for that gain stage (1 meg) - maybe this is the culprit? The 2 gain stages before and after the loop are also driven very hard, with small cathode resistors, and large bypassing caps.

Anything else obvious that I might be overlooking? The amp has been completely recapped and retubed, all electrolytics on the board replaced, and much of the 'fat' from the board removed (clipping diodes, channel switching circuitry, all ceramic caps in the boost channel removed, etc.).

Thanks,
.011

[Don Symes]

I could see (but only in theory) how a pair of triodes that are:
1) out of phase
2) on the same segment of filter string
3) with an iffy cap or a failing inlet resistor on that RC section, and
4) overdriven
might be able to make some subharmonics.

Pretty unlikely, though.

[Pedro Vecino]

The original switching circuit does not extinguish the signal of the clean channel suitably when lead is active. The extreme low end of the signal is not filtered completely across the 220n capacitor that is connected to IC and it appears when clean volume is to the max.
This can generate an effect similar to the one that you perceive...
In this amplifier I use always a 1uF capacitor paralleled with 220n to absorb these low frequencies and to obtain a complete mute in the clean when lead is active.
Regards

[Enzo]

The amp has had massive changes made, and now it does something unwanted. It may - or may not - be related to these changes.

But your opening line made me uncomfortable. I wouldn't consider it moot. If you rebuild it into a hardwired version, you might just move the problem over with the parts. I always recommend to someone thinking about reworking an amp to make it 100% BEFORE doing any conversion work. That way you know if any problems crop up that they are due to the new work. Otherwise you DON'T know that when facing a problem.

Speaking of the problem, I would not as a rule look to engineering problems such as component values, at least not initially, because if that were the problem, then ALL of them would be doing this. Obviously individual cases may vary.

Run a sine wave through the amp and set it up so the symptom is occuring. Look at the waveform at the output to see what the shape looks like. Now trace the signal stage by stage through the amp, looking for where the added shape change comes on.