Peavey Roadmaster SS Distortion issue

[glebert]

I've been working on this amp for a while and finally have everything mostly working, but the distortion feature has an issue where it dies out a bit soon and also varies up and down a bit as it fades out. The distortion is accomplished by the transistor drawn sideways that cross connects the bright and normal channels. It seems like it is mostly going to act like back to back diodes, but I don't know that I understand it completely. There was no transistor in that location when I got the amp (almost looked like there had never been one), so I put in a generic 60V NPN (MPSA05). When I fired it up the diode that goes from the collector to the base was showing as shorted, I replaced that with a 1N4005 that I had handy. The distortion actually sounds pretty good, except the lack of sustain and the fade in/out thing. I have no idea what this amp sounded in 1974, maybe it wasn't ever a very usable distortion, anyone have any thoughts?

edit: now that I think about it, I don't think it will be just like back-to-back diodes, because the base of that transistor is already going to be biased up to around the Vbe forward voltage. The AC signal coming into the base will then try to force that Vbe to be bigger or smaller which will result in a change in the collector current, but the diode from collector to base will try to force the base voltage back towards its nominal (I think), so it is a very non-linear inverting amp stage with a ton of NFB?

edit-edit: Looking at some classic distortion circuits this seems like it has some similarities to the original Fuzz Face.

Distortion circuit.


Full Schematic
Peavey-roadmaster-SS-11-74-Schematics.pdf

Link to user manual
https://peavey.com/manuals/roadmaster.pdf

[Enzo]

As much as I love these old PVs, those 2uf/35v caps they used never live long. I generally find every one is worn out. And a little DC leakage could surely cause your issue.

[g1]

From the owners manual I get the impression that there is no actual distortion circuitry, just various series and parallel combinations of the channels. It's fairly convoluted.
So my guess is that the sideways transistor (above where it says 'english normal preamp') is acting as a switch rather than a distortion generator.

[Helmholtz]

From the owners manual I get the impression that there is no actual distortion circuitry, just various series and parallel combinations of the channels. It's fairly convoluted.
So my guess is that the sideways transistor (above where it says 'english normal preamp') is acting as a switch rather than a distortion generator.

The manual shows the distortion block in the functional diagram for series input.

It is a common emitter circuit with a clamping diode in the collector-base feedback path, base connected the output of the English Bright input stage and the collector output feeding the English Normal channel input.

[g1]

The manual shows the distortion block in the functional diagram for series input.

Yes, you are correct. From the description it seems to be doing both things, creating a series connection of the channels as well as making distortion.
1) "to place the bright channel output signal through a distortion stage into the input of the normal channel"
2) "distortion button on the automixer/footswitch disables the series connection of the preamps"

[glebert]

I think on some of the other Peavey hybrids the distortion is just the bright channel fed into the normal channel and overdriving the snot out of it.

[J M Fahey]

Same or similar here, I see no dedicated distortion circuit there but they might very well put top first stage output into the bottom one input to overdrive it.

[Helmholtz]

edit: now that I think about it, I don't think it will be just like back-to-back diodes, because the base of that transistor is already going to be biased up to around the Vbe forward voltage. The AC signal coming into the base will then try to force that Vbe to be bigger or smaller which will result in a change in the collector current, but the diode from collector to base will try to force the base voltage back towards its nominal (I think), so it is a very non-linear inverting amp stage with a ton of NFB?

I think your explanation of the distortion circuit is essentially correct.

Not sure, but you might get somewhat better results with a small signal diode like a 1N4148, because these have a higher forward voltage and a smoother clipping characteristic than a 1N4005.