Marshall Artiste 2040 Cathode Bypass Caps

[bobloblaws]

Hello everybody, I have a 1971 Marshall Artiste 2040 here which needed some TLC. After replacing a power tube and a couple of pots and biasing I thought I might be finished, but then I noticed that there is way more volume available in the reverb channel compared to the normal chanel. The normal channel seems very quiet considering it is a 2 x 6L6 50W amp. Initially I thought I would need to troubleshoot that channel but then I noticed on the schematic that on the reverb channel there are cathode bypass caps on the two preamp gain stages whereas there aren't on the normal channel. Would the lack of vs. the presence of the bypass caps make that significant of a difference between the two channels? Or maybe that combined with the different tone stack?

B.

[Helmholtz]

Schematic?

[Chuck H]

While I'm not very familiar with that amp I will say that a gross differential in output for two such channels is something that would have been addressed at the design level. Sure, sometimes there's a little, but a lot? Marshall wouldn't do that (I think, but willing to be corrected). In other words... The amp is probably still broken.

Please link a schematic to help with further discussion.

[sugeda]

...

[loudthud]

One big difference between the two channels is one has a Baxandall tone network and the Reverb channel has the typical FMV (no mid) type stack. Bypassing the cathodes on the Bax channel will add perhaps 10dB of gain which will makeup most of the difference.

[bobloblaws]

While I'm not very familiar with that amp I will say that a gross differential in output for two such channels is something that would have been addressed at the design level. Sure, sometimes there's a little, but a lot? Marshall wouldn't do that (I think, but willing to be corrected). In other words... The amp is probably still broken.

Please link a schematic to help with further discussion.

Sorry guys, I forgot to include the schematic...here it is.

Marshall-Artiste-50W-2040-Schematic.pdf Marshall-Artiste-50W-2040-Schematic.pdf

[bobloblaws]

...

Thanks sugeda, yours is a clearer drawing. But I see at least one difference, V2A does not have the cathode bypass cap. It looks like it might have been photoshopped out, or maybe the circuit actually was was revised.

[bobloblaws]

While I'm not very familiar with that amp I will say that a gross differential in output for two such channels is something that would have been addressed at the design level. Sure, sometimes there's a little, but a lot? Marshall wouldn't do that (I think, but willing to be corrected). In other words... The amp is probably still broken.

Please link a schematic to help with further discussion.

I'm pretty sure it's by design. It's not as stark a difference as I originally thought, and for fun I clipped in a couple of bypass caps on the V1 cathodes. that really beefed it up similar to the other channel. I measured the output power before clipping on the normal channel at 50W so it should be good to go.

[bobloblaws]

I still have a potential issue. The reverb channel has more apparent volume than the normal channel, but the sine waveform on my scope has a distorted character even at low volume levels. I discovered that turning the treble pot back to 2 or less cleans up the waveform. The first pic is the normal channel at close to 50W output. The second pic is the reverb channel at a much lower level and the treble pot straight up the middle. Swapping V1 and V2 did not make a difference. Should I try changing the Treble pot or the 250pF cap?

[loudthud]

Was the Reverb turned up when you took those photos ? What's really strange about this amp is the overly large bright caps. They tend to really load down the preceding tube at low Volume settings.

[bobloblaws]

Was the Reverb turned up when you took those photos ? What's really strange about this amp is the overly large bright caps. They tend to really load down the preceding tube at low Volume settings.

Reverb pot was at zero.

[Helmholtz]

Were vol. pots fully up?

[bobloblaws]

Were vol. pots fully up?

No, norm channel vol pot was turned up to just before clipping, and rev channel vol pot was quite a bit less to illustrate that the signal is distorting even at low volume.

[Helmholtz]

No, norm channel vol pot was turned up to just before clipping, and rev channel vol pot was quite a bit less to illustrate that the signal is distorting even at low volume.

For max. clean headroom vol. pot(s) should be fully up when measuring power output.
Reduce input level instead.
Otherwise it's likely to get preamp distortion.

[bobloblaws]

For max. clean headroom vol. pot(s) should be fully up when measuring power output.
Reduce input level instead.
Otherwise it's likely to get preamp distortion.

That is interesting. I was told at one point to use a fixed test signal of about 100mV and turn it up to just before it starts to clip. So I've always done it that way. On the normal channel I'm measuring right around 50W, exactly what I expected.

In any case, there is clearly something different going on with the rev channel that is not there in the normal channel. And as I said, turning the treble down to 2 elimates the distortion even when turning the volume higher to compensate for any signal reduction. I suspect that the difference will still be apparent if I max out the volume pots and use the input as the variable. But I'll try it out anyway.