SF MV Pro Reverb, distortion in the background

[Matthews Guitars]

I haven't played my SF '78 MV Pro Reverb in a few months. I have so many amps to play with, it just happens.

Today I needed that lush sound only a SF Pro Reverb can offer me. So I fired it up and quickly noticed that now I'm getting a fairly subtle, but certainly noticeable, distortion that appears
"behind the note", you don't really hear it until the note is fading out and the volume is dropping. The distortion has a "hollow" sounding character to it. Sounds like a sawtooth waveform.

Interestingly, the level of this distortion tracks with the bass control setting.
The behavior is the same in BOTH channels.

The amp was recapped about three years ago. Not expecting that to be a factor.

I'm GUESSING that I'm going to find a leaky coupling capacitor in the bass circuit. Or at least I would if the problem was just in ONE channel.

With it being in BOTH channels, yet affected by only the bass controls, I need to think about this one. Or just ask if any of you have seen this before and know the answer.
I'm not afraid to learn by being told. It's a valid way to learn!

[pdf64]

Reseat all valves a few times and check for the issue.
If not sorted check through a different speaker cab.

[Matthews Guitars]

Doesn't matter what speaker cabinet I use. Reseating all tubes changed nothing. I did those FIRST before posting.

[pdf64]

These use the crappy 022848 OT?
Try opening the nfb loop.

[Matthews Guitars]

This is the ultralinear version, which has the most wonderful clean reverby floating in clouds tone of any amp ever. I sure hope that transformer is OK, and have no reason to suspect a problem with it as of yet. I think it's going to be a bad cap on the main board.

[pdf64]

I think the 'UL' OTs are probably much higher spec than the 022848.

[Matthews Guitars]

I suspected the ceramic disc PI coupling cap and replaced it. No change.

I may have to get out the scope to find this. I do think the problem is a capacitor, the problem appears in BOTH channels and the behavior is affected by the bass controls in the same manner, and there aren't very many parts that are common to both circuits that make sense to look at given the symptoms.

I'm not even going to waste my time worrying about the output transformer. It's fine.

I even tried a spare set of Sylvania 6L6GCs I've been saving for Fender amps if needed. No change.

I readjusted the output tubes matching and hum balance controls for minimum noise. That actually helped a little bit. But it didn't fix the problem.

[martin manning]

I do think the problem is a capacitor, the problem appears in BOTH channels and the behavior is affected by the bass controls in the same manner, and there aren't very many parts that are common to both circuits that make sense to look at given the symptoms.

Maybe the PI's non-inverting stage decoupling cap?

[Matthews Guitars]

Checked and replaced it. Nope. Not that one either. Definitely going to have to scope this one out. I don't have enough new caps on hand to just totally replace every small cap in the amp.

[WhopperPlate]

Doesn't matter what speaker cabinet I use. Reseating all tubes changed nothing. I did those FIRST before posting.

Have you replaced power tubes? Tubes are always number one suspect . I coincidentally just had sane issue with a Princeton that new power tubes remedied

[Stevem]

I have had this issue maybe 3 times over many decades and each time in the end it was a preamp tube.

Since you say it’s on both channels it could only be the 4th tube ( channel mixer tube ) or the last preamp tube the phase splitter.

I am assuming here that what your hearing is like a fuzz tone added to a note that you have played, but at a lesser volume.

[Matthews Guitars]

Found the problem. Turns out that both my external test cabinet and the speakers in the combo cabinet are all slightly damaged and that is the distortion.

When I plugged into ANOTHER cabinet, actually I tested with ALL my cabinets, I discovered that the amp is fine, it's rubbing speakers.

Now I'm on the hunt for a pair of Fender Blue Label 12" speakers with no cone rub, type 081810. I want to preserve the exact type match.

[Stevem]

That was the only other possibility for that issue and I did not mention it because you said you covered that bad speaker potential.