Silverface Fender Twin has nasty bass distortion

[Whymeworry]

Yeah, I tried another cab. More proof it is caused by pre-amp bass distortion.

[Jazz P Bass]

I know the caps aren't bad, as I changed back to the original caps after the problem happened to see, then to another set of caps, but the problem persisted regardless of the caps.

Confused am I.
Simply to clarify:
You did not have the bass problem.
You changed to new caps, now there is a bass problem.
You put back the old caps and....what?
You still have the bass problem?

[g1]

The photos are not real clear, but the dropping resistors have been changed. The second one appears to be a 100K, probably not the correct value.

Good catch. And I think your right. That resistor is clearly brown/black/yellow = 100k. And is certainly the wrong value. The other resistor looks to be brown, black, orange =10k. The correct values for EVERY SF Twin I looked at indicate the values should be 2.2k (where the 10k is now) and 10k (where the 100k is now). I think I would start here if "I" were fixing this amp.

52Bill and Chuck are correct. Don't assume the resistors are correct if you did not touch them. You are talking about a 100K shown on the schematic, where is it?
The ones that should be there are shown on the schematic right below V10 (2.2K 1W) and to the upper right of V4B (10K 1W). There should not be a 100K under the can.

[Whymeworry]

Thanks for pointing the resistor thing out then. Also yes, the processes was: 1. My amp was working fine, but had old Mallory Caps in it from at least 25 years ago. 2. I replaced the Mallory Caps, my amp developed bad bass distortion. 3. I put the Mallory Caps back in, but the distortion continued.

[Chuck H]

And, just to be clear, you didn't change those two resistors under the doghouse.?.

[Shawnobi]

Could the caps be a red herring and this is a bad coincidence? Have you try switching pre amp tubes? Unless you have damaged something with the heat when soldering the caps seem ok (by substitution). And if those resistors are wrong, then this problem should have been around before.

[Jazz P Bass]

The first stage cathode bypass cap could be removed to see if it is a preamp signal problem.
You may have a case of rather severe 'Fender Fart'

[Whymeworry]

Okay all, I have now changed the resistors to your recommend values. Nothing has changed, but I assume having proper values is worth it regardless.

[Whymeworry]

And yes, the Fender Fart is very annoying.

[Jazz P Bass]

Have you tried to lessen the bass response by removing the first stage bypass cap?
And how about interstage coupling caps.
Have you looked for unwanted Vdc on the non plate side?

[Chuck H]

The problem WASN'T there. Then it was. Even with the old caps back in. The only thing that could have changed is the wiring. It's a bad ground or miswiring. Is there a flaw in this logic?

And we never found out if the OP replaced the original rail resistors or not. It has not been articulated clearly. Even though they are now replaced with correct values, were the incorrect values installed by the OP or not? If not then the incorrect values were part of the circuit that the OP liked. And should probably be put back in.

Nick, did YOU put new rail resistors in the amp when you changed the filters? Please answer this question.

[Whymeworry]

No, I didn't install the new resistors. But the resistors didn't cause the problem I can say for sure. I have at this point quintuple checked the wiring under the doghouse so it is very unlikely that it's a ground issue.
Is it possible for new Caps to throw the amp bias out of wack? I originally suspected replacing the old caps (which likely had drifted horribly) with new fresh ones could have screwed up the bias. I don't really know if replacing Filter Caps can effect bias at all, or if bad biasing would cause this problem, but someone at one point suggested this to be a possibility, and I have yet to rule it out.

[überfuzz]

See if you can find any funky ground connection. I had a farting Twin Reverb on my bench for way to long. It turned out to be a ground connection in the tone stack.

[Chuck H]

I have read here, several times, of people replacing filters in their BF/SF amps and experiencing more fidelity. More LF, HF and better dynamics. And I've also read that this isn't always appreciated by the user. The tone changes such that the amps can become harsh or muddy when clipping. That you desctibe the problem as an out of tune artifact, and that the problem persisted with the old caps back in makes think there is an actual problem. I have never heard of new filter caps changing bias significantly. You may need to take this amp to a shop where a tech with proper bench gear can scope it to isolate the odd distortion. This sounds terribly frustrating for you. And diagnosing it without test results from a remote location is proving impossible.

[Steve Conner]

Check the plate voltages to all the preamp tubes, compare against the schematic. That will give us more info to go on.