Peavey Bass amp making "bad speaker" sounds at high volume, but speaker isn't bad

[wizard333]

Peavey VB-2. Blew the 1.6A HT fuse as is common with these, so replaced that. Looking at bias on the supposedly 'new' matched sextet (JJs) one was 50% higher PD than the others, which were not all that close to each other either. Coolest tubes were biased at 70% so the hotter ones were probably red plating, hence the blown HT.

In any case, after replacing HT fuse and biasing cooler, the amp makes 'bad speaker' noises when turned up, except I've tried several speakers and I know it isn't that. Doesn't seem to matter whether volume is provided by turning the master up, master down and pre-amp up, or just turning the volume on the bass up and hitting the strings hard. Changing bias doesn't affect it. Cleaned all jacks/tube sockets/switches. Did the chopstick test and didn't find anything obviously loose that would cause it. Did the dark room test didn't see any arcing. Electrolytics don't look bad in any obvious way. PS voltage isn't sagging in any appreciable way. Replaced each pre-amp tube individually, no change.

It's possible the output tubes are crapped out due to the stress, but I was told it came back from Peavey once for this condition already with the 'new' tubes and only worked for half a gig thereafter. In any case I don't have 6 matched EL34s on hand (how often do you see matched sextets required in an amp running 34s?? 6550s, sure.....) to check that.

Anything quick and easy I'm missing here to track this down? Anything less than quick and easy that should be obvious?

[wizard333]

Update: Also does this when plugged directly into the effects return.

[J M Fahey]

When you hear "bad speaker sounds" and it's not the speakers, it's time to dig the scope out and start taking a hard look at waveforms.
You'll find some ugly ones there.
Only after that we can advance, otherwise it's a shot in the dark.
PS: you don't *really* need matched sextets, you can use matched pairs if that's all you get, just use one from the pair(s) on each side.
What you really need is balancing both sides driving the transformer.
PS2: I don't really trust much vendor "matched tubes" anyway.

[jrfrond]

Yeah, it's scope time, and I am suspecting either arcing or oscillation on peaks. In any case, I wouldn't approach this problem without a power tube swap first. In addition, if it were MY bass amp, I'd dump those EL34's now and convert to 6550's, which are FAR more reliable and in my opinion, sound a lot better for bass. If you stick with EL34's, make sure you bias them on the cold side. There isn't a production EL34 made that will last long at higher dissipations.

[wizard333]

Solved.

[tboy]

and the solution is... ?

[Jazz P Bass]

Come on man!
Leave us all hanging.
The solution was..........

[R.G.]

I gotta start doing that. It produces so much sturm und drang.

[J M Fahey]

Solved.

We don't deserve this.

[Enzo]

Oh well, at least we don;t see all the posts deleted.


Reminds me, I never sent a thank you letter to my sister in law for my birthday present.

[wizard333]

Peavey re-tubed it when it went back to them the first time, but I shouldn't have assumed they used the correct tubes. They put a 12AX7 in the PI, which calls for a 12AT7 on the schematic. My fault for assuming Peavey didn't F it up 8p. It's other issues were due to the wildly mis-matched "matched" tubes which red-plated and shorted.