VHT Pitbull Super 30 strange oscillating

[TASBrendan]

Hi all,

I have a VHT Super 30 on the bench and it's doing some inconsistent oscillating I've never experienced. The owner says that he replaced the tubes- 4xEL34s and it started up again after maybe a month. He had replaced the speaker and the one he installed wasn't soldered in very well- the amp problems didn't start up when I first had in on the bench, so I assumed there was just an intermittent connection there. He couldn't recall if the problem ever happened before the speaker swap. OT tests OK so far - Primary sides are reading the same ohms and the CT is right in the middle.

I have only the power tubes in and using a chopstick, touching any component or anywhere on the chassis causes a brief screech while being excited. This can be any lead, anywhere on the chassis, or tapping the tubes or transformers. It's louder when tapping the tubes or any component on the power amp PCB. The power section is on it's own PCB and I've disconnected every lead coming from the preamp board. The only things I haven't disconnected on that board are the leads to the reverb transformer, but the tube is removed. Here's what I've tried so far, one at a time with no change:

I've resoldered all of the tube sockets.
I lifted the negative feedback resistor
I swapped the OT primary leads
I measured about 10VAC ripple off the 2 filter caps, tried swapping out both with new caps anyway
The ripple on the scope is almost a saw wave
Unsoldered impedance selector and hardwired the 8ohm tap to the output jack
I removed the two 47pF caps connecting plate trace to cathode trace on both power tube halves
Measured all resistors and diodes - all check out
Voltages look good- nothing sticking out. I'm using a lightbulb limiter.
I have a mismatched quad of tubes that I tried and that seemed to be better, but I'm reluctant to blame the tubes since he said he already changed them recently. He has Tung-Sols in and the ones I have in the shop are a matched set of JJs and a set of old RCAs. There's no red plating at any time.

I cannot find a schematic for this amp, although the power section isn't too complex.

Any help at all is appreciated! I'm sure I'm missing something.

Brendan

[Jazz P Bass]

I would give it a try with everything installed.
Also, I would ditch the lamp limiter.

[g1]

EL34 not EL84 ?

[The Dude]

Do you have a scope? What frequency is the oscillation? Is it audible, or are you seeing it on a scope (or both)? Does the oscillation stop once started? Could it simply be a microphonic tube or tubes?

[TASBrendan]

Sorry, yes EL84 quad, cathode biased.

It's definitely audible. If I don't use a limiter, it is slightly going all the time- there's some tube sputtering, but also a squeal/screeching that goes off when the chassis, PCB or tubes are excited (slight tapping, rubbing, etc)
When I use the limiter, there's an occasional sputter and the oscillation only happens when the chassis, tubes or PCB are excited.

I can see it on the scope- it's a square wave at any amplitude. I'm not sure what frequency. It's intermittent so hard to see- my scope doesn't save anything. Maybe grid or stopper resistors?

Thanks

[TASBrendan]

Say it is just the tubes- any ideas what might cause it to happen again a month after installing a new set?

[The Dude]

Was the PI tube also changed, or just the output tubes?

[TASBrendan]

They said just the power tubes. I did the usual pull one tube at a time until I only have the power tubes in, so I've only been focusing on those. I just realize I haven't scoped back through the signal path to see if it's present before the power tubes...I'll check that out, too

[g1]

If the PI is removed I'm not sure how a bad power tube would do this. They will rattle, but I don't think they will squeal like that. Maybe some ground fault?
Also, can you be more specific about the square wave you referred to?

[Jon Snell]

If all the valves are removed except the EL84s, replace the EL84s.
A loose screen/control grid can make it squeal, as can a bad solder joint in the output stage.

[TASBrendan]

Sorry, I was out for awhile...
I finally was able to test the tubes in another amp and it had the same problem, so I installed new power tubes. New tubes in and it sounded great, just hope it holds- especially after touching up every joint I could. Bias was good, so no signs the amp was set up to burn through tubes. The owner had swapped out the speaker and the solder joints on the speaker were really bad. Is it possible a bad connection there would blow the tubes and not the output xformer?
g1, what I meant with the square wave was that, when watching the noise on my scope as it was happening, the squeal was a momentary square wave at the output along with sputtering.