Carvin VTR 2800 distorted output

[xtian]

Carvin VTR 2800 sounds OK until you hit the guitar strings hard, then output snaps into nasty distortion.

Using sine wave and o-scope, I determined output from SS preamp board is good. But problem persists with a resistive load. In this video, the sine wave at the speaker output is shown, and looks good, until I bump the input sine wave from 0.1v to 0.2v p-p. You can see the distortion immediately, jumping all around. But what's odd is that, as I reduce the sine wave back to 0.1v, the wave form on the scope slowly "heals itself". It doesn't just snap back to good, it...slides back toward good over a couple of seconds.

Does this give some clue I'm missing?

carvin_vtr_2800_sch.pdf

[Stevem]

How much does the voltage on the main power supply filters dip when have the start of the issue take place ?

[xtian]

About 50v, from ~400 to ~350. What are you thinking?

[xtian]

I just clipped in another OT, and the behavior is the same.

Maybe the power tubes' control grid-to-ground ceramic caps are ringing and oscillating? Or the power tubes were subject to oscillation and that's why the snubbers were added?

IMG_2057 Large.jpeg

[xtian]

Thought I'd try pulling one pair of power tubes. Good idea! Output is clean at 40 watts! Swapped tested pair into outer sockets, still good. Thinking "bad tube," I swapped in the untested pair, and... they work also, in both inner and outer sockets. Hmm. Put all four tubes in, and...pow, back to ugly distortion. So...two tubes, good. Four tubes, bad. WTH? (And yes, I matched OT impedance to tube load.)

[Phil_S]

I think you've established it is not any of these: tubes, tube sockets, tube socket dependent components (maybe), OT.

The problem happens when you drive the quad hard. IMHO, the questions to answer: what gets stressed when you do this; are there any underrated components?

FWIW, my first guess is a weak filter cap. My second guess is a leaky coupling cap. Third is a resistor that is going south but hard to see.

[mhuss]

Try strapping an appropriately sized cap across the HT line. I've seen (in rare cases) voltage-sensitive motorboating. I'll bet the HT doesn't drop as much with two tubes.

[Stevem]

So at the start of this issue the power supply is already down 50 volts? And even with 4 tubes in it how many watts are you seeing at 50 volts down and what is the max wattage you can get and at that point what is the power supply at?

I think jumping in another filter would be the place to start.

[xtian]

I’m reading at the veterinarian’s. I replaced all electrolytics already. Reservoir is stacked 220u, so 110u first node, then two 47u nodes. More later.

[Phil_S]

I hope everything went OK at the vet. Here's some irreverent humor. I hope it's not totally inappropriate.

[wpaulvogel]

What do the phase inverter outputs do on the scope? Previous stages? Work back into the preamp and ensure that the waveform is good.

[xtian]

What do the phase inverter outputs do on the scope? Previous stages? Work back into the preamp and ensure that the waveform is good.

Yes, all good. The post-PI waves are less distorted than the amp's output, but show ringing when in fault state.

Owner has asked me not to invest any more time, so I'm wrapping up the job with just two power tubes installed.