ST70 Rebuild with VTA70 PCB Question

Hello All,
I have rebuilt a ST70 with a new VTA board from tubes4hifi. New multicap can and new 12AU7s. Output tubes have been tested and are OK. Transformers are original. It works, but I'm having an issue I can't figure out. All tubes bias OK, but the output with a 1K signal looks like this:


This is from one channel, but both are the same. The signal looks clean coming off driver board, but I'm guessing it has something to do with bias. Any insight would be helpful. Thanks.

I think maybe there is too much resistance in the B+ supply. There is less energy in the top half of the waveform than the bottom.

JeffLaurence wrote:I think maybe there is too much resistance in the B+ supply. There is less energy in the top half of the waveform than the bottom.

your comment doesn't make sense to me.  what do you mean by resistance in the B plus supply?   do you mean the supply is sagging? also to me the waveform looks symmetrical.  the distortion is at the  0 crossover.  I think your negative bias on the power tubes is set for excessively low quiescent current. check the tube data sheet for recommended bias current.

That looks like crossover distortion. Set the bias slightly above cutoff. Try increasing the bias level and see what affect it has on the waveform.

Just a general note, not necessarily related to what you're showing, often times elusive problems come from poorly soldered connections. It sounds like you've tested/ measured the components and tubes to your satisfaction?

What I meant was the positive portion of the waveform ( above zero) didn't have as much power as the negative portion.

If both channels do this, have you swapped tubes around? Cross-over distortion may be, but still, the comment about one side of the wave form being significantly greater that the other side also indicates an issue with the output being 'imbalanced'. Could be tubes, driver circuit, or even the OTs themselves. I would play around with reversing leads and see if I couldn't 'reverse' the wave pattern. May even be a problem with improper feedback resistor value. If so, that could present a 'cut' or 'boost' in one side of the PP output which could result in that kind of wave form....or so I believe!! FWIW.

Does the waveform change when changing the bias?

Can you post the schematic, Not sure what a VTA board looks like never worked on one.

Pictures, Pictures, Pictures...

Output transformer grounding of secondary center tap.

Dumb question, but are you sure the input signal is symmetrical?  If it is, I agree, looks like classic crossover distortion from low bias current.  Should be around .40vdc(40ma) per tube across each 10 ohm cathode resistor.

Look at both Pin 5 grid signals using two channels of your scope.

The waveforms should be 180 degrees out of phase from each other.

Since there is no response from the poster I guess he either gave up or does not need any help???