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I redid my ST70 with solid state rectification and VTA board. It sounds bad. I drug out my old Eico oscilloscope and looked at the waveform at 1000 Hz using my laptop generating the signal.

Bad alright, the sine wave appears like it is "missing" a line segment for probably 90 or so degrees. It is really not missing though, there is a much larger signal there at a much higher frequency.

I verified using another tube amp (not an ST70) with exact same connections and it is fine, good sine wave.

Also, the ST70 biases out good and does the same thing with both Sovtek EL34's and Westinghouse 6L6WGB.

I need to hook it back up tomorrow and trace out the signal and go from there...

In the meantime, any ideas?

Try disconnecting the feedback signal from the output transformer secondary, leading back to the input stage of your VTA driver board. If the horrible sound goes away, you might need to swap wires leading from the output side of the VTA board to the EL34 grids.

Hi,

tomlang wrote:It is really not missing though, there is a much larger signal there at a much higher frequency.

That's a parasitic oscillation.

When you went SS rectification, did you increase the resistors supplying B+ to the driver board so it saw it's normal voltage? If not, that's likely a source of grief right there.

Cheers!

Turns out I forgot to ground the output transformers. Connected black to ground on each one and works fine.

Woah!

I have only seen that in ham transmitters with haywire bypass caps :-o

Hmmmmmmm.......

Hmmm... it only happens when the NFB loop is connected, so it's got to be some kind of stability issue, right? It is weird how it is able to pass nearly one full waveform, then it breaks down into full on oscillation. You said the VTA board works fine with another set of power tubes and output transformers?

I have not tested this VTA board with another set of transformers but it does the exact same thing with a quad of 6l6wgb's in place of the el34's.

i'm not going to be of any help here, but just out of curiosity, you said it sounded "bad". what exactly does sound like? distortion? noise? overtones?
just curious. good luck

tomlang wrote:I did change resistors as required for SS rectification. I also swapped wires going from VTA to grids as suggested above. I also moved neg feedback wire to other outputs and changed the nfb resistor significantly.

What are the changes in terms of resistor values from old values to new values? Have you changed the capacitor value for the NFB accordingly as well?

DING, DING, DING, WE HAVE A WINNER. Kegger on AudioKarma wins! Great catch! You know I just wired the thing up without thinking about grounding the OPT's. Was probably thinking of car stereos, and gee, this is a trans should be floating, WRONG!

Listening to it now, works great.

AWESOME!!!! :-D

It's nice when problem has a simple fix :-)

*puts this note in his tech files*

Cheers!

Like the quote I posted on AK (apologies to the guy in the Hovercraft on TV).

"Gentlemen, we have all learned a valuable lesson here today!"