VTA conversion went fine till I turned it on, low to mid volume sounds great, past mid volume starts to distort and crackle

Bought a used ST 70 and began upgrading to VTA board. Replace all sockets, quad cap, and replaced power xfmr. Using new el34s and gz34 rectifier. Biasing the amp went fine. Turned it on and played great on low volume, as I increased the volume past half way it starts to distorting. Any ideas?

What's the typical voltages ( as per the st-70 manual ) on the el34 and gz34 ?

What are you using as a source, preamp?

Did you replace the 15.6 ohm resistor with individual 10 ohm cathode resistors at the power tubes?

Are you using low-gain tubes like 12AU7s instead of 12AX7s?

Voltages for the gz are pin 2 438v, pin 8 378v, pin 4 370v and pin 6 374v. and el34 pin 3 416v, pin 4 417v, pin 5 40vdc, pin 6 40vdc and pin 8 .4 vdc. I am using a tuner with pre outs. Yes the bias resistors with 10 ohm resistors I am using 6sn7s on the low gain.

donkim wrote:Voltages for the gz are pin 2 438v, pin 8 378v, pin 4 370v and pin 6 374v. and el34 pin 3 416v, pin 4 417v, pin 5 40vdc, pin 6 40vdc and pin 8 .4 vdc. I am using a tuner with pre outs.  Yes the bias resistors with 10 ohm resistors I am using 6sn7s on the low gain.

Is the volume on the tuner or the amp?

donkim wrote:Voltages for the gz are pin 2 438v, pin 8 378v, pin 4 370v and pin 6 374v. and el34 pin 3 416v, pin 4 417v, pin 5 40vdc, pin 6 40vdc and pin 8 .4 vdc. I am using a tuner with pre outs.  Yes the bias resistors with 10 ohm resistors I am using 6sn7s on the low gain.

These values seems to be on the spot.
If all el34 has 0.4V on pin8 ( catode ) then your power supply seems ok.

And it both channels that distort ? Then
Do you have any dc komponent from your pre ? st70 and vta70 has no cap on input, dc here will upset
the working point of the front tube.

Next i'll check groundings and B+ connections to the vta board.

volume in on the tuner. Will check the grounding and b+ connection on the vta.

You could just have a dirty volume pot. Have you tried tuner cleaner or DeOxit on it?

Sounds reminiscent of some issues I had here when I tried adding a meter circuit and floated the ground on one side of each power tube pair. Basically ended up pushing one end and missing the complementary pull, and ended up with half a signal on each channel. OK at lower volumes, but would get all sorts of strange when pushed a bit.

Double check the schematic and instructions, checking off each step.

thank you will try starting from the beginning.

Check the negative feedback connection from the transformer outputs to the low-gain board.

arledgsc you were spot on the feedback connection was a terrible soldering on my part.
Thanks a million.

Sounded like too much gain was the issue. Congrats on your successful conversion!

way to go, arledgsc, thank's for getting the right answer!!