ST-70 - Can Not Set Bias

I have an ST-70 that has been working well for a few years. Last week it started making a buzzing noise so I checked the Bias, It would not go over .3V using any of the VTA's pots, so I figured its the rectifier tube. I changed the rectifier tube still staying around .3V with the pots at max. There is still what I believe to be a transformer/amp buzz.

Any help would be appreciated.

Mark

Philagot wrote:I have an ST-70 that has been working well for a few years. Last week it started making a buzzing noise so I checked the Bias, It would not go over .3V using any of the VTA's pots, so I figured its the rectifier tube. I changed the rectifier tube still staying around .3V with the pots at max. There is still what I believe to be a transformer/amp buzz.

Any help would be appreciated.

Mark

You could have a leaking / bad can cap. Check DC voltages, if they are low you could suspect the can cap.

I just looked at the OP again and realized it said your bias was .3v and not 3 volts. Yes, the twist loc cap will cause the drop in B+ and therefore you
won't be able to bias it up to the 1.56v. You can also have a shorted EL34 or two that would load the B+. Check and see what your voltage is at pin 3 of each
output tube. Now if the bias voltage was 3v and not .3vdc, then the selenium rect and the two 50uf axial lead caps need changing. I would change all those caps
if they are original.

Did you try swapping output tubes?

I had the same issue a few years ago and it was the multi-section twist-lok capacitor. If yours still has the original cap in it (on top of chassis), I'd replace it immediately. the two 50uv@75V bias caps should be replaced, too. I'd use 100uv@100v.