ST-120 bias question

[denis14]

Hi everybody!
I have this bias adjustement issue on V3, I can turn the potentiometer all the way and still got only a 0.4V reading.
So I suspect the dedicated potentiometer to be faulty, am I right?
Changing tube placement do not change anything!

It’s a tight fit here, I did a custom casing so need a Mouser part number.

This one?

3386P-1-503LF Bourns | Mouser Canada

thanks’!!!
Denis

 

[rayma]

The tubes could be weak. Check the bias circuit voltage, and see if it is as specified on the schematic.
If so, then check the tubes.

Not real likely to be the pot.

 

[indianajo]

Who made this?
My ST-120 has no bias pots. Stupidly low idle current is why they were slammed as one of the worst amps ever made. I've installed the djoffe bias control circuit on one channel Dynaco Stereo 120...can be beautiful - diyAudio

 

[rayma]

I think he means this one.
tubes4hifi amplifier KITs page

Measure the DC voltage at the bottom of the grid resistor (not at the grid) and see
if it is very different from the other 3 sockets.

 

[denis14]

Thanks’ everybody.
Yes, I am taking about a VTA ST-120.
Long story short, the main fuse blow, the 10 ohm bias resistor blow. Change everything including a vintage Tung sol 6550 (ouch!$$!)
Everything is back to normal.
Here a pic.
Google Photos