bias issues and more, with new express build

[DaveMon]

hello all, I just completed an express build, and I am working through some issues to get it operational. I am seeing very high voltage at the bias pot (-175.5 ). I have searched around and seen someone recomend replacing the bias pot and 10uf caps and all the resistors. Short of that, I have removed the bias pot and tested it....seems fine. Does anyone know any other troubleshooting steps i need to take to track this problem down? I am posting a jpeg of voltages....the only way to get a clear layout it seems. heater voltages are normal, omitted from pic for clarity...thanks in advance

[DaveMon]

additionally, I have gone through each part of the circuit for accuracy several times, (caught several errors) tried to learn more about the actual workings of the bias circuit, and checked out other layouts, schematics, gut pics etc. Learning a ton in the process...the information here is very vast

[Jana]

It sounds like you have the wrong value resistor for the bias supply--the resistor that goes from the high voltage AC to the diode. What did you put there? it should be about 150K to 220K.

[DaveMon]

thanks for the reply Jana, I have verified that it is a 220K resistor.. however, I removed the turret that the 220k and diode join on (space reasons) , and connected them directly to each other without using a turret....that shouldnt change anything should it?

My voltages are: PT secondaries 593 VAC (MV leads on each tap from PT)
PT secondaries 294 VAC each (to gnd)

Immediately downline from the 220K resistor you mentioned, before the diode, i get 197 VAC. On the other side of that diode -185 DC

[Tillydog]

Check the connections to your bias pot (and its value). In particular, check that you have the link from the bias pot to ground.

0.02p

Andy

[DaveMon]

thanks tillydog, it turned out to be a grounding issue, i have a working amp now

[Tillydog]

Good news !

Enjoy