bias circuit problem

[mageerc]

I have a 1970 Twin Reverb on the bench that the bias circuit has been Blackfaced to AB763 specs. I can't seem to bias the tubes at more that 45% plate dissipation. I have changed the range resistor from 470 Ohms to 1K and dropped the value of the resistor soldered to the back of the bias pot from 27K to 10K and still no joy. The amp has a new bias cap and the values from the bias circuit to the phase inverter are correct. The only thing that hasn't been replaced in that chain is the diode. Where should I check next... maybe the bias tap on the transformer or possibly a ground under the bias board?

[Fuchsaudio]

What's the raw bias voltage to the 220-K bias feeds ? What kind of tubes, and where are you measuring ? Should be an easy fix.

If you can, add a one ohm resistor to each power tube cathode, which would allow you to see each tube. All it takes is a bad screen resistor or socket to turn off a tube or two. If you measure current for all 4 at he same time, you won't be able to know that. Once you know the individual currents, it should be easy to trim the bias pot values to get the proper range.

[Blue Strat]

What's the raw bias voltage to the 220-K bias feeds ? What kind of tubes, and where are you measuring ? Should be an easy fix.

If you can, add a one ohm resistor to each power tube cathode, which would allow you to see each tube. All it takes is a bad screen resistor or socket to turn off a tube or two. If you measure current for all 4 at he same time, you won't be able to know that. Once you know the individual currents, it should be easy to trim the bias pot values to get the proper range.

+1. Check the grid voltage on pin 5 of the power tubes and measure the 470 screen grid resistors.

[VicLesnaik]

Hello,

I am attempting to install a variable bias pot in my 1983 Fender Concert amp but cannot find the 38k resistor to get started. Reading a Schemaic leave me cold.

Help with this ... I have done some simple mods already but am stuck.

[skipm45]

38k is not a standard value.
in 5% tolerance resistors, a 39k is the closest value.
in 1% tolerance resistors, you have a choice of 38.3k or 39.2k

Here's a link to a table of standard resistor values in various tolerances.
http://www.logwell.com/tech/components/resistor_values.html


HTH

Skip
www.skipzcircuits.com (http://www.skipzcircuits.com)