Biasing Question

[Robert]

Hello All,

I have just finished my Express build and am having trouble getting the expected bias readings I want. I have wired pins 1&8 together on each power tube, then I connect both wires together at an external bias point which is then connected to a 1 Ohm 5 watt resistor going to ground. Setting my DMM step to 200ma and establishing a ground to chassis with black probe, I get a reading of 16.6 on the bias point with the red probe. Changing the step to 20 ma, which I think is the wrong step, I get a reading of 3.35. All other voltages are in line:
B+1=397
B+2=387
B+3=287
B+4=263
B+5=250.
My Bias circuit is a 150K resistor off the secondary HV, a 1n4007 diode connected to a 100uF cap(with polarity correct) and a 25k pot followed by a 24k resistor to ground.

I'm sure I have done something stupid and would appreciate a quick bit of educating.
Thanks for all the help.
Robert

[glasman]

Hello All,

I have just finished my Express build and am having trouble getting the expected bias readings I want. I have wired pins 1&8 together on each power tube, then I connect both wires together at an external bias point which is then connected to a 1 Ohm 5 watt resistor going to ground. Setting my DMM step to 200ma and establishing a ground to chassis with black probe, I get a reading of 16.6 on the bias point with the red probe. Changing the step to 20 ma, which I think is the wrong step, I get a reading of 3.35. All other voltages are in line:
B+1=397
B+2=387
B+3=287
B+4=263
B+5=250.
My Bias circuit is a 150K resistor off the secondary HV, a 1n4007 diode connected to a 100uF cap(with polarity correct) and a 25k pot followed by a 24k resistor to ground.

I'm sure I have done something stupid and would appreciate a quick bit of educating.
Thanks for all the help.
Robert

You need to set you DMM to voltage (MV) not MA. The resistor allows the voltage to be developed across it when current is flowing. 1mv / 1ma with a 1 ohm resistor.

I am guessing that you are probably shooting for something like 60 to 80 mV. Which will correspond to 60 to 80 ma on the two tubes combined.

Gary

[Robert]

Thanks, Glasman!

I am now getting 30-40 mv range adjustable by my potentiometer. Please correct me if I am wrong, but that corresponds to your range of 60-80 ma since we add together the value of the two tubes.

Robert

[dobbhill]

I believe your tubes are too cold, meaning the bias current is too low and your bias voltage (B-) is too high (too negative). With the cathodes of your power tubes connected together, you are looking to get 40mv + 40mv = 80mv total across the cathode (1ohm) resistor. You need to change the bias supply to give you less negative voltage by changing one or more of the resistors in the network.
Hope this helps, D

[Robert]

Dobbhill,

Can you more specifically reference the bias supply resistor network you are referring to from the A1a schematic and a method to use to arrive at the correct voltage. Could part of the problem be the Two Rock bias circuit I used? Also, would cold tubes explain some or all the observations I posted?

Thanks in advance,

Robert