Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Yosser on March 01, 2026, 09:44:36 am
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I'm attempting to bias a 50watt pp fixed bias tube amplifier using the "Measure Bias With the Output Transformer Resistance Method" however I am experiencing some difficulty as the reading fluctuates between reading the ct voltage and plate voltages, in that they change after noting them down - is it normal to that voltages are unstable as it doesn't take much to end up with a false result.
Also a -40 mv bias voltage gives a plate current reading of 37ma V1 and 47ma V2 which is above safe dissipation, but with a lower bias level the readings are more unstable.
There are no 12ax7 tube installed but I don't think that should affect anything.
Can anybody advise?
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Turn the bias down and let the amp warm up to stabilize before making your adjustments. The resistance will fluctuate temperature as will the current draw until the amp warms up.
If it before stabilizes then you have another issue worth addressing before worrying about bias.
The other option is to measure the voltage directly across the CT to primary instead of taking 2 measurements. The safest way to do this is to use leads with clips and to clip on while powered down. You can also monitor this voltage as an indicator of having reached stability. Then power down, drain the caps, and switch your meter to resistance.
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Turn the bias down and let the amp warm up to stabilize before making your adjustments. The resistance will fluctuate temperature as will the current draw until the amp warms up.
If it before stabilizes then you have another issue worth addressing before worrying about bias.
The other option is to measure the voltage directly across the CT to primary instead of taking 2 measurements. The safest way to do this is to use leads with clips and to clip on while powered down. You can also monitor this voltage as an indicator of having reached stability. Then power down, drain the caps, and switch your meter to resistance.
Sorry I don't understand this, could elaborate please?
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I am experiencing some difficulty as the reading fluctuates between reading the ct voltage and plate voltages, in that they change after noting them down -
Rather than taking three voltage readings just take two readings. Connect one probe to the plate socket pin using a gator clip lead. Connect the other probe to the OT center tap using a gator clip lead. That's one reading. Repeat for the other tube.
Some circuits will get upset when connecting a meter to the tube plate. This is one good reason I suggest to always use the 1Ω cathode resistor method.
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I am experiencing some difficulty as the reading fluctuates between reading the ct voltage and plate voltages, in that they change after noting them down -
Rather than taking three voltage readings just take two readings. Connect one probe to the plate socket pin using a gator clip lead. Connect the other probe to the OT center tap using a gator clip lead. That's one reading. Repeat for the other tube.
Some circuits will get upset when connecting a meter to the tube plate. This is one good reason I suggest to always use the 1Ω cathode resistor method.
That worked out fine, thanks
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One thing I forgot to ask about is the voltage difference value between the plate and center tap, I'm assuming a larger difference would equate to a healthier/stronger tube because there is more current flow - is that the case?
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One thing I forgot to ask about is the voltage difference value between the plate and center tap, I'm assuming a larger difference would equate to a healthier/stronger tube because there is more current flow - is that the case?
Larger voltage definitely means more current flow, but this does not necessarily equate to a healthier/stronger tube.