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    Building, troubleshooting and testing of these amplifiers should only be
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Correct measurement of filament voltage.

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I've got a tube preamp which uses two 6CG7 tubes.
If I measure the voltage across pins 4,5 of tube A, it reads 6.0vdc, however tube B reads 6.5vdc.

The heater supply for both tubes is from the same source, and if I swap the tubes then the readings are reversed accordingly. If I read the voltage from the heater + and heater - terminals on the PCB, then the voltage is identical.

Does this mean that if we read the voltage across pins 4 & 5 of a (small signal) tube, then we are also seeing a voltage drop due to resistance of the filament?
 
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Very strange - it is a Broskie circuit with jumpers between the heaters of the two tubes. The jumpers are quite thick. Although even if this is the case, it doesn't explain why the readings are reversed when I swap the tubes.

Is the 0.5v difference likely to be affecting the performance of the tubes?
 
[...]If I measure the voltage across pins 4,5 of tube A, it reads 6.0vdc, however tube B reads 6.5vdc.[...]
If I read the voltage from the heater + and heater - terminals on the PCB, then the voltage is identical.

If I understand you correctly, you can only measure a difference if you take the measurement at the tube pins not if you measure at the PCB?!?
This would indicate that one tube has a bad connection to the PCB (oxidized heater pin?) and some voltage is lost at that bad connection. That would also explain why the problem is moving with the tube. In your setup tube A would be the suspect.
Clean the heater pins and see if that changes anything.
Best,
Martin
 
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