Hello.
Regarding my PP amplifier, there is a problem with a one EL34 power tube - can't set the correct BIAS on it, there is no any voltage drop across the 10Ohm cathode resistor. Could you please advice how can I solve this issue? I must mention that the amplifier is working properly, anode-cathode voltage across this problematic tube is 440V, the control grid voltage -24v (from the power supply, this could be change by a pot on each power tube),I measured the resistance between the cathode and ground after 10Ohm resistor and is 10 ohm. Photos and schematic attached, please help, thank you, Rafal.
Regarding my PP amplifier, there is a problem with a one EL34 power tube - can't set the correct BIAS on it, there is no any voltage drop across the 10Ohm cathode resistor. Could you please advice how can I solve this issue? I must mention that the amplifier is working properly, anode-cathode voltage across this problematic tube is 440V, the control grid voltage -24v (from the power supply, this could be change by a pot on each power tube),I measured the resistance between the cathode and ground after 10Ohm resistor and is 10 ohm. Photos and schematic attached, please help, thank you, Rafal.
An externally hosted image should be here but it was not working when we last tested it.
If the amplifier is operating normally, the output tubes are certainly biased properly.
With zero bias, the output tube would draw excessive current.
With zero bias, the output tube would draw excessive current.
Sorry, not clearly written, problem is with one "tube socket", swapping tubes does not change anything.
Resolder the little jumper wire that goes to the cathode resistor. It looks like a cold solder joint with flux insulating it.
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Bad joint or faulty valve. In either case there is no current flow to create a voltage drop. For small signals you might not notice that the PP is only a P.
If U cathode res. is zero, what is U on cathode pin in socket or on pin of the tube? I will not wonder, if U has high voltage, if there isn't contact.
One other reason why a tube may not be conducting current - check that you have screen voltage on that tube (390 Ohm blown or bad solder join etc.)
Cheers,
Ian
Cheers,
Ian
Thank you everyone for your tips! There were two problems in this issue: unexpected current conduction of a jumper near cathode resistor and the one of tubes was almost dead. Strange, that it was not very audible, only one channel was quieter.
Thank you again!
Thank you again!
Cool, thanks for the report back. In a push pull when one tube isn't working well the other one is still puting out. So at half power the output dB is only down 3. That isn't enough to be super obvious, believe it or not.
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