I have this Darkvoice headphone amp that sounded great for about an hour until one of the Chinese tubes failed in one channel. Actually, it looks like BOTH tubes triode sections failed for the same channel at the same time. Neither the 6SN7 for that channel or the 6AS7 for that channel is drawing any current. The other channel works. Is it possible that the 6SN7 failed and took the 6AS7 with it, or the other way around? (see attachment)
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OK after a bit of checking it appears to be a failed 6AS7. With the tube plugged in there is only several volts on one of the cathodes. Unplug the tube and there's 80 volts on that pin. All the resistors and other voltages check out. Sound logical?
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The 6080 depends on the 6AS7 for proper grid bias.
So the 6AS7 must be present for the 6080 to function correctly.
Can you answer the questions in post #2?
I would swap the like tubes between channels instead.
So the 6AS7 must be present for the 6080 to function correctly.
Can you answer the questions in post #2?
I would swap the like tubes between channels instead.
Yes the filaments light up. 200R is not open. You have the tubes mixed up. Its 6SN7 for the preamp and 6AS7/6080 for the power tube.
Right, but still swap the 6080 output tubes between channels to confirm that.
The problem should follow the tube.
The problem should follow the tube.
There is only one 6080 output tube. Its a dual triode, one per channel. Its a headphone amp. I wish I had more tubes but I don't now.
Ok, at least swap the 6SN7, which should do nothing if it's the 6080 that is bad.
I do have one only NOS General Electric 6080. If you make an offer plus shipping, I'll probably accept.
I do have one only NOS General Electric 6080. If you make an offer plus shipping, I'll probably accept.
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Is it possible to supply the voltages of the circuit (supply voltage etc)? Just to get an impression. Leave the tubes in their socket.
Wondering if there ought to be a blocking capacitor on the input in case there was some DC there? The design looks to be vulnerable to imbalanced operation.
I would expect at least a protection diode between the grid and cathode of that output tube.
Here are two scans - the bad channel and the good channel with voltage readings - both tubes plugged in. I checked the plate resistor on the bad channel and it measures correctly. Unplug the 6080 power tube and the readings on the preamp tube are normal.
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What I notice is that the grid voltage of the output tube is 1V positive with respect to the cathode!
The voltage drop across the anode resistor of the input tube divided by 30k gives an anode current of around 5 mA while the current though its cathode
resistor is 0.5 mA. So there is a lot of grid current flowing into the output tube. Are you sure that your measured values are correct?
The voltage drop across the anode resistor of the input tube divided by 30k gives an anode current of around 5 mA while the current though its cathode
resistor is 0.5 mA. So there is a lot of grid current flowing into the output tube. Are you sure that your measured values are correct?
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Is it really possible to parallel the volume control with the grid leak resistor and not expect the unexpected?
I think there MUST be a coupling capacitor connected to the input or you can omit it when you know there is a coupling capacitor
at the output of some pre-amplifier.
at the output of some pre-amplifier.
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Looks like runaway of the 6AS7 here. I understand from data sheets that you have to be careful with this tube's biassing. I would expect that a 1k cathode resistor would take of bias stability but for a directly coupled amp you need to be extra careful and I wonder perhaps in this case if it's too low. A larger value cathode resistor (e.g. 1k5) could be tried out to see if that stabilizes the situation.
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