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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Direct Coupled Tube Failures

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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  • darkvoice 1.png
    darkvoice 1.png
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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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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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  • Darkvoice bad channel.pdf
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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?
 
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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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