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PostPosted: May 27th, 2020, 11:08 am 
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Joined: June 4th, 2013, 2:39 pm
Posts: 255
Okay. Here's an idea. Disconnect the bias voltage and try cathode bias (I think Hafler says 200 ohms?) and see if it still oscillates.


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PostPosted: May 27th, 2020, 11:23 am 
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Joined: February 28th, 2013, 1:19 pm
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FerdinandII wrote:
Do you have a cheap set of speakers you could attach to the outputs? It might help you identify whether this is a low frequency or a high frequency oscillation. The culprits would be quite different, I imagine. If it's motor-boating for instance, that's something you would hear right away.


I will not hook up a speaker unless I know it is behaving. The tubes are running close to 200 mA of current through them and my 100W 8ohm resistor is too hot to handle. If it is high frequency, it will kill a tweeter. I am trying to just turn it on enough for momentary troubleshooting checking.

David


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PostPosted: May 27th, 2020, 11:26 am 
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Joined: February 28th, 2013, 1:19 pm
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Grover Gardner wrote:
Okay. Here's an idea. Disconnect the bias voltage and try cathode bias (I think Hafler says 200 ohms?) and see if it still oscillates.


Grover,

I was considering trying self bias as a last resort, but I guess it might be at this stage in the game.

David


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PostPosted: May 27th, 2020, 11:38 am 
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Joined: February 28th, 2013, 1:19 pm
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David McGown wrote:
Grover Gardner wrote:
Okay. Here's an idea. Disconnect the bias voltage and try cathode bias (I think Hafler says 200 ohms?) and see if it still oscillates.


Grover,

I was considering trying self bias as a last resort, but I guess it might be at this stage in the game.

David


But I am still not convinced it is the bias circuit. The bias running the output section with no input (i.e. driver tube pulled), is stable. Voltage comes up fast. The problem occurs with the complete amp. I have removed the bypass resistor across the coupling cap (which would eliminate any LF leakage from the driver into the bias circuit via the voltage divider) but it did not change anything. Still, worth a try.

David


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