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RE: Ping Paul Joppa

Posted by Paul Joppa (P) on July 19, 2017 at 16:37:35

In Reply to: RE: Ping Paul Joppa posted by Caucasian Blackplate on July 13, 2017 at 16:32:11:

I agree with CB. Sorry I didn't post earlier - I wrote a response but apparently failed to hit the "post" button.

He's also right that the EL34 cathode resistor is way too large. The 880-ohm value is not even a standard part, but it was used in the widely-copied WE Model 91 with a 300B. I speculate this circuit was originally for a 300B amp, with the triode-strapped EL-34 swapped in without adjusting for the substantial difference in tubes.

The target operating point is about 350v/70mA for the TFA-2004 impedance of 3000 ohms. I get 360 ohms for the cathode resistor; a 5-watt rating would be marginally acceptable, though I would probably use a 10-watt part myself.

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