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In Reply to: RE: Ping Tre: " 12K driving 300b would be just fine" question. posted by Tre' on May 23, 2015 at 11:39:54
"66pf would have a reactance at 200kHz of 12k ohms. If you were swinging the grid 140 volts peak that would take 11.7ma of current. "I don't think he will need to drive the grid at 200kHz, but if he does, peak current will be approximately 8mA due to power factor. More important, peak current at 20kHz - the highest frequency likely to be reproduced - will only be a little more than 1mA. That's the value he has to budget for when designing the driver.
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Buy Chinese. Bury freedom.
Edits: 05/24/15 05/24/15Follow Ups:
1mA? Hell, a 12AX7 will do that just fine ;)
cheers,
Stephen
"1mA? Hell, a 12AX7 will do that just fine"
You're probably joking, but this does deserve further explanation. Tre' said his goal was to have phase undisturbed at 20kHz. He then determined that in order to do that, the 66pF output stage must be driven from a 12K ohm source. It's true a 12AX7 could provide the necessary current to drive the output stage to 140V peak (the hypothetical maximum grid voltage), but it won't provide the output impedance necessary to achieve -3dB at 200kHz. Both the minimum output Z and minimum drive current capability are necessary for the stage to function as Tre' described. In this case, a 'SN7 or similar would meet both requirements.
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Buy Chinese. Bury freedom.
Or you could roll off the lows by partially bypassing rk (if present) and with special expen$ive boutique caps and magically get adequate HF response. :-) heh heh. JH
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