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Vintage threads from the first ten years
| previous: Gus Bruce I don't understand. I know t... -- 8/17/2000 9:39 PM |  |
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Re: Bypass caps for bypass caps
 8/18/2000 11:35 AM |
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Dave H. | Re: Bypass caps for bypass caps The "R" in the 1/2piRC equation should be the cathode resistor in parallel with the impedance "looking into the cathode" of the tube not just the cathode resistor. A voltage change of 1V at the cathode will cause a voltage change of u x 1 volts across the tube internal Ra and external load RL. The current change is therefore I=V/R or u/(Ra+RL). The cathode voltage changed by 1V and the current changed by u/(Ra+RL) so the impedance looking into the cathode is R=V/I or 1/ [u/(Ra+RL)] which simplifies to (Ra+RL)/u. A 12AX7 with 100k RL will have Ra=62k and u=100 so (Ra+RL)/u is (62k+100k)/100 = 1.62k. That’s about the same as the cathode resistor so the "R" in the equation will be halved and the frequency doubled. Yuk! what a ugly explanation. Now you know why I gave up teaching. I’m sure Randall will have a more lucid and accurate explanation on his site. Dave |
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| Replies: |
| Bruce Boy, am I glad you typed all that f... -- 8/19/2000 5:28 AM Tracy Thanks Dave, Randall Aiken Actually, that's quite a good expla... -- 8/19/2000 3:03 PM |