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Cathode bypass capacitor related question
Cathode bypass capacitor related question
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Old 13th August 2018, 09:41 PM   #31
6A3sUMMER is offline 6A3sUMMER  United States
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tapehead ted,

1. Were you referring to my post # 16 where I talked about a KT66 with cathode self bias, and a 270k grid resistor? No problem when the plate + screen dissipation is less than 27 Watts, the maximum grid resistance is 1 Meg Ohm. And with cathode self bias, for greater than 27 watts plate + screen dissipation, the maximum grid resistance for cathode self bias is 500k Ohms.

Fixed bias KT66 specs are completely different, plate + screen dissipation less than 27 Watts, grid resistor is 220k Ohms maximum; and plate + screen dissipation more than 27 Watts, grid resistor is 100k maximum.

2. The thermal run-away I saw with a Red Plate was many years ago. I learned my lesson Then. An EL34 with fixed adjustable bias was set to 50mA cathode current. The grid resistor was 270k Ohms, all ran OK with that circuit. Then I decided to try other tube types. I substituted a 6550. With 270k Ohms in the grid circuit, I adjusted the cathode current for 50mA. It was not long after the amp was warmed up that I heard terrible distortion, and I looked at the amp and the tube plate was glowing red (I immediately turned the amp off). In looked at the grid resistor specs, I saw my mistake, the maximum specified grid resistor is 50k Ohms.

Some tubes will work if you exceed the maximum grid resistance, some tubes will not. Tubes of the same type, and even of the same manufacturer may not all be the same. If you exceed the specifications, your mileage may vary.

Now you see why I tend to be a little conservative when picking the grid resistor.

3. I was using a 12AU7, not a 12AU6 in my KT66 amp. The amplifier output hum and noise was less than 100 microvolts Both with the 12AU7 cathode bypassed, and without the cathode bypassed.

Last edited by 6A3sUMMER; 13th August 2018 at 09:51 PM.
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Old 15th August 2018, 04:13 PM   #32
hpeter is offline hpeter  Slovakia
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Quote:
Originally Posted by AllenB View Post
Such an impedance is likely to be swamped by the plate resistance, similar to the impedance of an LED being so small in comparison.

On the other hand, some cathode swamping has been known to linearise plate resistance, which, when loading the stage with a CCS, becomes the primary driving impedance of the stage.
if i understand correctly, you thinking that nonlinearities in cathode circuit does not affect high-gain stage much .
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Old 15th August 2018, 06:45 PM   #33
hpeter is offline hpeter  Slovakia
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i just finished one test with diode bias.
just inserted diode in series with cathode resistor - sudden white noise and sound was harsh...
removed it - again all OK.
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Old 15th August 2018, 10:43 PM   #34
tapehead ted is offline tapehead ted  United States
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Quote:
Originally Posted by 6A3sUMMER View Post

Some tubes will work if you exceed the maximum grid resistance, some tubes will not. Tubes of the same type, and even of the same manufacturer may not all be the same. If you exceed the specifications, your mileage may vary.

Now you see why I tend to be a little conservative when picking the grid resistor.

3. I was using a 12AU7, not a 12AU6 in my KT66 amp. The amplifier output hum and noise was less than 100 microvolts Both with the 12AU7 cathode bypassed, and without the cathode bypassed.
So people DO still say "your mileage may vary!"

Sorry, I knew it was a 12au7, I just have 12au6 on the brain just now. Finally located a few different triode charts for that one.

I haven't seen the red plate (somehow?!) and I haven't had a tube burn out before my eyes that I can recall, but I've seen some circuits that eat tubes and on inspection exceeding the max grid resistance was an obvious culprit. Are all culprits guilty? Anyway I've been eyeing that one with suspicion.
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Old 16th August 2018, 12:51 AM   #35
AllenB is offline AllenB  Australia
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Quote:
Originally Posted by hpeter View Post
if i understand correctly, you thinking that nonlinearities in cathode circuit does not affect high-gain stage much .
I was thinking a resistor (a carefully selected moderate value), multiplied by gain.
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