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71a Line Stage and Miller Capacitance
71a Line Stage and Miller Capacitance
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Old 1st November 2006, 06:38 PM   #31
dsavitsk is offline dsavitsk  United States
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Quote:
Originally posted by kevinkr
Sounds like your plate supply is noisy (lots of ripple?) or the power transformer is close enough to the tubes to cause problems. 01A are very sensitive to both magnetic and electrostatic pickup.
This is what is weird. the PS is about 2 feet away in a separate chassis, and the supply is reasonably clean, and the CCS should clean it further.

Quote:
Originally posted by Eusebius
Have you tried the cathode bypass cap yet?
Quote:
Originally posted by kevinkr
You should definitely add a cap to bypass the cathode bias resistor. Note that the nominal plate resistance of 8 - 9K may be triple this amount if the cathode resistor is unbypassed depending on the value of that resistor.
I'll try that tonight. I was really hoping to keep the electrolytics out of this thing, but I'll see. I'll also try adding a pair of 20R resistors on the cathodes. Would the higher plate resistance lead to more noise here?


I actually prefer fixed bias via batteries applied to the grid, I use something like a 220K grid resistor and a 9V battery - works really well.

Quote:
Originally posted by kevinkr
I have also tried nicads in the cathode circuit, they are ok, note that nimhs seem to sound really lousy. (Interestingly higher internal impedance than good nicads) Normal operation will keep them charged but they do need to be replaced every few years.
If all else fails, I'll give this a shot too.
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Old 1st November 2006, 11:56 PM   #32
Eusebius is offline Eusebius  United Kingdom
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I was really hoping to keep the electrolytics out of this thing>>

Sure - I use 47uF polypropylenes for cathode bypass.
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Old 2nd November 2006, 01:19 AM   #33
tomtt is offline tomtt
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Location: kansas city mo, and on occasion, around the world ...
Quote:
Originally posted by Eusebius
I was really hoping to keep the electrolytics out of this thing>>

Sure - I use 47uF polypropylenes for cathode bypass.

good idea


pair of 71a amps in here-


http://www2u.biglobe.ne.jp/~tossie/c...-E.010106.html
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