On Thu, 07 Aug 1997 03:55:22 GMT wrt@eskimo.com (Bill Turner) writes:
>Just fired up my new homebrew 8877 amp tonight after nearly two years
>in the works. And it works! I can drive it to 1500 watts output
>easily with about 70 watts from the TS-850S. It just sits there and
>cranks out the power. Nice!!
>
>My question is about the grid current vs loading. This is a
>conventional GG circuit, PI-output with designed Q of 12, 3000 volts
>on the plate, nothing fancy. Unlike my previous amps (3-500Z,
>3CX1200A7) the loading control has a major effect on the grid current.
>Once everything is properly tuned for max output, when I adjust the
>loading control a small amount, the grid current varies a lot. I can
>swing it from about 10 ma to 80 ma without appreciable effect on
>either the input power or output power. It's weird to see the grid
>current all over the map and yet the input power is constant. Is this
>normal for an 8877? For a high gain tube like this, maybe it is.
>
>Any comments?
>
>
>73, Bill W7TI
>
Ive noticed similar indications with 2 and 3 tube 3CX800 amps and also my
old NCL-2000 6M tetrode amp (screen current sensitivity).
It may have something to do with the Zp being quite low with lower
voltage, high current tubes. A little change in C has a big effect. Just
a guess.
A LK-800A for example ( 3 x 3CX800's) will show almost no change in
output from roughly 20 to 100ma of grid current.
Adding a Jackson Brothers 6:1 reduction drive, or similar, makes tuning
easier.
It will be interesting to read the "experts" comments Bill.
73....Carl KM1H
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