[AMPS] Grid current question

[Rich Measures]

>Just fired up my new homebrew 8877 amp tonight after nearly two years
>in the works.  And it works!  
good
>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.
The 8877 is designed to have low grid-interception, so the grid current 
can be very small compared to a 3-500z .  However, with any tube, if you 
use lignt loading, grid current increases.  
- Why look at the grid current indication?  The grid I meter is of no use 
in tuning the amplifier.  80mA of grid current equates to about 6w of 
grid dissipation.  The grid is rated at 25w.  
cheers, Bill
Rich---

R. L. Measures, 805-386-3734, AG6K   


--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm