[AMPS] PA-77 question, round two

[Phil Clements]

>>When a good 8877 is properly tuned (slightly heavy loading)
>>it will put around 2300 watts into the antenna with 55-65 ma
>>of TRUE grid current. I have had several Alpha 77DX/77SX's
>>and all of them read HIGH on the grid meter. I think Dick had
>>in mind that it would help to save tubes. NEVER let the grid current
>>on an 8877 exceed 75 ma under any circumstances. I set all my
>>grid-trip circuits to this value.
>>
>Eimac rates the 8877 at 20w max. grid dissipation.  Normal grid/cathode
>potential is 55Vrms.  With 75mA of grid current, dissipation is approx.
>4.1w.
>
>Why do you limit grid current to 75mA, Phil?
>
>-  Rich

Rich,
Eimac is in the business of selling tubes. I am in the business of having to buy
tubes. Prices are soaring, as I am sure you know.

Why push your luck on a very fragile grid when all you need is  50-65 ma Ig
for full output on a good tube? Also, a grid trip-circuit has a lot of slack in it,
(not as bad a a fuse, but slower than a QSK circuit.) The only time I ever get
a grid-trip is when I select a wrong antenna in the heat of battle. Setting it
higher
than necessary is not a very good idea unless you have a lot of cheap spares
on your shelf.  I want the amp off line anytime the Ig even approaches 15-18
watts on peak. I think this approach is the reason for my success with the
8877. I have a pair with date codes of 1979 that are solid black in color now,
but put out within  97% of a new tube (1999) with the same Ig.

When lightly loaded, by mis-tuning or wrong antenna, the 8877 Ig soars more
rapidly than any other tube I have used. Alpha 77 series owners will certainly
agree with this!

(((73)))
Phil, K5PC




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