>
>On Sun, 24 Aug 1997 22:29:39 -0400 (EDT) KA2YKC@aol.com writes:
>>I have a Commander HF2500 amplifier that seems to be experiencing high
>>grid current readings. I am wondering if I have some flaky 3cx800's, the
meter(s)
>>are out of calibration, or I am just worrying for nothing.
>
>You have a real concern.
>
- One concern is exceeding the grid dissipation rating of 5w per grid.
As I recall, the cathode-to-grid driving potential is roughly 30V rms.
Is 80mA total/40mA per grid going to cause more than 5w of dissipation
per grid?
- Another concern is why the grid current drifts. Barium migration
might be the reason. If it turns out that Eric's heater voltage is above
Eimac's rating--as has oft' been the case in Commander amplifiers--I
suspect that barium migration is the problem.
>>....snip....... However, my grid ma meter is showing approx. 80 ma when
>>I first key down and it slowly drops to approx. 60 ma or so after 15 -20
>>seconds.
>............... The best
>way to tune that amp is with the full 100W from your exciter and go for
>maximum output which will be in the 2500W range.
RE: 3CX800A7 Ratings: 100w of drive for two tubes exceeds Eimac's
recommended typical driving power. The maximum anode (plate) rating for
a pair is 1.2A. 1.2A x 2500v = 3000w. At 60% efficiency, 3000w in =
1800w out.
>
>THEN, use your exciter to drop power down to wherever you wish WITHOUT
>touching the amp controls.
>
I agree, Carl, but it might be safer to tune up within the published
rating--or add enough cathode feedback R so that 100w of drive produces
1.2A of I-anode.
...snap...
>>
>>Therefore, my questions are,
... ...
>>c: Am I worrying too much as the tubes are rated at 80ma MAx per pair
>>???
>
The tubes are not so-rated by their manufacturer.
> Your concern is completely justified. Drifting readings are usually a
>sign of something wrong with a tube or circuit components.
>
amen
...snip...
>> Could I have some odd 3cx800's causing the high grid current ?????
>
>Odd 800's blow fuses; they are not a very forgiving tube.
I have made-usuable 3CX800A7s that blow fuses by tapping on the top of
the anode cooler to move the microscopic sputtered-gold melt-balls--which
cause the internal arc, which blows the fuse--into the base. However, if
the tube is subsequently inverted, the gold melt-balls must be resettled
into the base to move them out of harm's way before blastoff. The best
way to perform this operation is with a high-pot to periodically check on
the internal leakage-current during the tapping process.
Rich---
R. L. Measures, 805-386-3734, AG6K
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