> my result are :
>
> 3500 volt , 1Amp , 70 watt input , 2025 watt output ,120 ma grid.
Your grid current is too high for sustained operation. Based on
your comments below, you are loading the amplifier too lightly for
the available drive. Eimac's data sheet values generally represent
the tube performance ONLY and do not include external circuit (input
and output circuits) losses.
> the question is :
> is my tube exausted ?
Not likely, see above
> is my tube defective ?
No, see above
> people that working in brodcast fm , has told me , that they use
> amplifier at 2500 watt whit 80~100 ma grid 2000 watt 40~60 ma grid
Those sound like good values. I would not run the 8877 at any more
than 100 mA on a regular basis unless I had an unlimited free supply
of tubes!
> If I disalign the load coming down from 2000 watt to 1800 the grid
> current go down to around 40 ma.
Again, this seems to indicate that the amplifier is underloaded.
Alternate between increasing the load to lower your grid current
and increasing the driving power. Your 70 watts of drive may be
a bit low depending on your input matching circuit. If your input
circuit is wrong and you lose even 1 dB (10% of the available drive)
it accounts for the difference between your results and what the FM
broadcast users see.
Your amplifier is delivering 14 dB *NET* gain (gain after circuit
losses) and the tube is rated for about 15 dB gain in grounded
grid operation. Can your broadcast friends tell you how much drive
they are using? In my experience with an 8877 broadcast transmitter
it used a 4CX250B IPA (driver) and was capable of more than 150 Watts
(depending on screen voltage) drive.
I would recommend (1) adjusting or modifying your input circuit
and (2) increasing the available (clean) drive then see if your
results don't improve (if you even need the additional power).
73,
... Joe, K4IK
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