Its a great amp; the tubes are 8874s (the aircooled version of the
8873/4/5 series); the resistor array mounted below the tube deck is a
padding arrangement to allow 100+ watt transceivers like Collins, Drake
to operate with the amp easily -- normally shouldn't take more than about
50 - 60 watts to max it out. With brand new tubes, you can get > 2000
watts out on 20 M -- more typically you should get around 1500. To get
1500 you need to get it tuned up very carefully --
1) if you use bandpass tuning, you might want to tweak the bandpass caps
to mate with your antennas;
2) if you use manual tuning, a two tone test and an oscilloscope are very
good for getting it properly tuned (e.g., TS950 series and SM230);
if your antennas aren't exactly 50 ohms and you use bandpass, you'll get
a little less power out, but it really shouldn't make much difference.
Make sure you get/have an instruction manual -- they are still available
from Alpha-Power (970-535-4173)
If your front panel is grundgy (they used an epoxy-velour paint
originally which decomposes in ozone) I have a spare for sale for $80 on
an exchange basis.
On Sun, 4 May 1997, Ernesto M. Grueneberg wrote:
> Hi guys:
> While watching the alpha 78's manual, I realized it has some resistors
> in parallel between the rf from the exciter and the 8875s Can't remember how
> many, but the value was 15 ohms. I wonder what purpose they serve, with such
> a low value and also wonder if they can be removed, to increase drive =>
> output power??
> besides, anyone can provide any hints about this fine amplifier. I'm
> only able to get 1,1 KW (with 100 W drive) and thought it should be at least
> 1.5 KW.
> Thanks for any answer.
> Ernesto LU6BEG
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