I am not a technician. I know just enough to get into trouble. I
recently acquired an amp that has an Alpha 78 front on it. I believe
however it is an Alpha 314A. It does have the bandpass tuning feature
and that works very well. It covers 160 thru 10 meters. Someone has
changed out the original tubes, and replaced them with a pair of
3cx800a's. They also placed an outboard metal box on the back of the
amp, and I was told that it may be a filament supply for the tubes. I
have not gone into it as it works and I dont want to mess with what
works. My main question is this: The drive required seems to be high in
relation to other 3cx800's amps I have used. In most 3cx800 tubed amps
it usually only takes about 30 to 50 watts of drive to reach legal
limit. In this amp it will not produce over 1400 watts with 100 watts of
drive. The output holds steady, and does not change up or down as the
amp heats up. The amp tunes well and seems to have no other problems.
The output in the passband mode is about 1kw, on all bands. Dont
misunderstand I am O.K. with the 1400 watts. it is the amount of drive I
have to use to obtain it that concerns me.The mulltimeter shows right at
2600 volts in the ssb output mode and 1800 volts in the cw output mode.
I thought that maybe the tubes could be weak, but like I said before the
output holds steady, and I thought weak tubes usually show a drop in
output as the tubes heat up. Is this assumption wrong. Can the tubes be
weak and still hold true to the amout of output, only at a reduced
level. Or do you think the higher drive requirement is due to the
modification done in the change from the different types of tubes? All
in all the amp works great as to tune up and smoothness of operation. It
does not run hot nor has it ever given any trouble of any kind. If this
is just the way things are, will it hurt the tubes to keep running them
with the near 100 watts of drive required to obtain the 1400 out? Do you
think a new set of tubes would take care of this problem? Or is the only
real answer to just run it till the wheels fall off? Thanks to any of
you that take the time to reply.............. W4TH...Tom Hix
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