Tom,
One way to tell which amp it was to start with is...
78A has vacumm output relay and hypersil transformer.
374A has standard open frame relay and standard open frame transformer.
In either case, with the modification, the output is gonna be about the same.
What you describe sounds very familiar to me...I have modified my 374A with a
pair of 3CX800A7's....had to mount the fillament transformer on the back
pannel.
On a good day it will on most bands run 1500 watts out without having to beat
the tubes up with drive. Your output of 1400 watts sounds good to me. In
comparing this pair of 3CX800's to other amps you have to take into
consideration one maybe two things...
one...the HV is about 400 volts lower than say the 87A or 89...
two... the modern 3CX800 amp uses a tuned input...which does increase
efficiency some.
The higher drive requirement in at least my amp is because I chose to keep the
original input toriod and swamping resistors in the circuit as I felt this
would provide a better match for the modern solid state exciters and I found
similiar to what you stated it takes around 80 to 100 watts drive for me to
get the 1500 watts out. This IS in the bandpass mode...maybe your bandpass
networks were not retuned for the new tubes ? I do not see much difference in
output power between bandpass mode (if near optimal frequency) and manual
tune.
If it is at all possible for you to take a couple digital pics of your
modified amp I would be most interested in seeing it. I will forward to you
privately a couple pics of mine as I don't know how the reflector would handle
the jpg files.
Hope some of this helps...
73,
Terry - W6RU
In a message dated 9/1/98 10:23:35 PM Pacific Daylight Time, w4th@webtv.net
writes:
<< 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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