Hi Tom...
1. The main differences between a standard (unmodified) ALPHA 78 and a 374A
are (a) 78 has three 8874 tubes while 374A has two, and (b) the 78 has full
break-in/QSK using a vacuum T/R relay while the 374A uses a conventional
T/R relay. The 78 power transformer has a core of tape-wound Hipersil, with
generally rounded contours and a shiny black encapsulant. Frankly I'm not
absolutely certain of this because I may be thinking of the older ALPHA
374, but I believe the 374A has a conventional xfmr with a big rectangular
laminated steel core and varnish finish.
2. Two 3CX800A7 tubes are virtually identical with three 8874s in
capability and characteristics (except heater voltage), so the conversion
is logical and safe now that the tube prices are about equal and Eimac has
chosen to emphasize the newer '800 for the long run.
3. Most likely reasons for your amp requiring perhaps somewhat high drive
power would seem to be (a) low heater voltage (more than 5% either high or
low also is likely to shorten tube life) OR, and probably more likely (b)
the input circuit. Most (or maybe all) current model 3CX800 amps have tuned
input and presumably are designed to meet FCC rules which stipulate that
1500 watts output MUST require AT LEAST 50W of drive (i.e., <15 dB gain).
The 78 (and 374A) have untuned inputs with resistive padding to reduce gain
to <15 dB AND to protect each tube from receiving more than ~25W drive with
the 100W tube-PA xcvrs typical in 1977 when they were designed. In the
2-tube 374A this pad was 10-470ohm/2w comp resistors (47 ohms/20W) in
parallel with the cathode rf input. The 78 had only 5-470 ohm resistors.
The pad was located under the tube deck and air-cooled by the tube blower.
If your amp was actually a 374A and has the 47 ohm pad, cutting out 5 of
the original 10 the resistors is desirable with a pair of 3CX800s and
should decrease drive power requirements somewhat.
Also, the untuned input makes gain and efficiency somewhat dependent on
drive cable length and band. The 78 and 374A generally work best on all
bands with a 3-4' (maximum) length of RG-58 from the transceiver. I think
you should be able to get ~1500W peak output (possibly slightly less on 10
and/or 160M) with 80-100W of drive.
Good luck!
73 Dick W0ID
-----Original Message-----
From: Tom Hix [SMTP:w4th@webtv.net]
Sent: Tuesday, September 01, 1998 11:20 PM
To: amps@contesting.com
Subject: [AMPS] output on pair 3xc800a7's
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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