Care and Feeding of Alpha 374A

Care and Feeding of Alpha 374A
Care and Feeding of the Alpha 374A
The best way to run the 374A is to tune up in the CW
position to about 500w output, then switch to it to SSB
(higher plate voltage) and continue your tuneup to whatever
output you desire.
The CW / SSB HV settings were made for the old USA FCC
power rules, which were 1.0 kw DC INput on CW or 2.0 kw PEP
INput on SSB. The ratio of the low to high HV is such that
if you tuned up at the CW limit using the lower HV (CW),
you could then just switch to SSB and the amp would be
tuned up correctly for the higher SSB limit.
It is better for the amp to run the higher HV all the time,
it keeps the grid and plate current low. Watch the grid
current, NEVER exceed 50ma per tube (as stated in the
For 1200w output, tune the amp with a keyer sending fast
dots. The grid current will be about 60ma, plate current
will be about 1A. Sending dots at 40 wpm gives about 1/2 of
the key-down reading on the meters.
When using the BANDPASS function on various bands, keep the
antenna VSWR below 1.5:1. Higher VSWR can damage the
capacitors in the BANDPASS circuitry, they are not voltage
rated as high as the much larger manual TUNE and LOAD
capacitors. Do NOT hold the key down at 1200w in the
It is normal to have slightly lower output on 10m than the
lower bands.
Information on tuning up on 17m is on our web site. Do NOT
use the 374A on 12m, the plate choke will burn up.
The 78/76A/374A amps were designed to run on 240vac. Using
lower voltage that is 10% less (like 220vac or less) lowers
the filament voltage of the tubes, lowers the output power
and will shorten tube life.
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