Alpha 374 BANDPASS ADJUSTMENT PROCEDURE

1)	Cooling of the 374 - combined with replacement tube 
prices - is such that we do not suggest exceeding about 400w 
maximum average total plate dissipation for more than 2-3 
seconds. That corresponds roughly to 1 kW average (or 2+ 
kW PEP) "DC" plate input, or about 1200 W peak output. 
That was the FCC limit and what they were designed for 
around 1974.

2)	Equipment required: electronic cw keyer, peak-reading 
wattmeter (P-3000 or equivalent), HF transceiver, 240vac Variac.

3)	Use 240vac from Variac to power amplifier.  All realignment
MUST be done with the amplifier in low plate voltage mode (CW), 
and with the top cover in place to allow airflow over the tubes.  
Attempting to adjust the loading capacitors in the bandpass circuit 
with the amplifier top cover removed will destroy the tubes.  The 
safe way to do it is to slide back the top cover just far enough to 
be able to remove the brushed aluminum top cover bar.  This will 
allow access to the bandpass capacitor shafts.  Then slide the top 
cover so that it is flush with the top of the front subpanel.  Use 
masking tape to cover the small slit opening between the cover 
and the subpanel.  The amplifier primary interlock must be 
depressed and then the 374 can be turned on.

4)	Adjustment of LOAD capacitors should be done in the middle of 
the portion of each band where operation is desired.  Do all loading 
adjustments into a 50 ohm dummy load ONLY.  Exciter drive should 
be reduced to zero before keying the amplifier.  Increase drive as 
required, the capacitor for each band is adjusted to maximum RF 
output of 750-900w (Ig up to 75-80 ma OK). 

5)	Output of 1.2kw PEP or keyed CW (fast 'dits') should be 
possible in SSB(HV).  Read power on P-3000 wattmeter.

NOTE : For all 76/76A/374A/374/78 amps (untuned inputs) use 
short coax between transceiver and amplifier, 4 ft or less. 

NOTE : Viewed from the front of the amplifier, the five LOAD 
capacitors for tuning the Bandpass sections are:


10m                     80m

          switch
           shaft

15m    20m              40m