Hi Matt...
The primary reason for providing the LO tap on earlier models was to allow
safe and efficient CW operation at 1 kW dc input, the FCC limit pre-19'77.
It's generally fine to run the older ALPHAs in the HI/SSB (high plate
voltage) mode on CW as long as you don't exceed about 1 kW d-c plate power
input AVERAGED OVER A FEW MINUTES. Heavy contesting "run" operation is
probably one of the toughest operating conditions wrt duty cycle, and the
smaller amps using 8874 tubes probably shouldn't be pushed to 1.5 kW output
under these conditions - 1000W is safer.
The ultimate criteria are transformer and tube seal temperatures. If you
keep the transformer below ~80 deg C (~176 deg F) and the tube seals below
200 deg C MAX you should be safe even though "pushing." Of course it takes
a bit of "savvy" to make these measurements, and as Carl often warns,
anyone who isn't really knowledgeable and comfortable working with
dangerous HV inside a big amp shouldn't take the chance!
77Dx and Sx series amps should be perfectly happy running 1.5 kW output all
day on any mode.
73, Dick
-----Original Message-----
From: Matt--K7BG [SMTP:aa7bg@3rivers.net]
Sent: Friday, September 11, 1998 10:53 AM
To: Richard W. Ehrhorn; 'doshan@vaxxine.com'; amps@contesting.com
Cc: 'broz@csn.net'; 'w0un@aol.com'
Subject: RE: [AMPS] Purpose of CW/SSB switch
At 09:59 AM 9-11-98 -0600, Richard W. Ehrhorn wrote:
>
>Hello Phil,
>
>ALPHA amps built before the FCC power limit was changed (about 1977-78)
>were designed for convenient "1 kW dc INPUT". That old rule, in effect
>since the CW & AM 30's, was ambiguous and about 90% misunderstood when
>applied to SSB. But it translated roughly into something like 2.5+/- kW
PEP
>dc plate INPUT, taking SSB duty cycle into account.
>
>So the amps' HI or SSB position provides plate voltage appropriate for
good
>linearity and efficiency at ~2.5 kW PEP dc input, which corresponds to
>about 1.5 kW PEP RF OUTPUT, which the post-1977 rules specify.
>
>The LO or CW position delivers a lower dc plate voltage which is optimized
>for 1.0 kW dc input on CW (as the old rules allowed). Also, the design is
>such that when the older amp is tuned properly for CW in the LO position,
>just switching to HI (SSB) sets it up for efficient SSB at 2+ kW PEP
>without any further tune/load adjustments [i.e., both conditions require
>approximately the same plate load resistance for the tube(s)].
Dick, So does this mean that one SHOULD NOT run older amps in the SSB/HI
position while operating CW mode? I'm still a bit unclear on this after
following this thread. I have been doing just that. OOPS?
73, Matt--K7BG
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