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)].
After the new rules came into effect allowing up to 1.5 kW rf output on all
modes, there was no compelling reason to incorporate a LO voltage tap for
carrier-type operations, particularly in amps like the ALPHA 87A, 89 & 91B
which provide a tune meter function so that accurate tune-up can be done
safely at low drive with full plate voltage applied.
It's important to remember that, if wishing to run less than full power,
one should NOT do it by decreasing plate loading. That can and will result
in excessive grid current, distortion, and possible tube damage or
destruction. Instead, reduce power by simply turning down the drive.
Efficiency is lower under these conditions but, at least with the
amplifiers mentioned above, it won't result in dangerous overdissipation or
heating. (After all, the point is lower output, right? 20M ops may realize
that this issue doesn't seem to come up very often!)
Hope this answers your question, Phil.
73, Dick W0ID (ex-W4ETO)
-----Original Message-----
From: doshhan [SMTP:doshhan@vaxxine.com]
Sent: Thursday, September 10, 1998 10:28 PM
To: amps@contesting.com
Subject: [AMPS] Purpose of CW/SSB switch
What is the true purpose of the CW/SSB switch on many of the older linear
amps ?
Was it simply to ensure that you stayed within the old power INPUT
regulations when in CW mode, or is there some other good technical reason ?
Obviously the reason I'm asking is that I can't see a good technical
reason for the switch. Actually I see good reasons for not having it but I
need someone to verify my understanding. (I wasn't planning on having one
on the 4-1000 amp I'm building).
Phil T. VE3OZZ
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