On Thu, 10 Sep 98 21:28:26 PDT "doshhan" <doshhan@vaxxine.com> writes:
>
>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 ?
Most of the older amps...Dentron, National, Heath, etc had the switch to
comply with pre-1979 FCC power rules. In most cases the amp still
operated in its SSB class mode, AB or B, but at reduced Plate and Screen
voltage as required. The bias did not change except for a few cases such
as the DTR-2000 and some Handbook examples. I know of no commercial
examples that went to Class C for CW but I'm certain others will point
them out !
Later post-1979 amps that have the switch are primarily to reduce the PS
load on CW when the design is marginal to start with and also for RTTY
operation. The B&W PT-2500A ( ex-Viewstar) is an example of the latter
case....it had a healthy PS and could run 2KW plus on SSB but the
designers felt that was a bit too much for a pair of 3-500Z's on RTTY or
heavy CW contesting.
>
> 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).
It might have some advantage dyring the debugging stage to have a simple
way to reduce plate or screen voltage in any amp. 240V Variacs at the 4+
KVA level are not always a junk box item.
When I used to build single and dual 4-1000A amps for people I used a
xfmr with a 2800 and 3900VAC tap that could be hot switched by an old WW2
surplus BC-375 ATU switch...5KV @ 25A rating and I skipped 2 positions in
the wiring. Phenolic mounting block and fiberglass shaft along with a big
Millen ceramic shaft coupler kept things very safe and failure proof.
Anyway...its getting too late in the early AM for more ancient "war"
stories Phil. Have fun with the project.
73 Carl KM1H
Amplifier Repair and 6M Conversions our Speciality
National Radio 1963-69; part of the NCL-2000 design team
>
>Phil T. VE3OZZ
>
>
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>
>
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