Vic K2VCO wrote:
> The TL-922 manual includes several warnings against operating the
> amplifier on CW in the SSB position (The CW/SSB switch changes the
> no-load plate voltage from about 2KV in the CW position to 3KV for SSB).
>
> However, if the mode switch did not reduce the voltage for CW, but
> instead increased the bias to limit the ZSAC to close to zero, the load
> on the transformer would be significantly reduced. The increased IMD
> would be irrelevant for CW.
>
That's exactly the modification I made to my SB-220 once the FCC changed
the power limit rules to specify 1500 watts max output on CW.
I changed the wiring related to the CW / SSB switch to always make the
HV the higher (SSB) value. Then I re-used the CW / SSB switch to add an
additional 26-30 volts of Zener bias to the stock factory bias in CW.
This is just enough to cut off the tubes with key up, but not so much as
to introduce key clicks. I "scientifically" (i.e., biggest one I could
find locally) selected a 50-watt Zener.
Admittedly, the SB-220 power transformer (or band switch or soldering,
etc.) is not the same as the Kenwood, but I believe -- as you are
proposing -- the total elimination of key-up idling current on CW
compensates for the higher power on CW.
I've run my SB-220 this way for as long as the FCC regs have been the
way they are. Never had to replace a power transformer yet.
One nice thing about the mod is that it's totally reversible and makes
no changes to the front panel!
Bud, W2RU
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