Mike, K6BR wrote:
>What modifications did you make to get 1500 watts out, was it just a power
>supply change?
>
>
1. Used the SSB transformer taps on CW.
2. Added a 50-watt 27-volt Zener diode in the filament center tap on
CW, to reduce key-up idling current to zero. Heat-sunk the zener to one
of the interior aluminum walls in the amplifier through a mica insulator.
3. Took all transformer wiring off the front panel CW/SSB rocker switch
and used that switch to short out the new 27-volt Zener when operating
SSB. (Do not short out the stock 3.2-volt Zener!)
Somewhere I think I have a handwritten single-page summary (in Heath
kit-buildling style) of the exact wiring changes necessary to implement
the high-power CW mod. I also use the Zener diode part of the mod in my
Amp Supply LK-550. Please understand that cutting the tubes off with
the 27-volt Zener should not be used on SSB, even though it helps reduce
total tube dissipation. At the edge of cutoff the tubes are operating
in a less linear region and increased distortion on SSB will result.
Related background:
I run QSK with an outboard electronic TR switch here. I don't use PTT,
VOX, or Semi-QSK on CW because I don't like to hear the sound of big
relays ker-chunking -- even once per transmission -- and I want to hear
the people I'm working between my dots and dashes. When I am operating
CW, my antenna changeover relay in the SB-220 is energized 100% of the
time I am using the amplifer; the Zener diode automatically cuts off the
tube when I'm not sending, thus eliminating any shot noise at the input
to the TR switch.
I don't operate much SSB but the simplest way to key the amp, of course,
is to have the amp relay in the exciter directly drive the antenna
changeover relay in the amp -- whether VOX or foot-switch (PTT)
operated. (But note -- not all solid state transceivers are capable of
directly switching the 120 volts DC that appears on the ANT RELAY phono
jack on the back of the SB-220.)
In years past I've also used a homebrew electronic circuit that biases
the grids of the 3-500s during "key up" (i.e., while in RX mode) in both
CW and SSB. Back when my exciter was a 32S3, my outboard circuit took
its input from the 32S-3 key line. My circuit instantly unbiased the
ampllifier *before* any RF was emitted, and it included a
pulse-stretcher to keep the amplifier keyed until *after* the last of
the RF had been delivered by the exciter. (I had modified my 32S-3 to
pull the key line to ground on SSB from inside the exciter; on CW, my
keyers or bugs did the same job.) Later, when I switched to Kenwood
transceivers, I found a pre-existing low-level signal that performed the
same function as my old pulse-stretcher circuit.
In my opinion, 3-500Z amplifiers are great. The circuitry is simple and
the tubes are hardy and inexpensive. Once I had an HT-33A (PL-172) and
another time an Alpha PA-76. Both were prone to failure despite all
sorts of protective control circuitry. Perhaps most important from a
DXer's point of view, 3-500Z owners don't have to wait three minutes
before they can use their amplifier....
Bud, W2RU
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