Hi all !
I have so far not contributed to this discussion, but as a long time ham
(1968) as well as a professional, I would like to say a few words:
I think it is possible to make a pair of 572B's work relatively well on 6
meters by careful design taking the tubes internal construction into
consideration.
In the early days of VHF there were not too many tubes available designed
for these frequencies. So manufacturers often operated tubes well above max
frequency ratings. I remember an RCA Nera TV transmitter operating near
200Mhz using a pair of tubes rated at a max freq of 50 Mhz for full ratings.
The construction was unconditionally stable and several units were produced,
indicating a good repeatability. The transmitter ran at abt 60% of max power
ratings given at 50Mhz. And this is what worries me most about the
conversions discussed here. They say they get 750 watts or more from those
units.
Manufacturers max rating for two tubes ICAS is 600 watts below 30Mhz ! It
is obvious to me that 750 watts ICAS at 50 Mhz is way out of all good
engineering and operating practices ! Say 350 watts and I would probably
accept it, but I would NOT recommend it. Why should we push all
limits when we have suitable tubes at reasonable prices that will give a
much more reliable unit ? If it is just to prove that it can be done, I
understand it,
but to make a bussiness out of it ? It is at best very questionable
engineering practices in a unit that is promoted for sale. But can it be
done ? Certainly !
Is it repeatable ? Most certainly ! Is it to be recommended ? Most certainly
not ! I have myself built a 2 meter FM transmitter using EL-95 as a PA.
That is an AF amplifier tube ! No freq of max ratings is given. It is
strictly AF amp. And it was run well above its max power ratings ! So sure,
it can be done,-
but would I recommend it ? NO !! And I would love to have one of those
converted SB200's on my desk, tune it to 750w, apply a two tone test
signal at 750watts PEP , and watch my spec. analyzer ! I would also love to
see it at 500w pep ! IMD -32dB ?? Hardly :-)
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