Rich raises some interesting points. I guess one question to ask is:
"Are we designing a commercially viable, going to make lots of, device
OR are we building one offs as amateurs?"
The answer to this has potential for running us down one of two routes.
Looking at Rich's points:
>IMO, it would be feasable to produce an amplifier that had:
>1. 2mS - 3mS switching.
>2. Rheostat-adjustable filament-potential.
>3. Filament voltmeter function on the multi-meter.
>4. Low VHF-Rp parasitic suppresson.
>5. Step-start.
>6. Tank circuit with appropriate RF current rated components.
>7. Good input swr with any transceiver.
>8. Glitch protection.
#1. 2mS - 3mS switching.
Some of the smaller vacuum relays are quite OK at 1Kw plus - carrying,
NOT hot switching. However, they are abt $75 in the US. A few years ago,
they were available here at $7.50 (yep seven dollars fifty cents!) at
the flea markets. I use a pair on my amp, but the output one also
switches the bias such that the PA is not biased into its operating
range before the output relay has changed over - the only 'hot'
switching is about 25mA of DC from about -150 volts down to about
-40volts. If you're building an amateur as opposed to a 'lots off', then
these relays are viable.
Question: has anyone been really successful with PIN diodes?
I've got 10 off 1000volt 2 microsecond PIN diodes on insulated stud
mounts, rated a 1A DC, and 10 of the same wire ended. One day, I'll see
what can be done. Interesting that W2VJN's company is avoiding PINs
because George is dubious about the reliability under amateur
conditions.
#2. Rheostat-adjustable filament-potential.
If you're building a one off, it's possible to pick up a low voltage
regulated PSU very cheap - I got a 6volt 20Amp DC output switcher for
$20 at a flea market. This way you can use the current limit to run at
constant current until the filament/heater warms up, and then gets
regulated. And there's a lot of 30amp, 5 volt switchers around. But not
for production...........there, the rheostat is better. I also argue
that step start on filaments is no bad thing either.
#3. Filament voltmeter function on the multi-meter.
Amen to that. It's not a difficult or costly thing to do, either.
#4. Low VHF-Rp parasitic suppresson.
I guess we all have our own opinions on this, and its implentation, but
the basic principle (as I understand it, the parasitic suppressor should
have a low Q at VHF) is agreed.
#5. Step-start.
An alternative that can be very cheap after a good flea market, is a
motor (or even hand?) driven Variac. Otherwise, I would say that step
start is a sine quae non. For production, it isn't that expensive,
either.
#6. Tank circuit with appropriate RF current rated components.
This surely comes down to deciding what the application is. If you want
RTTY at 1500 watts for half an hour or more, then it's very different to
non compressed speech with one minute messages every hour. If you want
a contest amplifier, then you're better off with the RTTY approach.
#7. Good input swr with any transceiver.
I must admit I don't understand this. If the input SWR is good
(<1.5:1?), then why should the driving transceiver come into it? Why
should the input impedance be dependent upon the driving transceiver? Or
have I missed something?
#8. Glitch protection.
I assume that by this, Rich means the 'spike' of energy of much higher
level that the transceiver can produce before its own ALC grabs the
output. Interesting one, this. Is the best approach:-
1. Build an amplifier that can handle the spike?
2. Arrange some method of clipping the spike so it can't overdrive the
amp?
3. Produce some fast enough acting system to overide the ALC and not
produce the spike?
One thing about old fashioned tube PA's in transceivers - that's a
problem they didn't tend to have.
For the one off, maybe the use of a 180 degree hybrid in the input, with
the amp fed from the isolated port, and one of the other ports fed
through a PIN diode to a load. In normal use, the PIN diode is off, so
the power is reflected into the PA. When a spike is detected, the PIN
diode is switched on, and the drive to the amp disappears by the
isolation of the hybrid. You could make a very fast ALC with that. A
delay line can be put between the hybrid and the amp. Not practical for
economic production. Also throws half the driving power into the load on
the other port, although I guess two PIN diodes could get round that.
But I believe that many modern transceivers can heavily overdrive modern
grounded grid tubes anyway.
>From all this, I think it's easy to see that there are two approaches
for most of these points. As I haven't yet seen anything available which
will allow me to get an amplifier which meets my additional criteria for
the perfect amplifier, which is a cost of less than 10 cents per watt
output, home brew is the answer. Besides, I like building things! It's
the only chance I get to do real radio engineering these days - work is
meetings and typing on a computer!
73
Peter G3RZP
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