VE7RF makes a pretty strong argument towards using bigger tubes like
YU156, instead of trying to squeeze a kW or 2 out of an 8877 or pair of
glass triodes. I think he is on to something as far as dollars per watt
goes. I prefer the Almost All Digital LCR meter, and have been going to
SK widow's yard/sales around here for years, to accumulate a decent
stash of iron. Just keep in touch with the local hams and you'll find
'em. Also, EPAY does come up with decent iron every few months, if you
don't mind paying twice (for the item and for shipping it). I've gotten
quite a bit of audio iron this way, for AM projects and ideas.
Broadcast stations are one my best sources of HV power and inductive RF
components for HF projects. Many have dumped or are in progress of
dumping their tube 1 kW rigs, AM and FM stations, so these usually have
something around 2500-3500 volts at just under an amp. And most are
tapped on the primary to give a range of voltages. And the best part is
that they were designed for class B or H, with 24/7 operation for years,
made by decent manufacturers like Basler, SNC, Electro/Stangenees, NWL,
Magtran and others. You can find them on ebay (take the whole
transmitter and gut it) or by meeting with the local station engineers.
One opinion that Jim and I differ on is the amount of capacitance for
filtering the HV, and that is basically a difference in design
philosophy, which has been beat to death the past week here. I build
pulsed RF amplifiers for a living, so high stored energy is a norm, when
you need 200 amps of plate current 10% of the time. But it really sucks:
the protection circuitry, always wondering if this will be the big bang
moment at turn on, 20 page safety procedures on shorting out the caps,
and annual training for all the technicians and engineers for the thing.
I can tolerate a little power droop or ripple, if I can get away with
it, in a continuous carrier RF system (or modulated). I have already
stated that having three phase primary power is quite a wonderful thing
for power supples. There is no concrete rule, to each his own on how far
one carries things. If you are making something commercial, then there
are overlying rules that the corporation will have that limit the amount
of over-design that an be tolerated, cost and time wise.
Carl's suggestion about thinking out of the box and looking a
line-operated doublers is another way. Be sure to breaker or fuse the
line properly so that if you get fireworks you don't cause a fire, since
you don't have the impedance of a transformer in there anymore. At work,
we have two VERY LARGE 750 kilovolt DC power supplies, that power the
proton injectors for accelerator. They are just large room-sized voltage
multipliers. We have a ~8 KHz audio power amplifier (pair of something
like 3CX20,000's push pull) feeding a Haefly oil-filled output
transformer about 4 feet tall. This outputs about 50,000 volts of audio,
which then feeds the multiplier stack. You may have seen it on Nova or
Bill Nye the Science Guy, seriously, they filmed it in the 1990s - big
science...that thing won't supply amperes, but will do milliamperes.
If you started with a 220/440 step down transformer (very common at junk
houses, building tear downs, and electrical suppliers) run backwards,
and then have a big multiplier stack with diodes, capacitors and
resistors, you got some plate voltage with plenty of current. And it is
isolated from the line in this case.
My pole pig is 10 KVA, 13.8 kV to 220. It is destined for a big tesla
coil, spark excited, when I retire. It is frothing with PCB oil! It does
take a tractor with loader to move such things (Kubota 22 hp in my case).
Start building amplifiers again, darn it! I hate seeing this craft
fading away with (us) old timers. There will always be a need for high
power RF to cut the mustard at some point. Hams were pioneers in this
art, and its sad to see people snapping up $5000-10,000 amplifiers
instead of whipping up something a little ugly in the basement with $500
worth of materials or less. Despite what we infer here, it ain't rocket
science at all! They are certainly a lot easier to design and build than
receivers and even exciters.
One caveat for me, has been finding time to build things at home
anymore, but thats another story. And having time to use radios.
73
K5PRO
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