The title of this doesn't reflect what it has drifted to, but
here is another nickel, comments on Jim's comments:
> && resonant chokes schemes are steam engine technology..and buy you
> absolutely ..nothing !
> Resonate the choke at 120 hz... and the peak V across both the choke and the
> resonating cap will be
> through the stratosphere. Both will blow up, the choke will be ruined, and
> the cap will explode.
If you don't design it properly, it will fail. The voltage across the resonator
section will go up, but not
like a Tesla coil. It is limited by the Q of the components, which isn't very
high with a iron core choke (maybe Q=2).
The plate voltage on the 3500 watt FM transmitter was 4.5 kVDC. Without the
resonant choke, the voltage could soar to 6 kV unloaded.
> && To build em correctly, you have to suck one helluva lot of bleeder
> current. At that point, the choke should
> be resonant ABOVE 120 hz. OK... now when you suck loads of plate current,
> the choke L will drop a bit...and
> resonance will INCREASE. IF the coke resonates BELOW 120 hz...with just
> bleeder current... you are in trbl.
> When sucking plate current, the resonance will increase, and when it passes
> through 120 hz....Kaboom every time.
The voltage measured across the resonant choke in the BE rig was 4kV peak to
peak, a sine wave at 120 Hz, when it was minimum (bleeder only) current.
This does not cause a kaboom with the components designed to withstand such
voltage and the corresponding circulating current. It was parked so that at
minimum
current, it was at resonance. Then it would shift upwards in frequency when
loading was added to the power supply. No surprises there.
> && Here henry used a 6000 vac xfmr....when a 3800 vac xfmr + FWB would
> have worked a helluva lot better.
> Even a 1900 vac xfmr and a FWD would have worked better. A 6000 vac xfmr
> is a poor utilization of it`s
> winding. Then toss in the huge,heavy choke + heavy oil caps. The classic
> resonant choke setup only filters 120 hz to boot. It won`t
> reduce the ripple at 240,360,480 hz. So they still need a mess of oil caps.
>
The Broadcast Electronics circuit ran a 5.3 Hy choke designed by SNC, with 0.45
uF, made from two series connected 0.97 uF 2500 VAC polypropylene capacitors,
the
kind used in microwave oven power supplies. Then there was a second stage
filter, with 3.5 Hy, and 4 uF oil cap smoothing filter to get rid of the
harmonic ripple voltage. The alternative design used a much larger choke, about
20 Hy, with 20 uF, and a lot of bleeder current, to get some
semblance of regulation. The bleeder current was terrible. With the resonant
choke design, it was much smaller, the bleeders were higher R.
Pappenfus discusses the circuit in the 1964 edition of Collins Radio's "SSB
Principles and Circuits". It was also found in Terman's textbook Radio
Engineers Handbook.
Tony, I0JX, has a good website on the technique.
http://www.qsl.net/i0jx/supply.html
> &&& A simple 3800 vac hypersil xfmr, or similar... + a BIG C filter will
> run rings around the resonant choke scheme. Why
> spend $$$ on plate xfmrs with massive oversized sec V ratings = smaller
> ga wire used= low current capability....then toss in the
> meagbuck heavy choke = 1921 technology.
> > later... Jim VE7RF
Jim is correct that a massive C filter capacitor alone will work very well and
make a nicely regulated HV supply for a tube amplifier, especially with varying
loading like SSB.
It will also add a lot of stored energy, that must be dealt with safely.
Protection of the tube becomes imperative against the spark that could happen.
This is something that
shouldn't be taken lightly. Jim has done all the right things, we've discussed
this together numerous times.
In March of 1982, large values of capacitance and high voltages meant a lot of
money and a lot of real estate. This was not competitive in a commercial FM BC
transmitter that
ran on single phase AC but the resonant choke was costly also. It solved a
stupid problem, of soaring HV with removal of RF drive, at a time that FM
exciters like the BE model
FX30 could be turned off and on simply. And it worked, was fun to research and
build for production.
73
John
K5PRO
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