Hi Jim, I see you remembered this! I learned of the tuned choke input
idea from the 1964 Collins Radio SSB book by Pappenfus, and also looking
at the Henry circuit, a 1950s edition of Terman Radio Engineering, one
of the Galaxy transceivers, and a few other commercial circuits that
used it. I had also talked to a designer at SNC transformers in Oshkosh
who knew the idea and was challenged and interested in making the
chokes. I had liberty to do it at Broadcast Electronics, as we could try
ideas that weren't conventional;we were trying to introduce products
that would change the industry at the time. The standard for FM
broadcast rigs was an L-C-L-C choke for single phase and just an L-C for
three phase power supplies. With FM being a constant power mode, it made
sense. The hitch was that if the exciter drive was removed, with a class
C final, then the load shifted to near zero current. And then, any
reasonable L input filter would cause the HV to soar tremendously.
Typical response was for the HV meter to peg the scale and scare the
engineers! FCC R&R had strong mandates that the final voltage and
current meters operate in the upper 2/3 of scale, not at half scale, for
the rated output of the transmitter. So when voltage soared because of
not enough load, the meter went above 100%. The only way around it was
to put in a huge bleeder that wasted hundreds of watts, otherwise. This
was to get enough load that a reasonable choke like 10 Hy would meet the
critical inductance criteria.
At the time (1980s) no one would have put a huge capacitor filter in the
transmitter, due to cost and space. Voltage for a single phase 3500 watt
transmitter was 4500 VDC. Series electrolytics did not meet the long
term 24/7 reliability goals. With a resonant choke, this was simplified.
It didn't need to do anything most of the time, and it was only going
into resonance with the parallel cap at zero load. That prevented the
voltage from soaring and pegging the HV meter when it happened. FDesign
of the choke then was simplified, as it had to have the exact value of
L, measured with low DC current. When loaded with more current, choke
inductance drops. Resonant freq rises above 120 Hz. (or 100 Hz for the
overseas model tap on the choke). And the voltage stays reasonable as
the load current is high enough to ensure critical inductance in the
filter.
As for exploding capacitors, I found quickly that the normal oil filled
paper/mylar filter capacitors were not appropriate for a resonant choke
circuit. I measured the peak voltage across the cap when it was resonant
and when it was off resonance, and indeed it was a large swing. Hence I
went to two HV caps in series with plenty of margin. It helped that my
cap manufacturer had a lot of microwave oven caps made with
polypropylene and low loss tangent, for a very low cost. Rarely if ever
did one pop. I designed it so that with no current drawn, it ran right
at resonance but never on the low side.
Many years later, (in the past decade) I have talked to some of the
owners of those transmitters still running them. They didn't understand
the circuit, ended up having a cap leaking oil eventually, and just left
them out or worse. So the thing had lousy regulation no load to load.
Thats the worst case problem once the cap fails. The chokes didn't fail,
SNC did a great job of building them for this circuit.
One other point, I learned that the resonant choke doesn't provide
adequate filtering for harmonics of the rectification frequency. 240,
360, 480, etc. They just roar through the L as it has a shunt C
bypassing it for the harmonics. So a second L-C section had to be added
to the design, for the higher order filtering. It was very small, not 10
Hy or anything like that.
All in all, it was an interesting project, but I would never do it again
today, well who would use a tube for FM anymore? It is probably not an
appropriate design for SSB or AM where the load is varying significantly
all the time.
73
John
K5PRO
From: "Jim Thomson" <jim.thom@telus.net>
To: <amps@contesting.com>
Subject: [Amps] Henry 2k-4 HV inductor
< On 12/13/2017 01:15 PM, Jim Thomson wrote:
> ## Although the tuned choke concept works, it really is a throwback
to the
> 1950s. IF the choke and parallel resonating cap just happen to
resonate at 120 hz,
> the peak V across that parallel tuned choke will skyrocket, and both
the cap and choke
> will explode. Typ the choke is resonated just a bit higher than 120
hz, like 123 to 124 hz.
> When u start sucking loads of plate current, the inductance of the
choke will DECREASE a bit,
> and the resonance of the choke + resonating cap combo will INCREASE
some more, like now
> up to 124-130 hz.
/* snip */
## Nope, the small resonating cap is wired directly in parallel with
the choke. That forms a
parallel tuned circuit, killing the 120 hz component. Any cap after
that, that is wired from B+ to
B- just kills any residual 120 hz components, plus harmonics of 120 hz.
HV filter caps are not wired
from B+ to chassis, they are wired directly between the B+ and B-. Any
HV meter is also wired
directly between B+ and B-. Old ARRL handbooks will depict HV meters
wired between B+ and chassis,
which is incorrect..and more fubar.
## John lyles designed a HV supply for a FM broadcast TX. For the
resonating cap, he used the
.9 uf at 5 kv rated small oil cap you will typ see in any microwave
oven. He used 2 of em in series,
so .45 uf at 10 kv. Both caps sent to the choke maker..who builds the
resonant choke around the
.45 uf combo. On any single phase setup, the component you are trying
to get rid of is always 2F,
or in north america, 2 x 60 hz = 120 hz. In japan, UK, etc, its 2 x
50 hz = 100 hz. For 3 phase
setups, it now becomes 6 F, or 300 hz in the UK and 360 hz in north
america.
## resonate the choke at 120 hz, and the caps will explode every time,
it has to resonate just above
120 hz. Ask anybody who has tried it.
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