[Amps] Alpha vs. Henry (tuned choke PS filter)

John Lyles jtml at losalamos.com
Mon Apr 4 16:08:13 PDT 2011


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




More information about the Amps mailing list