[R-390] Power supply data

[Jim Whartenby]

I was a list member several (15?) years ago but now that I have two beaten and abused R-390's, I'm back! 
I am in the process of collecting the missing bits and pieces for both R-390 receivers but while I await their arrival, I've been thinking about that big heat generator, the regulated power supply.  I have looked through the Pearls of Wisdom on the non-A power supply but find no hard data, just well intentioned guesses and opinions; mostly concerning the R-390A.  For example, it seems that no one knows the ripple voltage on the filter cap just before the pass regulator.  I calculate about 30 volts ripple assuming a 10 uF filter capacitor, 0.2 amps load current and a 1.5 mllisecond charging time.
So doing some rough "worst case" calculations, I find that the 26Z5's and 6082's are operating at pretty much the upper end of their respective specs.  
The 26Z5 pair dissipate 2 watts for each plate assuming a worst case 40 volts drop within the tube and 50 mA of plate current in each section for a total of 8 watts.  Heater power is 26.5 volts @ 0.2 amps = 5.3 watts X 2 = 10.6 watts.
The 6082 pass regulators are assumed to have, worst case, about 190 volts across them also with 50 mA for each section = 9.5 watts again times 4 for a grand total plate dissipation of 38 watts.  Heater power is 26.5v X 0.6A = 15.9 watts X 2 = 31.8 watts
Eliminating both pairs of rectifiers and pass elements would remove perhaps 88 watts of heat from the receiver, again, worst case.

I'm looking into an article published in "Ham Radio Magazine" in the July 1970 issue starting on page 52.  It just uses a single SCR plus a diode and a resistor to regulate B+.  Of course the rectifiers are also solid state and the filter capacitor is much bigger in value then one would normally expect.  If you want a pdf copy of this article, contact me off list.
The interesting thing about this circuit is that the power transformer secondary voltage just needs to be higher then the regulated voltage you want but there is no worry about how much higher the transformer secondary voltage is.  The SCR is biased OFF as long as the filter capacitor is at the set voltage.  Only when this voltage falls below that set by the gate resistor and series diode will the SCR again conduct and charge the filter cap.
There are a lot of unknowns such as the stiffness of the SCR regulator and the noise caused by the SCR at turn on.  A snubber network should help with the noise generated but how much noise is generated and how affective the snubber circuit will be are all unknows, at this time.
Since heat is the enemy of just about everything, eliminating it should help increase the life of the R-390.
Comments?
Regards,Jim

Too much agreement kills a chat.  E. Cleaver