[R-390] RE: Was variacs, now bucking transformers...

[Drew Papanek]

On constant voltage transformers Dave Wise wrote:

  "The output waveform is not a sine wave; it's closer to
   square.  This means that unless it's true-rms, your
   voltmeter is lying to you.  In any case, there's
   no right voltage.  The radio was designed assuming
   a crest factor of sqrt(2).  The filaments respond to
   rms, but B+ responds to peak."

With the choke input B+ filter in the R-390A, B+ responds to average voltage 
(which has strong dependence on peak).  If one were to change sinewave input 
to fully squarewave input then adjust for correct RMS to satisfy heaters, 
then B+ would end up about 11% high (neglecting changes in voltage drops due 
to altered peak currents and saying nothing of the changed "swing" of the 
input choke).

For a capacitive input filter, the aforementioned RMS condition would lower 
B+ about 29%.

As you say, there ain't no right voltage.  Situation Normal, All "Bucked" 
Up.

Here's fodder for a new dead horse:

How about running R-390A on a CV transformer with taps set for correct RMS 
to heaters?  Then tap into one of the rear panel fuseholders to insert an 
outboard soiled state regulator which would drop B+ back to normal.  If your 
'390A's power transformer didn't hum before, it probably would now with the 
harmonic content.  R-390A transformer would run a little warmer too, but 
hey, you'd be fully regulated, dammit.

For the R-390 non-A, one could set CV transformer taps for correct heater 
RMS, and let the 6082's take care of the B+.  With the lower voltage out of 
that capacitor input filter they'd run cooler.  If regulator circuit drops 
out on ripple valleys (not in The Land of Chuck), parallel some more 
capacitance across that 10 uF(?) filter cap.  If still not enough headroom, 
an external DC source could be connected in series to jack voltage back up.  
Along with the already regulated B+, you'd now also have regulated heaters.

  " Apropos of nothing: The circa-1960 IBM mainframe
   in my basement contains two CV transformers."

Decatron tubes?  Mercury delay lines?   Rotating drum memory?  Bearskins and 
stone knives?   You are one serious boatanchor collector!  Does that 
mainframe work?

Drew

"Repairing and modifying R-390(*) vicariously through advice to others"

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