[R-390] B+ current

[2002tii]

John wrote:

>The voltage concern comes from reading the Pearls - there is an emphasis on
>the 115 VAC vs. 120 VAC and using a variac to reduce the modern voltage.

If everything is healthy, no problem.  I typically use silicon 
rectifiers with no dropping resistor.  My power company delights in 
pushing 126-128 volts routinely, and I've had no problems of any sort 
with my personal 390A.  I rarely use it these days, but for a couple 
of decades it was on 24/7 and got heavy use for another ten years 
after that.  It has all but 3 original tubes, and they all test like 
new.  These things are not delicate, or fussy about tubes, resistor 
values, or much of anything.

>I do not have enough of my radio up and running to know if
>hum is an issue, or what my major sources might be.

As a practical matter, barring excessive heater-cathode leakage in 
one or more tubes, all of the residual hum is a result of using the 
chassis as a return for the heaters.  I once re-wired a 390A with all 
twisted-pair heater wiring and a derived center-tap, which dropped 
the residual hum not quite 10 dB (from just above the audio-section 
noise to well below it).  I would never bother doing that again, 
although I would also never design tube equipment that used the 
chassis as a heater return.

Best regards,

Don



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