[R-390] Second R-725 on ebay

[David Wise]

I don't have the schematic in front of me, but I'm sure
they simply added a resistive divider from B+ to ground,
with resistances set up so that the tap is, say,
+50V.  Then just connect one side of the (isolated)
heater supply to it.  The voltage is not critical;
it just has to be high enough to inhibit emission
and low enough not to puncture the heater insulation.

This is done in tube audio preamps that
don't take the more effective (and
expensive) approach of heating with DC.

It's also done, particularly in test equipment,
where there's a cathode at high voltage; then
you have to get the heater off ground to stay
within the H-K breakdown spec.  Check out the
power supply in 500-series Tektronix scopes.

Apparently, hum modulation in the PTO
was enough of a problem in the R-725
that they sought to reduce it.  That's
a special application.  It's never
occurred to me that it could matter
in a straight 390A or 390.

Since oscillator amplitude has little
effect on mixer output, I figure the
problem was FM not AM.  If you don't
get any hum when you tune to the edge
of IF passband on a CW signal, I'd say
it's not worth the effort.  And if you
do, it's cheaper to just try a different tube.

Dave

-----Original Message-----
From: Tom Bridgers [mailto:Tarheel6 at msn.com] 
Sent: Monday, May 11, 2009 12:26 PM
To: David Wise; r-390 at mailman.qth.net
Subject: Re: [R-390] Second R-725 on ebay

David,
How does one "float" the vfo tube's filament up to a positive dc potential?
Does that mean we should do the same thing to our R-390A's PTO tube's 
filament?
Thanks,
-tom