[R-390] 6080 in place of 6802 - RMS ???

[David Wise]

Sorry for the big quote
after my post; I fat-fingered this
little laptop keyboard and sent
by accident.

If you want to run 6080s, I see three
ways to handle the heaters.

1. Transformer.  Just a 6.3 or 12.6V
filament transformer on 120.  If there's
room, JUST DO IT.  It wins on simplicity,
cost, effectiveness, RF noise, and heat.
IMO it's the only way to go, but I don't
know the realities of the R-390.
Somebody give me one and I'll try it :-)

If a transformer won't fit, or
you [lunatic :-) ] insist on a diode:

2. Diode + resistor.  My off-the-cuff
numbers here are wrong, but they
make a reasonable starting point for
cut-and-try.  I said that the tubes
get 8.8Vrms each, so you need
to reduce it to 6.3, that's 5V .
The tubes pull 2.5A, oof! so I'd
try a 2-ohm 20W resistor and see
how it goes.  This solution generates
more heat than the 6082s.

3. Diode + Zener diode.  The zener
reduces the peak voltage to one
that results in the right power.
I haven't calculated the required
zener voltage, and it's actually
somewhat involved if you want an
exact answer (integration of trig
functions over less than 90 degrees).
Much easier to cut and
try until you get the right brightness.
To measure the power you'd need a
true wattmeter because the waveform
is weird.  Start with a 7- or 8V
20W zener.  It needs to be heat-sinked.
This solution also generates
more heat than the 6082s.

Regards,
Dave Wise