On Feb 22, 8:35 pm, ken scharf wrote:
TimShoppawrote:
On Feb 21, 7:27 am, Jack Schmidling wrote:
My new (first one in 50 years) is an 8000 modulated by a pair of 813's
running as triodes. The RF deck and Pw supply are up and running and I
am working on the modulator now.

My plans are to drive it with the Ranger and eventually do a stand alone
audio section. The Range gets poor audio reports in spite of months of
trying to resolve it.

I am wondering if anyone here as any experience using 813's as triodes
and would also like some help on the audio driver.

My sole experience is using them not in triode mode, but in pentode
mode. The RCA specs tell about using a pair in AB1 as a modulator.
Specs he

http://www.mif.pg.gda.pl/homepages/f.../079/8/813.pdf

In triode mode (plate connected to screen) you'd be limited to 750
volts on the plate if you follow the spec sheet religiously, which
will really hurt you in terms of AF power ability. I'm guessing that a
pair of 813's used this way would not put out more than 120W of audio
power. Many beam power tubes will take more volts on the screen than
the spec sheet will say without arcing over but I cannot tell you
about this mode on an 813.

In pentode mode you have to do a fairly stiff screen supply (which you
might have elsewhere in your line-up). Don't forget the -80V or so
grid bias too.

The "classic" way of doing all this is with a pair or maybe two pairs
of 811A's, which are zero-bias as a class B modulator. You can easily
get 250W of audio out of a pair or 500W out of two pairs. Driving
power isn't zero but it's very moderate (less than 10 watts).

The tubes for all this is easy - but you've got a modulation
transformer lined up already? Choosing impedances to match that
transformer is probably more relevant than any dickering about tubes.

Tim.

A better way to use the 813 in triode mode is to connect the two grids
together. This creates a HI-MU zero bias triode.

My experiments at audio have shown that this mode causes more non-
linearity than simple pentode mode. But... maybe not anymore than
you'd get from a similar-sized high-mu zero bias triode, as those are
fairly nonlinear too!

My experiments were with much smaller tubes (807's and 6L6's and the
like) than the 813's we're talking about here. Most of the extant
SPICE models do pretty bad when you tie the screen and control grid
together (not surprising!)

Tim.