On Mar 9, 8:02 am, Jack Schmidling wrote:
My Ranger driven 813 modulator is working very well but now I want to
move on to a self-contained mod.

I have ordered a 15 W solid state amp but need some help picking a
transformer for between it's 4 ohm output and the 813's running as triodes.

I am sure this has been done many times so perhaps someone can save me
the pain of trying to determine the impedance needed for the output of
the trans.

Thanks,

JACK K9ACT

Wild guess: in pentode mode, a PP pair of 813's in AB1 needs approx
160 v peak-to-peak to develop 250 watts of power.

15 watts at 4 ohms is about 20 volts peak-to-peak.

So you need a voltage step-up of 8 or more, round up to 16 for a
little headroom. Don't round up too much or you'll lose some current
ability. If you're looking for impedance ratios they are voltage
ratios squared, so 1:64 (250 ohms, sounds low) to 1:256 (1K ohms,
about right). Note that 600 ohms is a commonish value, you're in luck
if it has a center tap!

As a sanity check, typical 811A (high mu triode) class B pairs take
about 10 watts at 1000 ohm impedance for driving too.

Generally the voltage ratio is more relevant than any impedances
printed on the transformer. If you find a hi-fi or 70.7V PA
transformer that's great, but lots of people have pressed filament
transformers into use too. A center-tap on the output side (even if
it's really the input side!) is really nice, you'll note that
"international" filament transformers have two 120V windings that can
be put in series for 240V or in your case to make a center tap.

My question for you: what kind of modulation transformer do you use to
drive the plate for the 8000? I'm jealous that you apparently had one
just sitting around :-).

Tim.