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.

Jack,

I agree with Tim's comments, and will add just a few words.

One concern I have is that the 15 watt amp will see a load of varying
impedance from the 813 grids. If you are running them Class AB, their
input Z is near infinite until
grid current flows. That reflects back to the 15W amp, which may or
may not affect it.

I suggest putting a resistor across the secondary of the driver
transformer, so the amp always sees a load. The resistor then sets the
impedance of the transformer.

Some back-of-the napkin numbers:

As Tim says, 15 W at 4 ohms is about 20 volts peak-to-peak. To give
some headroom, let's use 10 volts.

Again using Tim's numbers, figure about 200 volts peak-to-peak at the
813 grids. That's a 1:20 voltage step up, or a 1 to 400 impedance step
up.

The output Z is therefore 1600 ohms or so. The resistor should be 1600
to 2000 ohms, 20 W. Could be a bunch of smaller resistors in series-
parallel. The 20 W rating is very conservative, too - 10 W may be
enough.

Besides the transformers Tim mentions, keep an eye out for Stancor
"rectifier transformers". These have been around forever, and have the
numbers ranging from RT-201 to RT-208 or so. They are all the same
windings, the difference is in the power rating. An RT-201 is more
than enough VA for this application.

The RT series are well-documented, common and not too expensive, and
have the desired dual primaries (which you use in series to drive the
813 grids). They also have lots of secondary taps so you can try all
sorts of ratios.

One request for your website: MORE PICTURES!

73 es GL de Jim, N2EY

....that 8000 is just beautiful, btw...