[AMRadio] Audio Driver Transformer advice

Donald Chester k4kyv at charter.net
Fri Jan 16 01:45:13 EST 2015

----- Original Message -----

>>>>From: Paul Baldock <paul at paulbaldock.com>

I'm building a modulator with 2 x 813's in pushpull (strapped as 
triodes) driven by a 20W Solid State audio amplifier. I need a driver 
transformer to go between the 813 grids and the 20W amp. The 20W amp 
can drive about 20V peak to peak. I think I need about 200V peak to 
peak for the 813's, so a 10:1 turns ratio ratio would be about right. 
So based on that an audio transformer designed to go from 600Ohms 
center tapped to 4 or 8 ohms would probably do the job. Have I got 
the math right? Does something like this exist?


Paul >>>>>

For best results, use the least amount of step-up possible to just barely
get the required peak-to-peak audio voltage, with a little bit of additional
head-room. Ideally, the driver amplifier will go into saturation (flat-top)
at the same instant that the modulator is driven to its maximum capability.
IOW, the driver and modulator stage reach saturation at the same time. I
would design in a little extra  margin, so that the driver is capable of
driving the modulator slightly beyond saturation, just to make sure you get
the maximum undistorted audio power out of the modulator before the driver
starts clipping.

A typical class-B grid impedance varies widely over the audio cycle, from
infinite ohms (on the negative half of the cycle when the tube is not
conducting, down to a minimum of about 500 ohms at the crest of the positive
peak. The grid voltage / grid impedance curve will likely be non-linear. The
total impedance of the whole mid-tapped winding would be something like 4
times 500 ohms, or 2000 ohms. Using the 8-ohm tap, the impedance ratio would
be 8 ohms to 2000 ohms, or a 1:250 Z ratio. That would be equivalent to a 1
: 15.8 turns ratio.  The 4-ohm tap would require more step-up, 1 : 22.7.

I doubt that a 600 ohms to 8 ohms transformer would give enough voltage
swing. It may be hard to find an output transformer with a primary impedance
as low as 2000 ohms at that power level, although I'm sure such a thing
exists somewhere. In the  real world, I would look for a good quality hi-fi
output transformer rated for at least 20 watts, and preferably 30 watts or
more, with as low a plate-to-plate impedance on the  primary as you can
find, but nothing much below 2000 ohms.

A (backwards wired) transformer with more step-up will deliver more
peak-to-peak voltage to the grid, but the effective internal resistance of
the source voltage will be increased, giving more distortion as the audio
source voltage regulation is degraded. Putting it another  way, using too
much step-up, the grids of the tube will cause the audio voltage output from
the transformer to sag. Ideally you might find a transformer with multiple
taps so you could experiment to find the optimum turns ratio, but you'll
have to make do with the best you can find.

Also, a transformer rated for higher power (and lower primary impedance)
will tend to have lower winding resistance, which would reduce the audio
voltage sag.

Once you have found what looks like a suitable transformer, try both the
4-ohm and 8-ohm tap and see which one gives the most satisfactory results
with the least distortion.

Don k4kyv

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