[AMRadio] AMRadio Digest, Vol 132, Issue 12

Donald Chester k4kyv at charter.net
Fri Jan 16 17:11:06 EST 2015

>----- Original Message -----
From: Paul Baldock <paul at paulbaldock.com>
Thanks for your input.

I finished up using two step up transformers with the 8 ohms sides in
parallel and the 500 Ohm sides in series to simulate a center tap. So that
is effectively 8 Ohms to 2000 Ohms, center tapped. Testing in to a 1K
resistive load the output looks reasonably distortion free, >>>>

That should work fine,  since each modulator tube is active over only half
the audio cycle, and each side of the midtapped secondary coil of  a real
push-pull transformer would be acting as a separate transformer driving a
separate tube anyway, while the other half idles with no load. Gates used a
somewhat similar configuration in the BC1-J transmitter, except that they
had a pair of what were probably 30-watt public address or jukebox
amplifiers, and used the 500-ohm output tap on each unit to independently
drive each 833A grid, and drove the input to each separate amplifier 180°
out of phase with the other one to maintain proper push-pull operation.

Your lash-up probably will have less distortion than would a 5K or 10K
push-pull output transformer wired backwards.  Hope it works well when you
actually connect it to the 813s.

Actually, the transformer impedance ratio would be rated as 8 ohms to 500
ohms, total primary to 1/2 secondary, even if one single transformer were
used. It  doesn't really act as a 2K winding, since only half the winding is
delivering power at any one time, unlike the  case of a class-A amplifier
where the entire winding works over the full audio cycle.  It took me a long
time to figure that one out, years ago. 


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