On Aug 30, 7:38 pm, "Harold E. Johnson" wrote:
"COLIN LAMB" wrote in message

... Push-Pull allows you to runn in other than class A, which is about 35%
efficient. However, class A can result in very good audio.

Some high end audio amplifiers are single ended class A. High power puts
out a lot of heat.

Colin K7FM

Back in the year one, Bill Orr, W6SAI, published an article (When QST was
still a technical magazine) on a single 304TL Heising modulator. I built one
to modulate a 4-400A.

Ran class A when you were talking to it, ran class C when you weren't.
Shifted the bias at a syllabic rate by rectifying a bit of the preamp audio
and applying it as forward bias when talking. Worked very well, quite
efficient, and the origin of EBC. The Electronic Bias Control bias shift
seen in 1970 and 80 Alpha RF Amplifiers to hold down plate dissipation for
class AB and B operation.

W4ZCB



Which Year One was that, Harold? Where I became a ham many years
back, there was a creative fellow in our club who designed a compact
AM transmitter using a couple 6146s, one for the RF PA and one for the
audio output. They were coupled not by a choke as in Heising, but by
a common audio output transformer, available back then much more
readily than a modulation transformer, and having the advantage that
the DC in the center-tapped winding was (nearly) balanced, avoiding
saturation effects in the transformer and allowing the use of a
relatively small transformer. I think Mitch designed that thing in
the mid 50's.

Cheers,
Tom