terry wrote:
On Aug 31, 2:17 pm, K7ITM wrote:
On Aug 30, 7:38 pm, "Harold E. Johnson" wrote:





"COLIN LAMB" wrote in message
... Push-Pull allows you to run 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- Hide quoted text -

- Show quoted text -

I understand that the answer to the basic question is 'no'; not
necessarily push pull for an audio modulator. Just a big hefty audio
amp and some means of plate modulating the RF PA! There are other
methods of AM modulation etc.

BTW the 6146 design above sounds ingenious; have a mental picture of
the total DC plate currents divided in opposite directions one for the
RF tube and the other for the modulator tube. Then I suppose the
modulator tube as it received audio, varied the plate voltage/current
of the RF tube thus modulating it. A sort of variation of Heising
(choke) modulation? Interesting.

While modulation transformers are getting a bit rare and expensive these
days there are ways to get around the problem. Years ago the ARRL
handbook used a filament transformer as a modulation transformer. The
center tapped secondary was used as the primary with high power
germaninum transistors (this was a mobile rig). The 115v primary was
the modulation secondary.

One could also use two tube type output transformers back to back.