On Fri, 15 Apr 2005 19:07:19 GMT, "Henry Kolesnik"
wrote:

I just noticed in this ad: http://hamstation.com/ybstkftmk5.htm that
Yaesu's FT-1000 finals are MOSETS in push-pull. I don't ever recall
seeing a pair of final tubes in an xcvr or linear in push-pull except
perhaps way back in a very old QST. Can someone explain why push-pull is
used in the Yaesu and why we don't see it in "modern" tube amps?
tnx

Cuz a single 3CX3000A is all you need?

Seriously, tubes are (relatively) high impedance devices and except
for distributed amplifiers, are usually tuned. The high impedance
step down to 50 Ohm requires large transformation ratios that are more
easily implemented with tuned circuits. Push-pull tuned circuits are
a pain in the ass to implement, especially when you want band
switching.

Bipolar and MOSFET amplifiers are by comparison low impedance devices
and are usually untuned. Instead of tuned circuits, broadband
transformers are used for coupling, impedance transformation and in
this case, power combining. The push-pull circuit requires lower peak
supply current and has inherent suppression of even-order harmonics.