Highland Ham 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?

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Tube amps with 2 output valves can be used in push- pull but then for 1 band
only unless you would switch multiple inductors.
A single valve or multiple valve in parallel can readily output into a
multi-band PI filter arrangement.
In QST -March 1967 , pages 11 - 15 there is an article on a 1kW amplifier
for 50 MHz with 2 pcs 4-125A or 4-250A or 4-400A push pull . This is a
single band amplifier with inductive coupling on in- and output.
Unless you would perform band switching by changing the inductors through
plug-in , switching for various bands would be very complicated.

Frank GM0CSZ / KN6WH.



There were dual band tank circuits for tube amps that worked on
both 6m and 2m without changing coils. The circuit was made in
both single ended and push pull versions. The push pull circuit
had two separate pick up link coils and basicly was series tuned
on 2 meters and parallel tuned on 6. The 6 meter tank coils were used
as rf chokes on 2 meters. The single ended version used a similar
trick, but was link coupled on 2 meters, and could either be link
coupled or a Pi network on 6 meters.
Source: old ARRL handbooks and VHF manuals from the 50's and 60's.