On Sun, 12 May 2019 11:04:06 -0000 (UTC), Cursitor Doom
wrote:

Gentlemen,

I've got this RF sig gen I'm testing at the moment. It was made in 1955
and is all valve (I say "all valve" but there are only two tubes in it
and only one of those generates the RF - the other's 400Hz for the
modulation).
Anyway, it has 8 ranges in total covering 100KHz to 240Mhz. The
oscillator tube is a double triode, a 12AT7. One half handles range from
100Khz to 30Mhz and the other takes care of 30 - 240Mhz. Now, it all
works great EXCEPT for one range (the 3rd lowest) which has appreciable
distortion present on the output. It looks a bit like it's being over-
driven on my scope. My question is, does a valve like a 12AT7 require
different DC biasing points for every range of frequencies? Obviously the
range switch is switching in different combinations of coils and
capacitors, but is it likely to be also switching in different cathode-
grid DC biasing at the same time?

I have a Radiosonde weather balloon transmitter which has a
single valve. The valve is a 3A5 twin triode with one being the
RF oscillator at 72 MHz and the other an audio oscillator which
was controlled by the sensors.

I did have a one valve test oscillator which was powered by
a nine volt grid bias battery.



TIA