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December 11th 03, 07:55 PM

In article ,
(Bruce Kizerian) writes:

I have seen remote cutoff pentodes such as the 1T4 used in
regenerative receivers. I'm guessing there is a good reason...perhaps
something to do with the decrease in gain with increasing signal
level...so that the pentode oscillator "fights" going into to
oscillation enough to give a very smooth transistion...rather than the
more abrupt transistion of some solid state circuits.

A 1T4 is an old directly-heated cathode tube. Played with those
a 56 years ago...in regenerative receivers...:-)

Looking back at those old magazine article projects, I'd surmise
that the "reason" a particular tube was used was that it was
AVAILABLE to use.

A regen operates at the hairy transition of self-oscillation where
positive feedback yields a tremendous increase in gain. Doesn't
matter whether it is vacuum tube or transistor. The only difference
between remote-cutoff and sharp cutoff characteristics (transistors
of the bipolar junction type are very sharp cutoff equivalents to
tubes) would be on the amount of spurious garbage created when
the regen jumps into full oscillation.

I know of no solid state equivalent (without some sort of AGC feedback
loop) for the remote cutoff pentode. Does anyone know if there is?

"AGC in a regen?" If you bias the control grid of a 1T4 about all you
will do is change the feedback setting for that hairy transition just
prior to going into self-oscillation.

And speaking of the 1T4...has anyone played with these enough to give
me some ideas for using them in regenerative radios.

The last time I played with battery-filament tubes (including the 1T4)
was in helping a friend restore an AN/PRC-8 Korean War era walkie-
talkie in the 70s (I wore one once in 1953). Nice VHF FM transceiver
very stable. The first time I played with a 1T4 was in a regenerative
receiver back in 1947 and it was very touchy on feedback setting
versus frequency on MF to HF bands (plug-in coil thing).

I also wondering
how much you can starve the plate and still get good results in a
regen receiver.

Experiment with it. There's not a large amount of data on regenerative
receivers any more and very little in the way of vacuum tube info.
A 1T4 power demand is minimal, 1.5 VDC filament at 50 mA and about
2.4 mA plate and screen current at 45 VDC supply. [183 mW total
supply drain] That's fairly "starved" considering the transconductance
is only 700 umhos (typ) at 0 VDC control grid. Even at "maximums"
of 90 V plate and 67 V on screen, the transconductance is only 900
umho.

A 1L4 is a close cousin to the 1T4 as a sharp-cutoff pentode, same
envelope, filament, etc., but the gm is about ten percent higher. A
3Q4 or 3S4 (a 1S4 was made but was not recommended for new
designs back in 1956) has a gm of 1500 umho at twice the filament
voltage and 8.8 mA plate+screen at 90/67 V. Those "power pentodes"
have a semi-remote-cutoff characteristic. [800 mW power supply
demand would be "power" from a battery pack way back then...]

Regenerative receivers were all the rage among hobbyists back
about 1930 when tubes were relatively expensive and lacked
transconductance for effective RF amplification. Such were a
cheap way to get more sensitivity out of a single tube stage...but at
the expensive of instability and lack of uniform selectivity. Once the
superheterodyne was into production, it proved far better overall.

For experimentation purposes, a high gain-bandwidth product op-amp
IC might produce some interesting results. The gain-bandwidth (or
0 db open-loop gain frequency) of some op-amp ICs is up at 30 to
70 MHz now and the DC open-loop gain is enormous in comparison
to vacuum tubes. I sense possibilities of an op-am regen or even a
superregen on up through HF. Just a thought... :-)

Len Anderson
retired (from regular hours) electronic engineer person