[Milsurplus] Low B+ receiver operation

[Mike Morrow]

Hue wrote:

>Something that occurred to me recently, if you lower the B+ to
>a fraction of the original B+, say as Dave did from 250 down to 65, will
the
>receiver exhibit a tighter IF bandpass, due to higher tube plate resistance
>in parallel with it? Right now i am thinking "yes, it will", but i am also
thinking
>it probaby would not be a radical enuff improvement of great benefit to
>operation. Maybe if you got down to a really low voltage, like 12 volts on
the
>plate. On the other hand, there probably is the limiting factor of the Q of
the
>coils themselves.

I have no answer to the questions you asked, but I'll mention one other
result of low B+ operation.

That's the vacuum tube's space charge effect on control grid bias under low
plate voltage conditions.  With a LV plate supply, electrons emitted by the
cathode that would otherwise be drawn to a high voltage plate tend instead
to accumulate around the control grid and drive its potential negative.
Some low voltage space charge tubes are said to be self-biasing with the
help of this effect.  But in reality, I'm not sure that any noticable bias
change effects would result from operating a tube in a circuit designed for
250 vdc at a tenth of that voltage.

Most of all those BC-1206 and BC-1333 type LF beacon and 75 mc marker beacon
receivers, and the receiver section of the Collins 18S HF RT, and the
R-392/URR, and others, used 28 vdc B+, on conventional HV tubes, except for
the occasional 28D7.  They must have worked well in that application.

73,
Mike / KK5F