[ARC5] [Milsurplus] The Search for a Retubed, Stable BC-375

[Dennis Monticelli]

Dennis,

You should be able to drive constant current by installing a cathode
resistor and putting either the BE of a HV bipolar transistor of the GS of
a HV MOSFET across it.  The collector (or drain) would then pull down the
screen dropping resistor to regulate the current via neg feedback.  For
triodes it's harder to do because more current and voltage capability (and
thus power handling) is needed by the transistor.  A second transistor (or
tube) could be used to drive the plate off the signal from the cathode
driven transistor.

The feedback loop would not be high gain nor would it have excess phase.
Hence it should be both fast and stable.

I've been meaning to try this idea myself, but never go around to it.

Dennis AE6C

On Fri, Nov 21, 2014 at 10:27 AM, Dennis DuVall <duvallddennis at gmail.com>
wrote:

> Been running a stock 375 for over 20 years here.  In my experience the
> frequency drift is negligible on 160M, tolerable on 75 but a challenge on
> 40.  Most of the frequency drift seems to take place immediately following
> key-down while that huge oscillator plate heats up thus changing the
> capacity seen by the oscillator tank coil.  On 40m the frequency shift is
> about 3kc downward stabilizing after about 45 sec.
>
> One Idea I came up with but never  implemented was to somehow provide for
> a constant current flow through the oscillator tube, key up (not
> oscillating) or key down (oscillating) thus holding the physical
> size/shape of the plate of the oscillator tube constant.  Never came up
> with a quick and dirty way to do this but might be worth a second look.
>
> Dennis D.  W7QHO
> Glendale, CA
>
> ***************
>
> > On Nov 21, 2014, at 6:47 AM, Mike Everette via ARC5 <
> arc5 at mailman.qth.net> wrote:
> >
> > There was an article in one of the Surplus Radio Perversion Manuals
> about using a Command Set to drive a BC-375.  I also seem to remember a
> similar article from QST during the late 40s.  Someone on line has a BC-375
> bibliography posted.  Might be Rich Post from Athens, OH (owner of the
> Boatanchor Pix site).
> > 73
> > Mike
> > WA4DLF
> >
> >      From: Robert Eleazer <releazer at earthlink.net>
> > To: arc5 at mailman.qth.net
> > Sent: Thursday, November 20, 2014 8:03 PM
> > Subject: [ARC5] [Milsurplus] The Search for a Retubed, Stable BC-375
> >
> > Dave, why would you not use a pre-hacked SCR-274-N transmitter VFO in
> your BC-375?  We know that the oscillator will work with an FET with little
> modification.
> >
> > Wayne
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