[R-390] 6082's and regulators

[David Wise]

The short answer (to which I alluded in the
final quote paragraph below) is that when
a tube is operating, its interelectrode
capacitances depend partly on the density
of the electron stream.  It gets really
complicated for pentagrid converters:
in some cases the capacitance is negative.
Some radios (the Zenith Transoceanic is an
example) deliberately introduce a small
capacitive coupling between oscillator and
mixer electrodes to neutralize it.

What continues to amaze me is how stable
Collins managed to get it.  Some serious
design horsepower there.

73,
Dave Wise

> From: Bob Camp [mailto:ham@cq.nu]
>=20
> Here's what I *think* is going on.
>=20
>     1) signal changes, drives AGC
>     2) AGC goes to the mixer tubes
>     3) Gain of the mixer tube changes
>     4) Input impedance of the mixer tube goes up as the gain drops
>     5) Oscillator load pulls
>=20
> If that's the case then the next question would be which=20
> oscillator pulls
> the worst. I would bet on the PTO being the one that moves=20
>=20
> ----- Original Message -----
> From: "David Wise" <David_Wise@Phoenix.com>
> >
> > Signal from 3.5uV to 350mV (100dB).
> > Result: About 5Hz.
> >
> > B+ from 210V to 220V.
> > Result: About 1Hz.
> >
> > So the tuning is affected more by AGC than B+.
> >
> > >
> > > This isn't the only source of frequency variance.
> > > AGC on the mixers causes small changes in their
> > > dynamic interelectrode capacitances, which reflect
> > > back to their respective oscillators.  I didn't
> > > say these are large effects, just that they exist.
> > > There's a fascinating section on this phenomenon
> > > in the Radiotron Designers Handbook.