[R-390] 6082's and regulators

[David Wise]

Question: How stable is the R-390A?

Test setup: R-390A with ultra-regulated VFO
filament, powered by external regulated B+
supply via the main B+ fuseholder, with SE-3
external BFO.  Receiver driven by HP 608D at 20MHz.
(Too much work to extricate the 8640B from
the bench it's on.)

608D frequency set for a few hundred Hz beat note
on its calibrator heterodyne output.  SE-3 BFO set
for almost the same note; actually a Hz or two off.
SE-3 BFO set to the same "side" as the 608D
calibrator, so generator drift affected both
notes equally.  Listening to 608D on one side
of stereo headphones, SE-3 on the other.
With one note in each ear, they beat together
in my brain.  I counted beats against a clock
second hand while cranking the signal up and
down, then the B+.

I could have run the two heterodynes into
a scope in "add" mode and timed the envelope
peaks and valleys against the graticule, but
I didn't think of it until just now.

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+.

I did not measure the oscillators independently;
this is a system result based on the combined
effects of the 2nd and 3rd oscillators.  They
might both be drifting; if so, they drift almost
exactly the same amount.

I did not check the first oscillator or the BFO.
The BFO is undoubtedly more sensitive than any of
the conversion oscillators, since its screen voltage
changes.

No R-390As were harmed in the course of this experiment.

73,
Dave Wise

> From: David Wise=20
> > From: Bob Camp [mailto:ham@cq.nu]
> > Can you even measure a change in the supply voltage as you=20
> > tune a signal and
> > if so how much?
>=20
> I'm sure you can.  As signal strength varies, so does the
> AGC and consequently the RF amp, mixer, and IF amp cathode
> currents, which make up a respectable fraction of the
> unregulated B+ load.
>=20
> I'll report back with hard data.
>=20
> 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.