[R-390] Carrier Level meter R-390 A

[Gary Gitzen]

Hi R-390,

David Wise gave his comments and insights
regarding a possible modification of the R-390A
carrier meter circuit to accommodate 100 ohm
replacement meters. It prompted some additional
thoughts and comments.


> Since the DC operating point, voltage gain, and
> plate resistance of V506A are changed, I'd expect
> some change in the LONG AGC action (MEDIUM too
> if you are using my AGC mod), but I believe
> you would not notice.

I pretty much agree on all points, especially that
the difference would be hard to see. V506A at this
point is acting pretty much as an AGC controlled
current source developing voltage across a small
R548. The cathode voltage, thus effective grid
voltage, will change by maybe 150-250mv if R548 is
changed from 27 to 160 ohms. That's an almost
trivial grid voltage change.

The specific action of the AGC-variable suppressor
grid voltage on V504's (6AK6) cathode current is
unknown. Can anyone provide insights? Will it result
in any significant cathode current change, thus
meter action; or is V504 merely a convenient current
source?


> getting rid of R537 eliminates the problem with
> the stock carrier pot where the whole adjustment
> range is squished into the last 5% of the pot,

Yea!!!!! And please see below for original meters.

> Because the pot is a larger percentage of the
> 6AK6's cathode bias resistor, IF gain is more
> sensitive to the carrier pot setting than before.
> This could be ameliorated by rewiring the pot
> as a pot instead of a rheostat.

Excellent suggestion! Please consider my earlier
thoughts on the subject so modified.
[Translation: Why didn't _I_ think of that?
Programmers note: it's called "stepwise refinement"
& peer review.]

> Reducing R524 will preserve the original IF gain.

Yes, and if R523 is rewired as a pot instead of
as a rheostat when R537 is removed, R524 (6AK6
cath res) definitely needs to be reduced to around
600 ohms. A 5.1K shunt across it will do this.


Carrying David's "pot thought" forward, replacing
the R523 & R537 combination with a 20 ohm pot
wired as a pot while using an original 17 ohm meter
would appear to be a Very Good Thing. [Please see
caveat #2.]
This raises the following question: were 20 ohm
AB pots with 10% tolerance available in 1950-55
for a reasonable price? This circuit appears to
beg for one, instead of the R523/R537 kludge.


> The meter's scale factor (as opposed to its
> zero point) is as sensitive to the carrier pot
> setting as ever.

This implies that the AGC-variable suppressor
grid voltage to V504 _does_ significantly affect
cathode current. Thanks for that insight.


> The main objection, in my opinion, is that your
> deck is now incompatible with the stock meter.

Absolutely agreed. Please see caveat #2.
 >> Caveat #2: If you do this, _please_ document it
 >> on both the chasis and the IF module.

> If I had exactly one radio, no spares, and no
> intention of acquiring more, I'd do it.  It's
> very much worth a try, and anyone who does it,
> please post your findings.

Thanks to David for his comments and insights.
It's obvious they come from someone who has "been
there and done that" as opposed to my theoretical
musings.

If I had even one R-390A without an original
carrier meter, and had on hand a 100 ohm replacement,
I'd give it a try. And I'd carefully document exactly
what was done, and why, on both chasis and IF module.

I second David's request for results from anyone
who makes this mod to hopefully allow use of
100 ohm carrier level meters in place of the original
17 ohm meters.

Just so it will all be in one place:
  Install 100 ohm replacement carrier level meter.
  Replace R548, 27 ohms, with 160 ohms (150 ohms if
   it's all that you have).
  Remove R537, 22 ohms.
  Replace R524 with approx 600 ohms, or shunt the
   existing 680 ohm R524 with 5.1K.
  Rewire R523 as a pot, with the meter lead coming
   off the wiper.
   If needed to get adequate adjustment range, add
   22-47 ohms in series with the ground side of R523
   and reduce R524 by the same amount. (39 ohms with
   R524=560 ohms should work nice.)
  Document what you have done, and why, on both
   the chasis and IF module.

All relevant constructive comments welcome.

Gary Gitzen
5/24/2006