[R-390] R-390a AGC Issue/Observation

[David Wise]

On my '54 Motorola, gain is exactly the
same between AGC and MGC with no signal.

Are R547-R549 okay?  What's the
no-signal plate voltage, V506 pin 1?
I see about 25V.

If you want to ship me that 5814A, I'll
be happy to report back with measurements.

It's normal for control grids to want
to go negative.  The effect is called "contact
potential".  The grid is swimming in a cloud
of electrons surrounding the cathode, a few
hit the wires.  In AA5's this is how the 12AV6
gets its bias, contact current through that
10 meg resistor.  But there should not be
so much current that it overcomes the
R544 delay.

I had the opposite problem; my AGC went
positive with no signal.  This is bad
for the tubes, runs them too hot.  When
I installed V506, the positive voltage
was reduced but not to 0.

The suppressor grids on the AGC line are an
attempt to clamp it to ground without adding
a diode or another 12AU7 wired as a diode,
but they aren't spec'ed for this and individual
tubes vary a lot.  I added a silicon diode -
problem solved.  (Won't help you.)

I don't see how that 12AX7 you put in can
have its grid at +1.2V, it should draw
grid current at less than that.  Unless
the cathode is high.  Where's pin 3?
Should be 50-60mV with no signal.

Dave Wise

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Tony Casorso
Sent: Sunday, April 12, 2009 3:39 PM
To: R-390 at mailman.qth.net
Subject: [R-390] R-390a AGC Issue/Observation

I have been annoyed for quite a while with the fact that I get a big drop in 
gain when I switch from MGC to AGC, even when tuned to just background 
noise.I checked the AGC bus voltage and found that it was about -.9 volts in 
AGC and 0 volts in MGC. I wasn't sure if -.9 was reasonable for the AGC with 
no signal but I suspected that it was not becasue of the big drop in gain 
and the fact that this radio is supposed to have delayed AGC.

I first checked the resistors on the IF deck (R544, R545, and R546 if I