[R-390] R390 B+ Current?

[jgedde at optonline.net]

I found the problem and fixed it.  Apart from the two charred resistors in
the cathodes of the 5651's, it was a slightly leaky C608 and one other bad
resistor (R616).  That one was tough to find as it tested at 86k (versus 82k
it should be) when it was cool.  As the radio heated up, it changed in value
- a lot.  I should have changed it from the start because I didn't like the
look of it.  It was dull in appearance not shiny like the rest of the
resistors.  Weird.

I also brought my AF deck up to MOD 4.

I now have a B+ supply of 181.7V with 160 mV p-p of ripple (and a room
heater to boot.)

John

-----Original Message-----
From: Jacques Fortin <jacques.f at videotron.ca> 
Sent: Sunday, May 26, 2019 10:01 AM
To: jgedde at optonline.net; 'r390a' <r-390 at mailman.qth.net>
Subject: RE: [R-390] R390 B+ Current?

Hello John,

>From what you describe (the hum balance drifts as the whole radio is heating
up) I guess that it is the 6BH6 transconductance that changes when it is
heating up, which is more or less an indication of an end-of-life tube.
The "gain" of the 6BH6 should change as it is heating up: the hum balance
adj. role is to vary the open-loop gain of the whole regulator, balancing
the hum part of the voltage feeding the 6BH6 plate with the amplified
version of the output hum coupled by the C608 to the grid.
Maybe it is the time to find a "real" tube tester of the mutual conductance
type, at least.
Or better, a parametric tester.
Or to find someone nearby that can test the tubes for you.
It can be a leakage internal to the 6BH6 also, and tubes testers are not all
equal at detecting that type of defect.

73, Jacques, VE2JFE