[R-390] Super simple R392 audio mod

[Charles Steinmetz]

As discussed previously, the R392 runs its 26A7 output amplifier tube 
(V608A/B) with only about -0.25v of contact bias developed across the 
470k grid resistors and drives the 26A7 grids through coupling 
capacitors from split-load phase inverter V607. Thus, when there is 
audio the 26A7 grid current charges the coupling capacitors on 
positive signal peaks, thereby rectifying the audio signal and 
causing massive distortion.

I proposed a super simple mod to correct this gross distortion; 
however, since I do not own an R392, I needed volunteers to test it 
and verify that it works.  The preliminary results are in, and it 
does indeed work.  I'm still waiting for detailed reports from some 
of the beta testers, and will publish a fully documented modification 
after those reports are in.  However, we appear to know enough to let 
everyone in on the fun.

Parts required:  One 3.9v, 500mW  Zener diode  (1N5228B, 1N748A, 
BZX55C3V9, etc.)

Procedure:  V608 Pin 2 (the dual cathode of the 26A7) is grounded by 
a wire that runs across the tube socket to Pin 7 (grounded end of the 
heater), which is connected to a ground lug on the tube base.  Remove 
the wire between Pin 2 and Pin 7 (leave Pin 7 connected to the ground 
lug), and replace it with the 3.9v Zener diode.  The cathode (banded 
end) of the Zener should go to Pin 2, and the anode to Pin 7.

Test:  Turn the radio on, and check that Pin 2 measures ~ +3.9v DC 
with respect to ground (the exact voltage will depend on the 
tolerance of the particular Zener diode you use).  If this checks 
out, you are done.  [If you measure ~ +0.5v to 1v, you probably got 
the diode in backwards.  Be aware that every now and then, 
manufacturers mark the wrong end of a diode.]

Possible issues:  I do not know if all R392s are built exactly 
alike.  It is possible (but I think unlikely) that some R392s use Pin 
2 as a ground tie point for other circuitry.  If V608 Pin 2 on your 
R392 has anything connected to it besides the wire to Pin 7, the 
other wire(s) or component(s) will need to be detached from Pin 2 and 
connected to ground independently (that is, they should NOT be 
connected to the Zener diode -- only Pin 2 should be connected to the diode).

R629:  Some R392s were built without R629, or have had R629 removed, 
for operation with the official solid state output module.  This 8.2M 
resistor provides overall NFB around the audio amp 
(V606/607/608).  If your R392 does not have R629, you should install 
it for use with a 26A7 at V608A/B.

NOTE:  This mod is intended to solve the problem of horribly 
distorted audio in the R392.  It does NOT extend the capabilities of 
the R392 audio stage in any other way -- it still puts out only a 
couple of hundred milliwatts into a 600 ohm load.  Like the original, 
it will not drive a low-impedance speaker or phones directly 
(anything less than ~300 ohms).  Note also that V607 produces some 
nonlinearity before V608 clips, due to the very low B+ (28v).  The 
super simple mod described above fixes the horrible distortion 
produced in the power output stage of the stock radio, but does not 
address the much lower distortion generated by V607.

In short, the mod makes the R392 work like it should have worked from 
the factory, given the parts that were used.  As a side benefit, it 
reduces the plate dissipation of the 26A7 by about 70%, so 26A7s will 
last much, much longer than they do in a stock R392.

My thanks to the beta testers for all of their help!

Best regards,

Charles