[Collins] 32S-3B

[Dr. Gerald N. Johnson, electrical engineer]

ALL resistors other than precision resistor change value when heated.
Some varieties more than others.

The two 68K resistors act as part of a voltage divider from the +275
volt supply along with R88, a 1500 ohm resistor, to set the cathode bias
for the relay driver tube. That 1500 ohm resistor is shunted by a 470
ohm resistor to manually key the stage. The tube acts as a simple
comparitor to key the relay when the grid is driven positive or the
cathode pulled towards ground. The two 68K resistors dissipate a total
of 2.15 watts at the rated 275 volts low voltage supply. A higher low
voltage supply will increase their power dissipation and also raise the
cathode voltage of the relay tube. Higher voltage should make it take
more dc voltage on the grid to pull in the relay.

A tube with grid emission will tend to drift the grid positive,
especially with the 10 megohm grid resistor. A tube with heater to
cathode leakage will lower the cathode bias. Either will lead to the
relay pulling in. Low voltage from the low voltage supply will reduce
the cathode bias and make the tube more sensitive. My 32S! schematic
says the cathode bias should be 4.5 volts. If its significantly less
than that, they key line is closed or some other resistors have shifted
in value.

With not audio input, the grid voltage on the relay tube must be zero.
And positive voltage will come from grid emission, rectification of hum
in the audio stages (but that can be removed by turning down the VOX
gain) or leakage in C112.

The 6U8A is not a good DC tube, its an RF tube. I don't know of a good
DC rated substitute.

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.