[Collins] Collins 75S-1 V8 6BF5

[Dr. Gerald N. Johnson, electrical engineer]

Many an S-line has darkened paint on the inside of the lid over the
6BF5. It does tend to run fairly hot. But there are a few things in its
circuit that can make it run hotter.

Its run with fixed bias and a bit of cathode resistance. The fixed bias
comes from the receiver bias supply (used for RF gain control also). I
think its possible the S-1 used a little Sarkes-Tarzian enclosed
selenium rectifier rated at 20 milliamps. I know early 516F-2 power
supplies used that selenium. It is a little black rectangular package
with a mounting stud and a couple terminals and a red and white paper
label the best I recall. It will have developed a high forward
resistance over the half century and will cause the bias voltage to be
low. The best replacement is a two terminal (both insulated from ground)
terminal strip and a 1N4007. A leaky bias filter capacitor will also
cause lower bias voltage. With low bias voltage, the 6BF5 will draw
excess plate current.

Any or all of the carbon composition resistors in the circuit can have
changed value including those in the bias supply to upset the proper
voltages on the 6BF5.

Its also possible that the S-1 used a black beauty molded oil filled
paper capacitor for the coupling capacitor from first audio to 6BF5
grid. Its sure to be leaky, if its a black beauty. Later receivers used
disk ceramics there and if the bias supply is OK but the grid voltage
isn't as negative as the voltage charge calls for, that coupling
capacitor is leaking and causing the 6BF5 to draw excess plate current.

The excess plate current tends to char the paint above the 6BF5, to
shorten the life of the 6BF5, and likely that of the audio output and
power transformers.

I have seen a 6AQ5 in the 6BF5 socket, but the AQ5 won't have the proper
load and isn't capable of the audio output power of the 6BF5, both
characteristics lead to more audio distortion. Its probable that the
bias requirement of the AQ5 are also different.

On the CRA page, there is an article about a solid state replacement for
the 6BF5, I understand it runs much cooler but may add a little
distortion to the audio. I have a copy but I've not yet tested it.

73, Jerry, K0CQ, Technical Advisor to the CRA
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.