72 What about AF power amplifier bias circuits?
Description
This article is from the Antique Radios And Phonographs FAQ, by Hank van Cleef vancleef@netcom with numerous contributions by others.
72 What about AF power amplifier bias circuits?
You can do a little inspecting to see what your radio uses.
a. By far, the most common circuit is to use a cathode resistor
with an electrolytic capacitor for AC bypass. This is what you will
find in all of the transformerless sets. AC bypass is less critical in
push-pull output stages, although most of them operate class AB (i.e,
both tubes biassed near cutoff). If the capacitor is shorted, the output
tubes will over-dissipate and their plates will glow red in a few
minutes. If the capacitor is open, audio output will be low and
distorted.
b. Back bias. I was somewhat surprised in checking Terman
"Radio and Electronics Engineering" 4th edition (1955) not to find this
circuit. It uses a power resistor in the B- return to develop a bias
voltage, typically 10-30 volts, and may be used in conjunction with the
cathode resistor self-bias circuit. The center tap of the power
transformer will be connected to one end of the power resistor and B-
circuits will be connected to the other end. On sets using filament
power tubes, the filament supply may be connected here, and the power
tube grids returned to the power transformer center tap. Most of the
bias voltage is developed by output tube plate current. If there is a
leaky electrolytic here, it will generally overstress this resistor and
burn it out.
c. Separate "C" bias supply. In this case, the set will have
second rectifier tube, filter, etc. These are not common in home
entertainment equipment, much more likely to be found in theater and
public address amplifiers.
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