For historical purposes, be it still a usable circuit, I thought I would
write a little in detail about using a tube type shunt regulator. In
most of the RCA receiving tube manuals, under electron tube
applications, a good explanation is given. Back in the days of yore,
tube color tv sets needed a good regulated supply to serve the
horizontal output circuit. A typical tube used was the 6BK4 triode.
There are two versions of this tube with different anode dissipation
ratings. The 6BK4A had a 30 watt dissipation and the 6BK4B a 40 watt. It
was connected the same way a transistor or FET is used to regulate
through a dropping resistor (Rs). The difference being is the cathode
was held to a positive potential with respect to ground. The grid was
fed by a resistor divider from the anode to ground. This same divider
can have a potentiometer added for a variance in the output voltage.
When the output voltage (Eo) rises as a result of a decrease of load
current, a small fraction of the additional voltage is applied to the
grid of the tube by the divider network consisting of two resistors. The
center tap connection of the divider resistor going to the control grid.
The increased voltage causes the tube to draw increased current, and in
turn, causes a voltage drop across the high internal impedance of the
unregulated supply dropping resistor (Rs), which tends to counteract the
original rise of voltage. The grid circuit could also be ran from a tap
on the boost supply to eliminate the high ohm resistor divider network.
This had a function of making the total load fairly constant.Other tubes
can be used too with success being ran as a triode such as tetrodes and
pentodes.
While reading up on this, I took a look at the audio section about IMD.
It really was just the same as what we had already discussed here. Now,
I'm not saying we should use tube regulators now but they could be used
if someone would want to. The only reason to think about them is that
tubes can sustain more abuse than some semiconductors. The regulator
circuit described above was a simplified circuit. Other regulators back
then used regulator tubes or zener diodes with the tube as in the
"hybrid" sets. One good way to see how some of this old technology was
done is to find some old Sams Photofacts or Riders manuals on TV and
radio servicing. You can find it also in older service manuals for older
amateur gear. The main reason for mentioning this is servicing older
equipment and keeping them original. Sometimes the collector price can
be effected by modifying the equipment.
Will Matney
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