[Amps] Tube shunt regulators

[Will Matney]

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