Another KCS47 Restore...No High Voltage/B+ Down 25V

[Electronic M]

Tubes don't just amplify AC and aren't just on and off. The plate current, with no signal, in anything with a control grid (other than a thyratron) is proportional to the DC grid bias. The exact bias voltage to achieve these following these 3 states varies with the tube type number and wear, but here goes. A tube with strong negative grid voltage (for instance -50V) is in cutoff and no plate current flows, a tube with almost no negative (or even positive grid voltage is in Saturation (maximum plate current it can physically pass... sometimes in excess of ratings is flowing.. often hundreds of mA), and if a tube is biased somewhere between the two it's probably in the linear region where a fraction of a volt grid change results in a mA change in plate current.... often the linear region is somewhat non-linear as it approaches the other regions.
In a single ended class A audio amp DC bias is critical to getting maximum volume...You want bias in the exact center of the linear region or the grid swing will clip either the tops or bottoms off the waveform before maximum achievable amplitude is reached.... Also audio output transformer DC saturation at high current can force reduction in plate current bias because the transformer hits saturation before the tube. In a push pull audio amp you want to avoid class A operation as you waste power and don't get maximum amplitude. In push pull the DC transformer saturation from each tube cancels the other and each tube can handle half the signal, but can be in cutoff for the other half without issue... However a little bit of zero signal plate current is necessary to avoid clipping from a messy handoff between tubes at the 0 crossing of the signal....If that distortion doesn't matter Class B can be used.

Horizontal sweep systems use class C where the load has resonance in the desired frequency range (like a bell) and the tube only conducts for a small portion of the AC signal sufficient to ring bell, keep it ringing and at the right ring strength.... Horizontal output systems Instead of using straight forward DC bias circuits that resistively divide the DC supply get their DC bias power from the signal. They use the grid leak detector principle where grid-cathode rectification of the AC signal creates a DC bias that regulates tube current.

Triodes, terrodes and penthouse act the same as signal amplifiers with regard to DC and AC control grid behavior in single input amplifying or switching applications...The only thing the extra grids do is change the slope of the linear region relationship between grid voltage and plate current...They increase gain which is useful enough to merit the additional cost and complexity. With extra grids you can achieve 2 input amplification or switching which can be seen in color TV color demodulation and some gated sync separator/AGC circuits respectively... Superheterodyne converter and or mixer stages also use this... You shouldn't see this in a horizontal output circuit unless it's the infamous single tube horizontal Muntz circuit in that one model they made...