[BC] Tubes, we got tubes

[RichardBJohnson at comcast.net]

Tube trivia:

A question I remember FCC First Class test, element IV:

What is the result if there is excessive RF voltage swing in the plate circuit of a tetrode RF amplifier?

(1) The tube arcs.
(2) Excessive screen current flows.
(3) The plate circuit has too high a Q.
(4) Grid current goes negative.
(5) None of above.

Now, I wanted to have an answer that said "All of above," but it wasn't there. However, the answer was embedded in
the very next question.

What limits the RF voltage swing in the plate circuit of a tetrode RF amplifier?

(1) Grid emission.
(2) Drive power.
(3) Screen voltage.
(4) ....
(5) ....

This turned out to be very important stuff because many years later, I actually designed tetrode RF and AF amplifiers.
The plate voltage swing in the downward direction is limited by the screen voltage. No matter how hard you try, the
plate voltage will never go below the screen voltage because the screen will be taking all the available emission current.

Many broadcast transmitter manufacturers used modulation transformers that were initially designed for triodes! This meant that the full plate-voltage swing was available when calculating the primary impedance. Not so with tetrodes. This is why many such rigs would not modulate much over 100% unless you added a separate supply for the modulators. The designs I produced would modulate 125% without any such tricks because I remembered that old test question in the FCC 1st Class test and designed accordingly.

Cheers,
Richard B. Johnson