RE: [TCML] Tube question

[<sparks@xxxxxxxxxxxxxx>]

> Does anyone (maybe in the over 50 crowd)

That's me, for sure! <G>

> remember about
> tube characteristic curves and how to use them to determine
> the proper operating points for VTs? It seems that 45 years
> ago this was the right way to determine the proper bias values
> for tube circuits, but I've never heard anyone on the list ever
> discuss doing a proper analysis, so maybe it's a lost art that I
> only vaguely remember?

It still works just fine.  I used the charts a few weeks ago to calculate the operating point and design the plate tank circuit for my KW-level HF amplifier using a large, gently-used TV transmitter tube.

In order to do this, you need to know the plate voltage you will use and the average plate current the tube will handle during the conduction period.  Then you can look at the curves and determine the resting (no signal) bias voltage to set the idle plate current (for linearity, if that is important) and the total grid voltage swing to drive the tube from plate current cut off to the desired plate current.

If you are driving the tube in pulse service, you will have to use the average current during the ON time of the pulse for your calculations.  Generally, unless the tube was designed for pulse service, the curves will not go that high.  Simple extrapolation of the existing data curves may not work, as the tube may saturate and "flat-top" before you reach the desired plate current.

Note that if the tube is a low-mu tube (generally triodes) you may not be able to drive the grid sufficiently far enough positive to obtain the desired plate current without damaging the control grid of the tube.  Both the control grids (G1) and the screen grid (G2) in triodes and tetrodes are much more delicate than either the cathode or the anode, and may easily be destroyed by even a brief application of excessive current.

It's best to keep the fireworks on the outside of the tube!

73,

Ralph   W5JGV - WD2XSH/7

_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla