[Milsurplus] BC-348's

[Bob Camp]

Hi

Well to say the least it's been a while since they taught tube design. 
The last crew I knew of went through the process in the USA did it  in 
the 1980's. They got a full boat subsidy  from the DOD and AEC guys.

Simply put all tubes obey a weird little item called the 3/2 power law. 
The plate current is a constant times a formula based on the grid 
voltage taken to the 3/2 power.

For a diode it looks like:

I_plate = K * V_plate ^ (3/2)

Needless to say that's only part of the story for a diode.

For a triode it looks like

i_plate = K ( V_grid + V_plate / u ) ^ (3/2)

I have always thought that putting the mu of the tube in that equation 
was the wrong location ....

for a tetrode is looks like:

for a pentode it gets really ugly:

i_plate = Ka * ( V_grid + V_screen / m) ^ (3/2) * arctan( V_plate / Kb)

The arctan term is the one that mostly gives you the "remoteness" of 
the cutoff.

There are a *lot* of different forms of these equations and an enormous 
amount of debate about subtle issues with each implementation of the 
formulas. Most of the circuit design was done using graphical methods 
rather than using the formulas.

On a triode the closer you space the grid to the cathode the higher the 
transductance of the tube. The closer you space the grid wires together 
the higher the transcductance. The more of the plate to cathode area 
occupied by grid wires the lower the transductance (since the plate 
current all goes to the grid ...).  Simply put real small grid wires 
very carefully spaced apart are a good thing in a high gain triode.

In a pentode the screen pretty much acts like the plate. as far as the 
control grid is concerned. Keeping the screen close to the control grid 
is a good thing. Again you have a mechanical issue packing all that 
wire into a small space.

By changing the spacing of the grids and to some extent the geometry of 
the plate (rectangular or circular) you can change the coefficients in 
the magic equations. More or less rectangular with tight spacing has a 
sharp cutoff characteristic if I remember correctly.

The full explanation of all this is about a 30 credit hour program at 
the graduate level at your local university. Needless to say that's one 
I seem to have missed out on. Since we are unlikely to design any 
pentodes from scratch any more it's not really clear how useful the 
information would be anyway ...

	Take Care!

		Bob Camp
		KB8TQ




On Aug 6, 2004, at 10:27 PM, J. Forster wrote:

> Can anyone explain how the grid(s) is/are physically structured in 
> various tubes
> to make them high or low transconductace, remote cutoff, etc. I missed 
> tube
> design by a year or two and just learned about them as voltage 
> controlled
> current sources.
>
> Thanks,
> -John
>
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