[ARC5] Smart People: Triode Connecting Pentodes

[Kenneth G. Gordon]

On 14 Nov 2013 at 5:23, Fuqua, Bill L wrote:

> kinetic energy and will not produce much heat. However, the screen
> should not be operated above the specified voltages and currents.

Yup.

> So
> that limits the plate voltage you may apply. You will still have about
> the same number of electrons striking the screen and if you increase
> the voltage of the screen-plate you may exceed the screen's
> dissipation.

Again, yup.

> So, the triode connected tube should be derated  to
> perhaps to 1/4 or 1/3  of the of the plate voltage that you may
> otherwise apply if it was a tetrode.

At least, that is the way they do it with audio amps.

> I never much cared for this
> particular way of triode wiring a tetrode

Me either!!!!!

> but it has been done for
> quite some time with success, mostly for audio applications.

And at "lower" voltages. In fact, all I have ever seen have been with 
audio applications...

> It is the
> simplest way to get a tetrode or pentode to behave as a non-zerobias
> triode. You can make a tetrode or pentode behave as a triode if you
> have some means to assure that the screen voltage changes proportional
> to the plate voltage. But this is not always simple. And at RF
> frequencies it could be very complicated if not impossible. 

Well, as far as I know, it has been done, but it supposedly ain't 
easy.

What I suggested was that David connect a bypassed 20K resistor 
between the screen and the plate. The resistor would keep the screen 
voltage down to something at least reasonable, and the paralleled 
bypass cap would keep the screen at the same RF potential as the 
plate. The resistor would also allow the screen to follow the 
flucuations in plate voltage, although the relationship between the 
voltages would probably not remain linear.

Although I have never tried this, it seems to me that this would, at 
least, keep the screen from over-dissipation. As he is now connecting 
his tubes, I am certain the screens are "taking a long hike."

I still think that connecting the screen to the GRID (G1) might be 
very useful, at least as an oscillator.

Ken W7EKB