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alexmoose 26th March 2013 01:16 PM

Pentode-like Triodes

There are a few Triodes here and there (811a for example) who's curves look like Pentode curves. Why is this?


DF96 26th March 2013 01:53 PM

It has very high amplification factor (160) so anode current depends heavily on grid voltage and only weakly on anode voltage. Notice too that under normal conditions the grid is positive, so you could say that it is acting more like a screen grid. It is a 'zero bias Class B' triode, so you use it in much the same way as you would with g2 drive to a normal pentode.

alexmoose 26th March 2013 02:54 PM

Ah! makes sense. So can we find a Pentode with a very low Mu? would it have Triode-like curves?

DF96 26th March 2013 04:50 PM

Mu doesn't really mean very much for pentodes; you have instead to think about inner mu and outer mu. To get triode-like curves you would need a rather loose g2 pitch, which means it isn't really working as a pentode.

pmillett 26th March 2013 07:29 PM

307A comes to mind. Very low Rp for a pentode. I''m sure there are others...

Ketje 26th March 2013 09:19 PM

A penthode in UL-configuration also acts more or less like a triode,depends on where the tap is placed.At the + it's a penthode,on top it's a triode (anode connected to g2)and ererything in between :-)

alexmoose 27th March 2013 02:15 PM

Thanks for the great replies. I get excited about theoretical stuff like this.

Wavebourn 27th March 2013 09:53 PM

You may use almost any pentode in such mode connecting all grids together.
It is called "Right Hand Triode"

tcqanh 28th March 2013 02:45 AM


Originally Posted by Wavebourn (
You may use almost any pentode in such mode connecting all grids together.
It is called "Right Hand Triode"

C3g and 4P1L are the example where G2,G3 connect to anode. Souns is great :)

I wonder G3 is safe when it straps to anode.

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