[ARC5] Smart People: Triode Connecting Pentodes

[Fuqua, Bill L]

   An very interesting connection used with the "forbidden" tubes for the hi-mu connection is
to connect the control grid to the cathode and apply signal to the screen grid. This produces
a very odd result. A low-mu zero-bias triode. 
  You can make a nice grounded grid amplifier with 4CX250B tubes this way but it requires
more drive since the power gain is only around 5 or so. But just about all the input power
goes straight to the output and the screen dissipation is practically zero. 
  This is mentioned in one of Bill Orr's articles on super-cathode driven amplifiers. 
  http://mikea.ath.cx/eimac/eimac08.pdf
  This triode configuration with the right tube will allow you to operate class A with positive
grid bias and very little grid current. But the low screen current is due to the geometry of these
4X and 4CX tube where the screen grid elements are in the shadow of the control grid elements.  
73
Bill wa4lav

________________________________________
From: Kenneth G. Gordon [kgordon2006 at frontier.com]
Sent: Thursday, November 14, 2013 12:06 PM
To: Fuqua, Bill L
Cc: ARC-5 List
Subject: Re: [ARC5] Smart People: Triode Connecting Pentodes

On 14 Nov 2013 at 16:39, Fuqua, Bill L wrote:

>    I believe by connecting the grids together you produce a high mu
>    zero bias tube.

Yes. According to Bill Orr in a document he wrote for Eimac, connecting the
grids together results in a hi-mu triode, while connecting the screen to the
plate results in a low-mu triode.

> 813,803, 4-400, and many others are done this way for grounded grid
> applications.

Yes, but as Bill points out in that article I mentioned, SOME tubes don't work
well in that condition, while others do. He lists several of each.

> Bill Orr wrote a number of articles on this.

As I said, published in an Eimac document which is available as a PDF on
line somewhere.

>   The down side is that this technique does not work well for class A
> or near class A operation since the grids have to have a positive bias
> on them to get the needed quiescent current.

Makes sense

> And the grid supply has to
> provide considerable current.

Again according to Orr, in tubes such as the 4X150 and 4CX250 family, even
operating in Class AB2, grid current can be as high as 3 or 4 AMPERES !!!!

>   I once used four 6AG7's in grounded
> grid as an amplifier following my CE 10A.

I remember that circuit. The CE-20A uses two 6AG7s as linear amps. They
work well.

Ken W7EKB