[Amps] Grounded grid 8122's

[Gary Schafer]

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Bill, W6WRT
> Sent: Tuesday, July 21, 2009 9:38 PM
> To: Ham-amps
> Subject: Re: [Amps] Grounded grid 8122's
> 
> ORIGINAL MESSAGE:
> 
> On Tue, 21 Jul 2009 21:00:00 -0400, Herzog <herzog at frontiernet.net> wrote:
> 
> >I dont believe there is any way to run a 4CX250 style tube in GG without
> >destruction. Very few tetrodes have the necessary grid dissipation
> required.
> 
> REPLY:
> 
> GG does not require any more grid dissipation than grounded cathode. GG
> does
> require more driving power, but not for the grid. The extra driving power
> "modulates" the cathode current. Inside the tube, the grid-cathode voltage
> determines the amount of grid drive and in this context the tube doesn't
> know or
> care which element is grounded and which is driven.
> 
> I am not saying that GG and GC are identical -they definitely are not,
> mainly in
> the phase relationships between elements and the feedback present in the
> circuit. But in terms of grid dissipation, they are the same.
> 
> 73, Bill W6WRT

Well, no they are not the same. There is a difference in what we all call
"grounded grid" and "cathode driven". With a tetrode in grounded grid mode
you are driving the grid and the screen. In order to develop enough screen
voltage to make the thing work the cathode needs to be driven hard enough to
develop the screen voltage and that puts high energy between the grid and
cathode which makes the grid draw current in most tubes.

With a so called cathode driven tetrode, placing screen and grid voltage on
the tube, it can be driven in AB1 just like it can if you are driving the
grid directly. Thus grid dissipation can be held to zero as it is done in
the Collins 30S1 amplifier.
If the screen and grid voltage were removed in that amp and the cathode
still driven it would cook the grid in a millisecond. 

73
Gary  K4FMX