[AMPS] grounded grid configuration, grid 1 location

[Ian White, G3SEK]

Skipp wrote:
>
>re: g-grid circuit options... #1 grid potential 
>-
>Hello there, 
>-
>I'm working on tetrode and pentode grounded grid circuit 
>design for an amplifier rebuild project. 
>-
>There are grounded grid circuits where the #1 control grid 
>is tied to the cathode, others where it's removed from 
>ground and cathode potential and operated with a dc bias, 
>while the other grids remain grounded.  These circuit layouts 
>are related to the type of tube and the ability of the #1 grid to 
>deal with excessive dissipation at some drive levels.  
>-
>The 4CX250 is one such tube in which text's mention g-grid 
>operation with the #1 grid at ground is not the best choice. 
>Excessive grid dissipation at high drive levels might damage 
>the tube.  Other tube types tend to be better suited to all 
>grids grounded g-grid operation. 
>-
>I'd like to know if anyone can provide reference material - 
>text name sources where I can obtain more detailed 
>information about the placement of the #1 grid.?  Most 
>of my Handbooks just breeze over the details.  
>-
>Please reply direct or through amps to the group. 

We have to be a bit careful what we mean by "grounded" - either grounded
to RF but biased at DC, or nailed directly to chassis ground.

The applications where both grids are tied to chassis ground are the
ones that result in excessive G1 dissipation. Tubes like the
4-125/250/400/1000 series which were originally designed for class C
with a large G1 dissipation will survive (although IMD is questionable).
More modern tubes like the 4CX250 series will not stand that abuse to
G1.

On the other hand, if you bias the grids to the same voltages that the
manufacturer recommends for linear operation, you can drive either G1 or
the cathode and get good results. All voltages are measured relative to
the cathode, and with a fully floating G1 and G2 power supply you can
choose to DC-ground either the cathode, G1 or G2.

The main types of connection are:

1. G1 driven, DC-grounded cathode, RF-bypassed G2. This is the
"standard" connection (the cathode may have a small negative feedback
resistor in HF amps).

2. G1 driven, DC-grounded G2, RF-bypassed cathode, eg the 144MHz 4CX1000
amp in the ARRL Handbooks. 

3. Cathode driven, DC-grounded G2, RF-bypassed G1. This is a favorite
for big tetrodes at UHF, where effective screen bypassing can be
difficult. The 4CX1000 in the Collins 30-L1 has DC-grounded screen, RF
drive to the cathode and almost RF-bypassed G1 (with some RF negative
feedback through a capacitive divider). 

4. Cathode driven, DC-grounded G1, RF-bypassed G2. Also used with big
tetrodes at UHF.

They all work, because the voltages of G1 and G2 relative to the cathode
are correct - but you still need the two grid supplies in addition to
HV.

There's something about this in my article in November 1997 QEX, which
is downloadable from www.ifwtech.com/g3sek/boards/tetrode-3.htm 
There is also a chapter in the instruction manual that comes with the
Tetrode Boards, but I'm afraid I haven't had time to rewrite it as a web
page.
 
73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.com/g3sek

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