The AM Forum

Hi All,

I must be really dense when it comes to trying to grasp the 'grounded grid amp' concept. Can someone illuminate me on just how it works?

Thanks, Roland........

Hi Roland,
There's a good description here:  http://wb0nni.dakotamade.com/ggbasic.html (http://wb0nni.dakotamade.com/ggbasic.html)

An article in the February 1969 issue of 73 Magazine (page 10) has inspired me to build a grounded grid amp using two 813s: https://archive.org/stream/73-magazine-1969-02/02_February_1969#page/n11/mode/1up

I am also contemplating building a power supply for it as described in the May '65 issue: https://archive.org/stream/73-magazine-1965-05/05_May_1965#page/n39/mode/1up

I have a filament and a plate transformer I could use for the project.

Easy:

"Regular amp": drive the grid, that controls the flow between the cathode and the anode (plate). The cathode stays (more or less) fixed.

"Grounded Grid": drive the cathode and the cathode moves with respect to the grid - which stays fixed. The tube more or less does not know the difference between moving the grid, and moving the cathode! It only cares about the difference between them...  ;D

Thanks for the info.

Does there need to be a rather large current flow between cathode and plate all the time? This is part of what I'm trying to understand. I get that the drive signal is applied to the cathode. How (in)efficient is the GGA?

Roland.....

Is there a pictorial which shows current flow in the tube with and without a drive signal?

Depending on the tube used, resting currents range, in general, 'tween 50 and a couple hundred milliamps. This will increase with drive.

For example, my pair of 8874s rest at 50ma. With 20 watts of drive they draw 350ma. At full drive it runs 750ma.

GG amps are no more or less efficient than grid driven amps for a given
Class of operation. GG amps will require more drive power. But most of that power feeds through to the output.

More info:

The hidden benefit of running GG vs: grid driven is the inherent (built-in) negative feedback. This will make the amplifier run cleaner with less splatter vs: a grid driven version running at the same parameters and no neutralization.  (both AB2)

In addition, the grounded grid element acts as a shield between the cathode and plate to make the amplifier stage much more stable.  For example an un-neutralized  grid driven amplifier might take off into oscillation on 10-20 meters whereas by simply putting it into a GG config would probably make it stable.  (But has less gain and is harder to drive)

The best of all worlds might be a properly neutralized grid driven amplifier using regulated grid and screen supplies in AB1. Neutralization is a form of negative feedback, though not as much as GG.   I run a pair of 4-1000As in grid driven mode (in AB1) and find it as clean and stable as a GG amp. However, it puts out somewhat less power than GG because it is run AB1. (no grid current, less efficient)

T