[Top] [All Lists]

[Amps] Grounding the grids

To: <>
Subject: [Amps] Grounding the grids
From: "Tom W8JI" <>
Date: Mon, 23 Jul 2007 22:58:07 -0400
List-post: <>
Lou Wrote:

>>> Don't expect miracles by grounding the grids.   Lou

Carl Wrote:
> Lou, it will be a miracle if a tube survives a flashover 
> without the grid
> "fuse" or a healthy surge resistor.

Let me inject a little factual history of WHY Heath and 
others used that asinine idea of floating the grids with a 

Collins came up with a very good idea in the 30S1. That amp 
used a cathode driven tetrode that could NEVER be allowed to 
have any grid current. The tube had a rated grid dissipation 
of zero watts.

Collins cure was to float the control grid through a low 
value mica capacitor, and to have the grid return through a 
high impedance. The small capacitance from grid to ground 
and the fact the tube never drew grid current meant a high 
impedance capacitive divider would reduce the power applied 
to the control grid. Since the cathode to grid impedance was 
always very high (the path was never in conduction) the 
divider worked perfectly. The GK capacitance formed one 
branch and the small C to ground the other. There was no low 
impedance shunting the capacitive divider in proper 
operation, because the tube never drew grid current in 
normal operation. It was a good stable linear divider that 
reduced power applied to the grid.

If the tube ever started to draw grid current the impedance 
from cathode to grid would drop like a rock, and the grid 
would effectively be tied to the cathode. This would reduce 
grid drive and protect the grid.

This scheme really only works in an AB1 tetrode, because the 
screen shields the input from the output and the control 
grid never draws current.

That very good idea got all screwed up when someone thought 
"gosh, that works with the 30S1 so well we might do that in 
a 3-500Z". What they forgot is the grid to cathode impedance 
is not high, the grid path conducts. It is AB2, not AB1. Now 
the upper section of the divider is a variable resistance in 
parallel with a capacitance, and the divider is not so 

This is why, if you do a two-tone test on an amp like the 
SB220 or L4B, you will see the IM products vary from low 
bands to high bands and are not as good as possible. If you 
simply remove the worthless grid chokes and capacitors and 
ground the grids the amp is more stable at VHF, and has 
better HF IM performance. The gain is also more frequency 
stable between ten meters and 80 meters.

Now one might wonder why people used that method. The answer 
is Bill Orr.

When I did an amp for Heath without the floated grids, Bill 
was right on the phone. He pushed hard to use his "super 
cathode driven" idea to give negative feedback. So I tried 
his idea, and found it made both stability and IM 
performance worse. Bill went over my head, and so I showed 
someone else at Heath the test results. We took an SB220 and 
pulled the grid chokes and bypass caps and grounded the 
grids, and both gain flatness with frequency change and IM 
performance improved. We could remove turns from the 
parasitic suppressor and the amp would remain stable, but 
when the mica's and chokes were added the amp would take 
right off.

Now Carl seems to think, as some people do, that the grid is 
a good place for a fault protection fuse. That's actually a 
horrible idea, as just a little thought will show.

Say a tube has an anode to grid fault from gas, and the grid 
is momentarily tied to the anode with a low impedance. Now 
the choke opens.

What happens now is the grid, after it explodes the mic 
caps, rises to B+. (If it does not rise to B+, it cannot 
limit current in that path.) So we have a grid biased 3000 
volts in an environment of plasma (that's what started the 
arc) and the filament is only a very tiny fraction of an 
inch away. Now the arc that formerly went to the grid goes 
through the grid to the filament, and the filament and 
everything in that circuit takes the full impact of the arc.

It's really a very dumb place to put a "fuse", because it 
simply moves the problem into more areas.

If you want to have fault protection for the tube, put a 
current limiting resistor in the anode. That resistor should 
be at least a few times higher than the supply ESR to be 
effective, or to make a useful change. Leave the grid at 
ground to at least afford some protection for the cathode 
system. The grid will have the arc anyway, you can't stop it 
by lifting the grid after the fact. Even if you do, a 3000 
volt positive grid will simply transfer the problem through 
the grid to the cathode, so you really don't fix a thing.

Unless there is some compelling reason to do so, floating a 
grid in a GG amplifier using a triode (especially a AB2 
triode) is not the best idea. It certainly does not protect 
anything, and it certainly does not make the amp work 

It really all got started because one guy made it his 
campaign to make sure manufacturers used his "super cathode 
driven" circuit.

73 Tom

Amps mailing list

<Prev in Thread] Current Thread [Next in Thread>