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Re: [Amps] Grounding the grids

To: "jeremy-ca" <>, <>
Subject: Re: [Amps] Grounding the grids
From: "Tom W8JI" <>
Date: Tue, 24 Jul 2007 12:28:36 -0400
List-post: <>
> However Ive never seen a simultaneous blown choke and grid 
> cap in the decades Ive worked with the SB-220 series or 
> any other amp using a similar design.

I certainly have.

> Early production SB-220 grid caps of the postage stamp 
> variety will split at the seams from age and moisture 
> ingress but the dipped micas seem to last forever.

....and you know it is age and moisture, and not an arc?

I've got boxes of 50 year old micas here of the same style 
that aren't split from moisture.

> OTOH, The Ameritron 2 x 3-500Z AL-82 and the AL-80 with 
> directly grounded grids have reputations of taking out a 
> tube and plate choke when there is an arc.

...and you somehow know the arc takes out the tube, and not 
that the bad tube makes an arc?
How would you know that?

The FACT is the HV in the AL82 is 3600 volts or so, while 
other amps are down around 3000 or less. This is why very 
rarely in AL80's do you see damage.
When the AL82 was designed 3-500Z's almost never arced. The 
Eimac's were great. This isn't the case today with the 
foreign tubes.

Truthfully if the AL82 was being done today it would have a 
20 ohm or so limiting resistor. Why 20 ohms? Because the ESR 
of the filter caps and choke combination is already ten 
ohms.That limits surge to 360 amps or less, since the arc 
path itself has some impedance. Adding ten ohms would make 
it 180 amps or less, but adding 20 would get it down to 120 
amps or less.

As for the idea a grid choke limits current, let's go back 
to basic Ohm's law. When the anode faults to the grid from 
gas, a plasma forms in the tube. See:

There are only two ways the plasma is extinguished and the 
fault current stops:

1.) The HV is lowered until the current flow stops.
2.) The grid and cathode rise to the anode voltage, so the 
current stops.

NOTHING else will magically stop the current.

If we assume nothing in the anode lets go, then the only 
other option is the grid must rise to anode potential. When 
it does that the little 500V rated  mica caps have 3000 
volts on them, and they arc. Now the grids are grounded 
again despite the choke opening. The capacitors actually try 
to protect the chokes.

If the grid actually has a successful choke opening and the 
little caps magically can hold off six times their rated 
voltage, then the grid rises to near anode voltage. Now the 
plasma simply passes current through the grid to the cathode 

It's almost silly to consider that some sort of fusing 

The only proper place to add a fuse or series resistance is 
the anode.

>And that is supposed to be designed to perfection.

No, that would be the National. :-) I don't know why they 
still aren't around since a perfect product would certainly 
have a longer sales life than the marginally designed AL-12 
series, which only have run 25 years now.

On the other hand the AL82 and virtually all other amps on 
the market today or in the past were designed to give the 
best value for dollar of cost. Not a single penny was spent 
that wouldn't benefit the customer for the target goal. Most 
engineers understand it is very difficult to build something 
that is priced to dominate a market but doesn't cause too 
many problems.

One of the very best amps on the market was the SB220, it 
had everything on the edge of being almost too cheap. When 
the power limit was raised to 1500 watts output, the whole 
game changed.

73 Tom 

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