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Re: [Amps] Grounding the grids

To: "Gary Schafer" <>, <>
Subject: Re: [Amps] Grounding the grids
From: "Tom W8JI" <>
Date: Tue, 24 Jul 2007 20:58:31 -0400
List-post: <>
> Why do you suppose that Collins put those little 47 ohm 
> resistors in each
> grid on the 30L1 amplifier?

I can't speak for Collins since I wasn't there and have no 
first hand knowledge, but in the AL811H and AL572 I put the 
resistors there to equalize drive.

The resistors eliminated the need to match characteristic 
curves of the tubes, making anode dissipation more even.

That was one of the most difficult choices to make in the 
entire amplifier, because floating the grid up on resistors 
increased the likelihood of damage to exciters from tube 
arcs and damage to the silver micas in the input circuit. 
Everything in life is a tradeoff.

With closer characterized tubes, like 3-500Z's, it wasn't 
necessary but with 572's and 811's it was tough finding four 
close enough to prevent hogging by one tube.

I would guess Heath and Collins did the same, since the 
engineers most likely experienced the same problems. There 
isn't any other logical reason I can think of for floating 
the grids through a resistor.

73 Tom

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