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Re: [Amps] Alpha 77Dx -Adding Grounded Grid Filament Choke

To: Paul Christensen <w9ac@arrl.net>,Amps reflector <amps@contesting.com>
Subject: Re: [Amps] Alpha 77Dx -Adding Grounded Grid Filament Choke
From: Vic Rosenthal <vic@rakefet.com>
Date: Fri, 04 Mar 2005 09:06:30 -0800
List-post: <mailto:amps@contesting.com>
Paul Christensen wrote:

> Whoops - my mistake.  The 77Dx's 8877 does not tie the cathode and filament
> together.  One side of the filament is grounded.  Would it still be
> advisable to add the common-mode filament choke anyway?

On first consideration, one might think that the capacitance between the 
grounded heater and the cathode might shunt some of the drive to ground on the 
higher bands, making it harder to drive.  If this were true, it might help to 
isolate the heater from ground with a choke.

On the other hand, this capacitance is actually part of the input network, and 
one would expect that the designer has taken this into account.  So unless the 
capacitance is high enough to make it impossible to get the correct value of 
output capacitor for the network, there's no problem.  This implies that if you 
do isolate the heater, expect to have to retune the input networks or even 
increase the value of some of the output capacitors.

In some amplifiers, a component failure can result in a large voltage between a 
grounded heater and cathode.  I've seen suggestions that to protect the tube in 
this case it might help to isolate the heater from ground.  I don't know the 
77DX so I can't say whether this is reasonable.

-- 
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco

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