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Re: [Amps] filament choke questions

To: Amps <>
Subject: Re: [Amps] filament choke questions
From: Don Kessler <>
Date: Thu, 8 Oct 2009 19:57:51 -0400
List-post: <">>
On Thursday 08 October 2009 07:43:29 am Roger Parsons wrote:
> Whilst the explanation below makes perfect sense, I have 1kW and 500W ex
> commercial FM (100 MHz) broadcast transmitters made by CCI - the former
> uses an 8877 and the latter a 3CX800A7. Neither of them had filament chokes
> and I presume they worked OK! (The 1kW has been modified to form the basis
> of my 160m remote station and the 500W awaits a project.)
> 73 Roger


I recently completed redesign of several amps, all are single and two-tube 
amps using the Eimac (CPI) 3CPX800A7.  

The datasheet for this tube as well as for the 8877 states that both filament 
leads must be RF isolated.  The original design had one filament lead tied 
directly to the chassis and this concerned me so I called and spoke with Reed 
Brandon at CPI.  According to CPI, while the amp "may" work without filament 
isolation, the "better" design methodology is to isolate both filament leads.

We use a bifilar choke, 3" long, 1" diameter wound on ferrite with good 
results.  With 37W drive into a pair of 3CPX800A7 tubes on 6m we are getting 
1500W out CW..  Anode voltage we are using in this particular amp is 2900V.

>From a conceptual perspective -- especially at VHF, I think the capacitive 
coupling between cathode and filament could be significant.  Why shunt 
cathode drive to ground via the filament? 

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