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Re: [Amps] filament choke questions

To: "Don Kessler" <>, "Amps" <>
Subject: Re: [Amps] filament choke questions
From: "Carl" <>
Date: Thu, 8 Oct 2009 20:24:04 -0400
List-post: <">>
----- Original Message ----- 
From: "Don Kessler" <>
To: "Amps" <>
Sent: Thursday, October 08, 2009 7:57 PM
Subject: Re: [Amps] filament choke questions

> 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
>> VE3ZI
> Interesting...
> 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?

The same answer applies to the 8877 family. On HF its usually not required 
and ham amp
 manufacturers are notoriously cheap. On VHF it makes a difference 
especially in a 3 hole
3CPX800A7 conversion of a LK-800TNY to 6M with 120W of drive. No, its not 
I just build them (-;


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