Amps
[Top] [All Lists]

Re: [Amps] filament choke questions

Subject: Re: [Amps] filament choke questions
From: Martin <martin@ok1rr.com>
Reply-to: martin@ok1rr.com
Date: Fri, 09 Oct 2009 15:33:40 +0200
List-post: <amps@contesting.com">mailto:amps@contesting.com>
The Harris AM7224/URC (RF-353) employs the 8877 also without any 
filament choke. Works.

73 Martin, OK1RR

Carl napsal(a):
> ----- Original Message ----- 
> From: "Don Kessler" <ki6sz@sbcglobal.net>
> To: "Amps" <amps@contesting.com>
> 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 
> mine,
> I just build them (-;
> 
> Carl
> KM1H
> 
> 
> 
> 
>> _______________________________________________
>> Amps mailing list
>> Amps@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps 
> 
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
> 
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps

<Prev in Thread] Current Thread [Next in Thread>