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
>
>
>
>
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