When you put a pot in series with the primary of the transformer, it acts
like a step start resistor. As the filament warms up, it draws less current.
I used a 150 ohm pot 15 watts in the primary of my homebrew 8877 filament
transformer. My transformer is from MFJ/Ameritron and is from a AL-1500 amp.
73
Jim W7RY
----- Original Message -----
From: "Bill, W6WRT" <dezrat1242@yahoo.com>
To: "Paul Decker" <kg7hf@comcast.net>
Cc: <amps@contesting.com>
Sent: Friday, May 15, 2009 1:54 PM
Subject: Re: [Amps] filament transformers
> ORIGINAL MESSAGE:
>
> On Fri, 15 May 2009 20:40:02 +0000 (UTC), Paul Decker
> <kg7hf@comcast.net> wrote:
>
>>
>>So my questions are: Is it acceptable to use the higher current rating in
>>the single tube version of the amp? Would current inrush be an issue? Is
>>the 3A version enough, should I get the 4A version or something even
>>larger? Or, is there something else I should be thinking about using?
>
> REPLY:
>
> The three amp version would be my choice, especially if you can place
> it somewhere in the amp or power supply where it would get a small
> amount of air flow around it. I would place a rheostat in series with
> the primary of the transformer to allow for exact voltage adjustment.
> If you decide to use the rheostat, you might consider a slightly
> higher secondary voltage such as 15 volts. That would give you more
> options to choose from, more range of adjustment and might be cheaper
> as well.
>
> Inrush current is not an issue with indirectly heated cathode tubes
> as far as I know. Eimac makes no mention of it in the data sheet for
> this tube.
>
> 73, Bill W6WRT
>
>
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