[Amps] How best to reduce filament voltage?

[Jim Tonne]

Gary, Colin, Bob and The List:

I sent an explanation of my ignorance on that key
point and so far it has not been posted.

- JimT


----- Original Message ----- 
From: "Robert B. Bonner" <rbonner at qro.com>
To: "'Jim Tonne'" <tonne at comcast.net>; "'Tom W8JI'" <w8ji at w8ji.com>; 
<w2cqm at juno.com>; <amps at contesting.com>; "'Norman Hockler'" 
<sanorm at columbus.rr.com>
Sent: Friday, February 02, 2007 11:37 AM
Subject: RE: [Amps] How best to reduce filament voltage?


> Jim you missed the key issue this is a ONE TRANSFORMER does all situation.
>
> You fire up the high voltage and you don't want that resistor dimming the
> whole AMPLIFIER..
>
> BOB DD
>
> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Jim Tonne
> Sent: Friday, February 02, 2007 9:51 AM
> To: Tom W8JI; w2cqm at juno.com; amps at contesting.com; Norman Hockler
> Subject: Re: [Amps] How best to reduce filament voltage?
>
>
> Tom wrote:
>
>> This will not work. Don't do it.
>
> in reply to
>
>>> Install a resistor on the primary side of the transformer.
>
> which is what I would have suggested.
>
>> It will totally hose the voltage regulation under load,
>
> The load is constant so "regulation" in the usual sense
> (same output voltage with varying load value) is not an
> issue.  A series resistance would limit inrush.  A series
> resistance at the transformer primary would limit inrush
> to the transformer as well.
>
>> the resistors would have to be huge.
>
> Seems to me the resistor WATTAGE value would be
> the same regardless of whether it was in the primary or
> the secondary of the filament transformer.
>
> Another way around this whole issue is to use a
> relatively small bucking voltage (obtained from, say,
> a filament transformer) in series with the transformer
> primary to reduce its input voltage.  That would of
> course be very efficient.  By putting a variac or
> equivalent on the primary of the buck transformer
> the filament voltage could be tweaked to perfection.
>
> - JimT
>
>
>
>
> ----- Original Message ----- 
> From: "Tom W8JI" <w8ji at w8ji.com>
> To: <w2cqm at juno.com>; <amps at contesting.com>; "Norman Hockler"
> <sanorm at columbus.rr.com>
> Sent: Friday, February 02, 2007 10:32 AM
> Subject: Re: [Amps] How best to reduce filament voltage?
>
>
>> This will not work. Don't do it.
>>
>>> Install a resistor on the primary side of the transformer.
>>> Much
>>> lower current and the resistors will act as a
>>> soft start for the filament to damp the inrush when you
>>> first turn on
>>> power.  Set the resistor value with the
>>> tube on and warm  There are many sliding tap resistors
>>> available from
>>> the distributors or a fixed one will
>>> work as well. Ohms law calculation to determine the value.
>>
>> It will totally hose the voltage regulation under load, and
>> the resistors would have to be huge.
>>
>> 73 Tom
>
>
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