[Amps] How best to reduce filament voltage?

[Robert B. Bonner]

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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