Several times over the years I needed a special filament transformer.
I used old variac cores from ones that had broken brushes or some burnt
turns on the
ends.
Wrap the old core with some Teflon plumbing tape to insulate it and use
some well insulated wire as a secondary and wrap it thru the core.
Make a few turns to establish the volts per turn using an AC voltmeter and then
wind your secondary. They range from .5 V/turn to around .25V/turn.
If you have a large core you can place spacers on it and wind your
low capacitance secondary. That way if you are operating GG you don't need a
high current choke.
73
Bill wa4lav
At 03:12 AM 5/25/2011 -0400, Ian White GM3SEK wrote:
>Roger (sub1) wrote:
> >On 5/25/2011 12:03 AM, Jeff Blaine wrote:
> >> Glen,
> >>
> >> You may want to rethink that headroom. The transformer will tend to
> >>have a bit higher voltage if running at less than rated load.
> >> And higher than optimal fil voltages equate to shorter tube life. In
> >>addition, there is less inrush trama to the filament if you
> >> have a matched transformer. I think the latter is subject to some
> >>debate however.
> >>
> >> In any event, you would want to trim the actual fil voltage with a
> >>series resistor for max life.
> >>
> >
> >I look at it as you need the extra headroom allowing for trimming the
> >voltage to one that is ideal. Of course with the line variations we run
> >into today, ideal might be right at the bottom of the permissible range
> >to keep the top within the permissible range.
> >
>
>Another option is a cheap packaged switch-mode supply, eg one of the
>'Mean Well' series from eBay suppliers. This particular brand are very
>solidly built with good filtering and shielding, and a straightforward
>electronic design using a TL494.
>
>I just took delivery of a 12V 12A SMPS for a 1296MHz triode PA. At that
>frequency the heater will need to be operated at the low end of its
>voltage range to minimize back-bombardment, leaving no tolerance for any
>further reduction due to line voltage variations.
>
>A stabilized SMPS might try to drive a lot of current through the cold
>heater at switch-on, so it may be necessary to 'slug' the voltage
>control loop with a capacitor. (I'll report back in due course.)
>
>
>--
>
>73 from Ian GM3SEK
>http://www.ifwtech.co.uk/g3sek
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