Its done sort of this way in larger rigs, usually an off the shelf
unit for 120/240 VAC is used, a ferroresonant type with 3 windings
and a capacitor. Then a standard filament transformer or special
current limiting type (2 x) with a small rheotstat or variac in the
primary side to set the exact voltage. With this arrangement, the
line v is regulated.
Should consult with your tube manufacturer if you plan to use
nonsinusoidal filament voltage. Of course its the RMS heating that
matters, but some waveforms are mechanically 'harsh'.
In 1998 I designed a saturable reactor filament ramp ciruit for a
pair of 250 kW tetrodes. Works very well, still using a more
conventional filament transformer though.
K5PRO
>From: BEAR <bearlabs@netzero.net>
>To: "amps@contesting.com" <amps@contesting.com>
>Subject: [Amps] Speaking of filaments...& regulation
>Thinking about the issue of keeping the filaments cooking at +/- 5%
>of nominal rating for longest life...
>
>Seems to me that a saturable core type transformer would fit that bill
>perfectly... certainly would regulate the voltage. You know, similar
>to those 120v AC supplies that do the same trick.
>
>In fact, I think I have some that are 6.3v saturable core trannys from
>something or other... obviously you'd need some that were wound *for*
>the desired voltage.
>
>Anyone got any thoughts on this?
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