Hi Harold,
As long as you are running a bridge...
You could substitute SCR's (2N683 or similar) for the two diodes in
the positive leg and build a little phase control circuit (See an old GE
SCR manual) to ramp the filament up for whatever time period you wish
then, just turn the SCR's on full cycle for normal operation.
Add an RMS detector (AD536 or later vintage part) to sense the output
V, provide for some feedback and you would have a regulated filament
supply with soft-start.
73 & Good evening,
Marv WC6W
On Thu, 26 Aug 2004 20:01:08 -0400 "Harold B. Mandel" <ka1xo@juno.com>
writes:
> Sirs,
>
> The new amplifier will be running a pair of 3CPX1500A7 triodes
> with floating filaments through bifilar chokes, powered by 5 volts
> Direct Current through a floating bridge rectifier and filter.
>
> In order to reduce the thermal shock to the filaments in the tubes
> a method of reducing the inrush current is sought.
>
> Please note the filament transformer, (5vac @ 22a) also provides
> the
> 22vac @ 1a and 44vac @ 1a for the various control, logic and for
> the
> G3SEK triode board. I would prefer not to vary the input voltage to
> the
> above transformer, as in the usual step-start method of injecting
> resistance,
> but would much prefer to step-start the D.C. 5 volt lines to the
> tubes
> with a
> time delay relay and a DPDT 30 amp contactor that would insert
> resistors
> on
> both polarities.
>
> So.
>
> Do you think that a series resistance of let's say 10 ohms at 50
> watt
> dissipation
> for some period of time would suffice to lengthen the tube life
> with
> regard to inrush
> shock? How long should the tube be in the inrush current-limiting
> state?
>
> I understand that ceramic tubes have quite different filament and
> electrode
> geometries and that some say an inrush protection is not necessary.
> However,
> when new tubes are $1400 I want them to have a sheltered childhood.
>
> Thanks for your advice.
>
> Hal Mandel
> KA1XO
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>
>
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