[AMPS] Adjusting fil voltage

[Radiodan W7RF]

Rich responed to Dan......

> >
> >I can't imagine why anyone would do this in the secondary?
> >
> €  When the amplifier has a combination hv / fil. transformer, like the
> SB-200 and 2KD-5, such resistors must go in series with the fil. sec.
> winding.

Yes, of course. I was thinking of separate transformers.

> >I used this method (primary) with a 25 ohm 50 watt wirewound
> reostat in the
> >primary of my old Henry 4K to lower the filament voltage on an expensive
> >5CX1500A tube.
>
> €  The reported filament inrush  current in the Henry 4K was 400 peak
> amperes.  Did you add a step-start?

Yes, I did. I think I use a 10 ohm 25 watt in each leg of the primary.
Further dusting off of my brain reveals this was after talking to you and
getting your recommendation.

>
> >Actual numerical voltage value is not that important, I just
> turned it down
> >to where power output dropped off and went back up about .1 or .2 volts
> >(reading secondard voltage).
>
> €  This is the right way to adj. the fil. potential for a thoriated
> tungsten cathode.  One really doesn't need an accurate fil. voltmeter.
> Howeven, this method can destroy heater/oxide-cathode type tubes.

I must have only used this method on those tubes types as I never thought
about it. I think what the problem Rich is getting at would be operating the
tubes BELOW the needed filament voltage for safe operation of the tubes and
warm up time (?).

>
> >This also acts as an inrush current limiter as a bonus.
>
> €  ... only when the rheostat happened to be set at max. R at startup.  .
>  A step-start  always limits fil. inrush current.
> -  later

I would imagine that anywhere the control was adjusted where there was SOME
resistance (not all the way towards zero resistance) it would indeed drop
the voltage and absorb this inrush current for the limited amount of time
need to bring filaments up to operating temperature (a few seconds). This
method worked well for me, but one wonders how much actual time do you buy
extra for your tubes if you run just slightly above the needed minimum
filament voltage?

Kind of like regular oil changes for you car, if you never keep it past
100,000 miles you might never know.

73, Dan W7RF

>
>
> Rich...
>
> R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
>
>
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