>
>Rich responed to Dan......
>
>> >
>> >I can't imagine why anyone would do this in the secondary?
>> >
>> Ä 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.
>>
>> Ä 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.
>
? R, R
>>
>> >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).
>>
>> Ä 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 (?).
>
? only the min. specified heater potential.
>>
>> >This also acts as an inrush current limiter as a bonus.
>>
>> Ä ... 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
? And if the mains V was low and the rheostat were set for 0-ohms?
> 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?
? The formula is (e2/e1)^23.4. Using the 2-500Z:
(5.25v/4.75v)^23.4 = 1.105^23.4 = 10.4 times more emissive life by
running the fil. at 4.75v instead of 5.25v. If one gets free tubes, it
matters not.
>
>Kind of like regular oil changes for you car, if you never keep it past
>100,000 miles you might never know.
>
? There was a telephone operator where I used to work who Never changed
oil. She added oil when the red light came on, and traded in her Mustang
every three years on a new one.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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