[AMPS] Adjusting fil voltage

[Rich Measures]

>
>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  


--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm