[Amps] filament voltage question

[R.Measures]

Tony --  The problem with 0 - 115% variable transformers is that it is 
not necessary to be able to adjust the filament V more than +/- 8%.  A 
rheostat does the job nicely.

On Oct 29, 2004, at 6:26 AM, Tony King wrote:

> An easy solution would be to obtain one of the small variable 
> transformers (I have a couple I bought on Ebay) and put it in the 
> filament transformer primary so you can make the necessary minor 
> adjustment.  73, Tony W4ZT
>
>
> At 08:47 AM 10/29/2004, Jim Smith wrote:
>> I'd install a buck/boost transformer to correct the problem with 
>> voltage drop rather than working around it, which may lead to still 
>> more complications.
>>
>> Jim Smith, KQ6UV
>>
>> ----- Original Message -----
>> From: TexasRF at aol.com
>> To: amps at contesting.com
>> Sent: Wednesday, October 27, 2004 9:34 AM
>> Subject: [Amps] filament voltage question
>>
>>
>> I am looking for some advice on adjusting the filament voltage on a 
>> TH347
>> tetrode used in my 1296mhz cavity amplifier. The TH347 is a directly 
>> heated
>> cathode tube.
>>
>> Due to a long underground run for the 240vac line (approx 900ft of 4-0
>> 3/cond) there is a voltage drop of about 4% when the amplifier is 
>> driven key  down.
>> The hv power supply is a capacitor input circuit and draws approx 21 
>> amps
>> off the 240vac line. I suspect the peak current is over 100 amps 
>> during key down
>>  periods.
>>
>> The tube manufacturer suggests setting the filament voltage to 5.8vac 
>> for
>> operation at these frequencies due to added cathode power from back 
>> bombardment.
>>  The normal service is for continuous television transmission and 
>> average
>> plate  current is about 1 amp. When used on cw the keydown plate 
>> current is about
>> 1.75  amps.
>>
>> My tests indicate that 5.8vac is too low for cw as the plate current 
>> is
>> about the same at the beginning and at the end of a long 
>> transmission. In other
>> words, there does not seem to be any significant added cathode 
>> emission due  to
>> back bombardment during cw transmission. On the other hand, with a 
>> steady A0
>> carrier, the plate current does rise as does the power output over a 
>> 10 to 15
>>  second time frame. This suggests that there is an increase in cathode
>> emission  under these conditions.
>>
>> When I raise the filament voltage from 5.8 to 6.0 volts the cw power 
>> out
>> rises from about 1600w to 2000w and remains near 2000w under most test
>> conditions.
>>
>> A complication is that the filament voltage drops almost .25v just 
>> due to
>> the line voltage drop from keyup to keydown. This means that the 
>> voltage has to
>> be set at 6.25v keyup to measure 6.0v keydown. I have read comments 
>> about
>> tube  life being shortened when the filament voltage is elevated. 
>> These are
>> fairly  expensive tubes and good pulls are almost non-existent. 
>> Obviously one would
>> like  to make the tube last as long as practical.
>>
>> Does anyone out there have input on the effect of elevated filament 
>> voltage
>> during periods that the cathode is not emmiting? In other words, is 
>> tube life
>> compromised during standby periods or is it only for operating 
>> periods that
>> matter?
>>
>> Adding a filament voltage regulator is complicated by the 34 amp 
>> current
>> requirement for the tube. It would be fairly easy to switch in a 
>> small value
>> resistor at the filament transformer primary to reduce the 
>> receive/standby
>> filament voltage but I wonder if this is a viable solution?
>>
>> Any comments/suggestions are appreciated.
>>
>> Thanks/73,
>> Gerald/K5GW
>> _______________________________________________
>> Amps mailing list
>> Amps at contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>> _______________________________________________
>> Amps mailing list
>> Amps at contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>
>
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org