[Amps] filament voltage question

[Tony King]

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
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