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[Amps] filament voltage question

To: amps@contesting.com
Subject: [Amps] filament voltage question
From: TexasRF@aol.com
Date: Wed, 27 Oct 2004 12:34:22 EDT
List-post: <mailto:amps@contesting.com>
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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