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@aol.com
To: amps@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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