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