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Re: [Amps] filament voltage question

To: TexasRF@aol.com
Subject: Re: [Amps] filament voltage question
From: R.Measures <r@somis.org>
Date: Wed, 27 Oct 2004 11:28:56 -0700
List-post: <mailto:amps@contesting.com>
Gerald -- The 8169 amplifier on my Web site has N.O. relay contacts across a 6-ohm resistor in series with the filament-V adjustment rheostat that's in series with the primary of the 9v, 43a filament transformer. During Rx, the contacts open and the filament V decreases by c. the same % that the mains-V decreases during Tx when the contacts are closed. Thus, the filament-V stays pretty much the same on Tx and Rx.
- cheers


On Oct 27, 2004, at 9:34 AM, TexasRF@aol.com wrote:

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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