[Amps] filament voltage question

[R.Measures]

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 at 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