It seems that the sag can be used to compensate for the increase in voltage due
to back bombardment. If you set the voltage at 6.0 volts, for example, and the
filament voltage drops under the load of the plate supply, at some point the
back bombardment will increase the voltage again. So, you have built in
compensation. Very crude, of course, as it would change with key down time -
but it could be used as an approximation.
In other words, the voltage drop and the voltage increase can be self
compensating to some degree. May not be perfect, but perhaps it is close
enough to make do - which is half of amateur radio.
At least it is a thought - laziness is the mother of doing less work.
Colin K7FM
-----Original Message-----
From: "R.Measures" <r@somis.org>
Sent: Oct 29, 2004 6:58 AM
To: Tony King <amps@w4zt.com>
Cc: Amps@contesting.com
Subject: Re: [Amps] filament voltage question
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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>
>
> _______________________________________________
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>
>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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