A plate voltage of 2700V is a bit low to make any serious power with a
3-1000Z. 3500V is a better place for it to run. Maybe try the 208 VAC tap?
Sincerely,
Dr. William J. Schmidt, II K9HZ
Trustee of the North American QRO - Central Division Club - K9ZC
Email: bill@wjschmidt.com
WebPage: www.wjschmidt.com
"If computers get too powerful, we can organize them into a committee; that
will do them in." -- Bradley's Bromide
----- Original Message -----
From: "Dennis" <pegasus@mho.net>
To: <amps@contesting.com>
Sent: Sunday, February 20, 2005 11:06 AM
Subject: Re: [Amps] 3-1000Z Filament Voltage Question
Hello Dennis..
I´m not sure you´re getting actual answers to your questions on here so
far. Yes, excessively low filament voltage will result in low power
output. It´s not particularly good for the tube to run it that way,
especially with an indirectly heated cathode. The 3-1000, however, is
not such a tube. The question about best filament voltage can only be
answered with precision after you know how much life is already on your
tube. Ideally, you would have a variac on the line to the filament
transformer. Then, while operating the tube at normal voltage and the
tube running at the high or maximum output you anticipate normally, you
would slowly decrease the filament voltage until the power output
begins to decrease. Then run it back up a tenth of a volt or so. This
guarantees that the filament is running at the lowest voltage necessary
for full efficiency. Then periodically this test is done again, since
as the tube ages more filament voltage becomes necessary. This is how
broadcast transmitters do it and an important reason they can get 20 to
30,000 thousand hours out of a tube. (there are other techniques used
to extend life though) With a thioriated tungsten filament, such as
yours, filament voltage is a BIG factor in tube life.
7.08 is indeed too low for a 7.5 volt filament, but MAKE SURE you´re
measuring with a good RMS meter. If your tube is new then 7.3 volts
should be plenty, 7.6 or more will shorten the tube´s life substantially.
Soft start circuitry is also very important for tube life. There was a
long term study done some years ago that seemed to show that every time a
filament was lit that approximately 60 hours of life was taken from the
tube.
Usually ham operators are stuck with a fixed filament transformer,
sometimes one that is combined with the HV transformer. On top of that
we deal with line voltages that vary greatly. None of these things are
good for tubes. If I were you I would do what is necessary to get the
voltage up to low-normal for the filament. If using the 208 volt tap is
necessary, then make sure you´re not overheating the transformer. I
have an old Alpha that had excessive voltage on the tube. I constructed
a resistor (2 feet of no. 20 wire wound around a ceramic form) to bring
it down to a few tenths below spec. A "dropping resistor" for this
application only has to be a few tenths of an ohm. Easier to make than
buy. Another question... is your low filament voltage experienced only
while under load??? If it´s normal unloaded, then drops that much under
load, then you have a more dificult problem.
I hope this gave you some things to think about... Sorry I don´t know
the design of your amplifier. That would have been more helpful to me.
73,
Dennis
NØSP
>>>>Dennis Sokol wrote:
>>>>
>>>>>I have a BTI LK-2000 using a 3-1000Z operating on 160. I can get
>>>>> 1250 watts out with 2700 volts and 750 ma key down. The tube seems
>>>>> to be limited in current draw. I have seen more power out at a
>>>>> different QTH with higher line voltage.
>>>>>
>>>>>I have measured the filament voltage at full load to be 7.08 volts.
>>>>> This is a bit low from the spec of 7.13 to 7.87 volts. Is the low
>>>>> filament voltage a possible cause of reduced power out?
>>>>>
>>>>>There is a 208 volt tap on the filament xformer but I would have to
>>>>> put in a dropping resistor in the primary to bring the voltage down
>>>>> to 7.5 volts. Is it worth the effort? Am I hurting the tube with
>>>>> the lower filament voltage or am I extending the life?
>>>>>
>>>>>Thanks and 73,
>>>>>Dennis W0JX Milan, OH
>>>>>_______________________________________________
>>>>>Amps mailing list
>>>>>Amps@contesting.com
>>>>>http://lists.contesting.com/mailman/listinfo/amps
>>>>>
>>>>>
>>>>
>>>>
>>>>
>>>>_______________________________________________
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>>>>
>>>>
>>>
>>>Richard L. Measures, AG6K, 805.386.3734. www.somis.org
>>>
>>>_______________________________________________
>>>Amps mailing list
>>>Amps@contesting.com
>>>http://lists.contesting.com/mailman/listinfo/amps
>>>
>>
>>
>>
>>
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