Adjust the filament until the output power just starts to drop. Measure that
voltage to see if it is within the low side spec range. If so increase it
.2V and dont mess with it every hour, day, week, month, or year. If the
voltage is below spec you have a super good tube, bring it up to .2V above
minimum and also quit playing with it.
Keep a log of power out and readjust when it drops.
Broadcasters do it in .1V increments which gives them a lot of increase
steps before its time to change.
My 3-500Z's are the ones that were in the amp from the factory in 1986.
After a hard life of contesting and DXing they still put out the same 1200W
power and Ive still got several more adjustment steps to go. I also keep the
drive to 80-85W.
A MLA-2500 got the same TLC when I got it over 20 years ago and its also on
the original 8875's.
Thats been shelved as a backup and the 2nd station amp is now a 3 holer
Alpha 76PA that I got with very soft tubes. Its running with $200 worth of
8874's from removed from service as obsolete equipment cell phone amps and
running strong at 6.1V.
Im a cheap SOB and dont like buying tubes (-;
Carl
KM1H
----- Original Message -----
From: "Roger (Sub1)" <sub1@rogerhalstead.com>
To: <amps@contesting.com>
Sent: Friday, January 14, 2011 5:33 PM
Subject: Re: [Amps] A Filament Query
> On 1/14/2011 3:42 PM, Dennis OConnor wrote:
>> Here are a few references I have bookmarked in my electron tube folder...
>>
>> Black heat is normally ruinous to tubes - ymmv -
>>
>> I ruined a really nice 3CX1200 when the carefully adjusted filament
>> voltage right at the lower limit of mfr's rating got further decreased by
>> a sneaky power company trick of dropping their voltage a few percent
>> during the post 11PM to 5AM low usage time period - when I was
>> transmitting at full-bore, war-emergency power on topband...
>>
>> Twere me, I would lower the filament voltage at rest by 3% which gives a
>> huge increase in filament life without undue risk of poisoning the oxide,
>> and then pop it up to full rating for a short period, perhaps 2 seconds,
>> before the 1 second operation...
>
> In this particular instance with the short on time every minute we are
> looking at 1440 heating cycles per day. With that many cycles I wonder
> if just leaving it alone (or running near the minimum voltage) would
> give better life than cycling it 1440 times a day 10080 times a week,
> 40,320 times a month or 438,840 times a year.
>
> 73
>
> Roger (K8RI)
>>
>> http://www.cpii.com/docs/related/22/AB18.pdf
>> http://www.w8ji.com/vacuum_tubes_and_vaccum_tube_failures.htm
>> http://www.archive.org/stream/VacuumTubeDesign/Rca1940VacuumTubeDesign_djvu.txt
>>
>> denny / k8do
>>
>>
>>
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>>
>
>
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