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Old 18th January 2006, 01:22 PM   #1
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Question Signal Tube Longevity ?!?

OK, I recently purchased a Fisher 800C, and last night I decided to check all the tubes to see what my shopping list would look like to get the 'ol girl up and running.

My tester is nothing great, an old Heathkit IT-21 emission tester, but it has been pretty good at finding bad tubes.

Anyway, I was pleasantly suprised to find all nine OEM Fisher branded Telefunken smooth plates that came with the receiver tested not just good, but very good...like as good as a NOS tube. All the tubes had very close section to section match as well.

So..just how long are these little bad boys expected to last ? I mean dang, these things are fourty years old (unless the unit was re-tubed with OEM tubes),and this unit looks like it was USED.

Casey
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Old 18th January 2006, 01:27 PM   #2
SY is offline SY  United States
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Tube or SS rectifiers in the Fisher?
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Old 18th January 2006, 01:45 PM   #3
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Hello Sy,

Solid State rectifiers.with the exception of the phono section, the filaments are AC.
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Old 18th January 2006, 01:59 PM   #4
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Some tubes seem to live forever. My Guild guitar amplifier still has ALL of the original tubes in it including the 7591's. It was made in 1969 and is definitely used.

I found an old Sparton radio at a flea market for $20. It had a hand written note taped to it that said that "all tubes replaced 2-14-31" The output tubes (NX-481's) are playing happily in a TubelabSE stereo amp that I made about a year ago. They are 75 years old. How much of todays electronics will exist in 75 years.
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Old 18th January 2006, 02:52 PM   #5
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Casey, thanks. One more data point for my beliefs about warmup and cathode stripping...
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Old 18th January 2006, 02:56 PM   #6
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Quote:
Originally posted by SY
Casey, thanks. One more data point for my beliefs about warmup and cathode stripping...
For or against? Real or imaginary?
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Old 18th January 2006, 03:29 PM   #7
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Hello tubelab,

Quote:
Some tubes seem to live forever.
So it would seem

Sy,

Quote:
One more data point for my beliefs about warmup and cathode stripping...
Would you care to elaborate on your "belief system". Cathode stripping is an area I know little (read nothing) about.

I decided to whip out the 'ol calculator and figure out the operating points of the various stages of the amp in search of clues to its long life. This is what I found...

Phono
Sect. 1) V=108 I=.3mA P=30 mW
Sect. 2) V=88 I=.58mA P=50mW

Line/Tone
Sect. 1&2) V=157 I=1.05mA P=164mW

PA
Sect. 1(VA) V=130 I=.45mA P=56mW
Sect. 2(Splitter) V=178 I=1.94mA P=345mW

...So, with the exception of the splitter, these tubes seem to be operated VERY conservatively. Even the splitter wouldn't be considered "hot".

Thoughts ?
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Old 18th January 2006, 04:39 PM   #8
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Hello valveitude

I have some tubes from the 1950's and the early 60's that are very used and they are still going strong. A few pulled from old B&W TV's. As long as they are/were used conservatively, well within ratings, they should have quite a few more years of service left in them. IMO and others, tubes way back then (Sherman: "Gee, Mr. Peabody!") were of higher quality as opposed to some that are of current manufacture. What we need is a way-back machine! And more care and less automation with better QC!

From looking at the schematic, the filament supply (V16, V17, V18, V19) is DC via a Selenium bridge. And the HT is half wave rectifried, maybe Selenium also. I would exchange for some Silicon but the DC output will be higher, so some circuit values will need to be adjusted. But hey that's right up our alley! The Fisher 800C is a nice receiver with lot's of possibilities for upgrading and improvement! Unless you would want to keep it in stock condition as possible.

Cheers
Wayne

Edit: P.S. The HT rectifiers are labeled as CR, so they may well be silicon.
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Old 18th January 2006, 05:13 PM   #9
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Quote:
Originally posted by dhaen

For or against? Real or imaginary?
Neither or both. It depends on the tube and the voltages, but there's a reason for the max Va(b) ratings on datasheets. For most common preamp tubes at common voltages, I've not seen any great difference in tube life from holding off B+ until warm as long as those ratings aren't exceeded.

High voltages (over 1kV) and transmitting tubes are a different story. Cathode stripping is real and must be prevented.
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Old 18th January 2006, 05:36 PM   #10
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If the HT rectumfriers are silicon, they may be marginal in voltage specs (PIV and current rating) as sand technology was in it's infancy, a 1N5408 would be a good and cheap replacement.
Just my

Wayne
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