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Old 17th September 2019, 12:20 PM
Riggers Riggers is offline
Join Date: Aug 2012
Location: Brisbane, Queensland. Aus
Posts: 66
Default Testing valves

Hi all
This might be a stupid question and I know Dr John has covered the subject of valve life before, but due to the fact valve testers are hard to come by in my part of the world (and bl@#*y expensive given units for sale are decades old and uncalibrated)....... Is it possible to get an accurate read on valve strength based on power output of the amp. You see I DO have a sig gen and ‘scope. If I measured a decline in output (say comparing two readings 6 months apart) would that aid in knowing when it’s time to replace valves?
Cheers once again
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Old 17th September 2019, 05:12 PM
bob orbell bob orbell is offline
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Join Date: Nov 2009
Location: kettering northants.
Posts: 2,049
Default Re: Testing valves

Not sure about the gen scope idea Phil, but a good signal is the very upper HF start to get weaker. Bob
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Old 17th September 2019, 10:10 PM
Richard Richard is offline
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Join Date: Dec 2005
Location: Notts
Posts: 4,981
Default Re: Testing valves

Hi Phil,

Yes, as Bob says, you'll notice when they're wearing out. The sound becomes "bleached" with weak bass and treble. You'd pick it up on a tester (I have one) but I haven't tried scoping bandwidth with old valves so don't know if it would be noticable that way, I imagine it should but a bit of a faff.

Work on 2-3000 hours for power valves run in class A near max dissipation (the likes of Quad II run theirs a lot gentler and will last much longer) and 20-50000 hours for small signal triodes which may often be run at 1/10 max dissipation
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