Testing 12ax7s

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Glaw343Glaw343 Frets: 4
Hi everyone, following a post I made previously (http://www.thefretboard.co.uk/discussion/137116/valves-on-their-way-out#latest) I took the advice of @ICBM and swapped V4 for V1, I also swapped V3 for V2 and that seems to have sorted my issue, with that in mind, I suppose it is likely that v3 and v4 are fine, I also have a few 12ax7s left over from a previous re-valve, can I test them to see which ones are faulty? if so, how would I do that? I'm going to get a new set as I'm not crazy about how these sound but it would be handy to have some known good spares :) cheers
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  • WazmeisterWazmeister Frets: 3722
    edited August 13
    I bought an Orange Vt1000 valve tester a few years ago, and it has been invaluable to me 
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  • Glaw343Glaw343 Frets: 4
    I bought an Orange Vt1000 valve tester a few years ago, and it has been invaluable to me 
    would that tell you if a valve is microphonic?
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  • springheadspringhead Frets: 91
    Glaw343 said:
    I bought an Orange Vt1000 valve tester a few years ago, and it has been invaluable to me 
    would that tell you if a valve is microphonic?

    No valve testers I know of will do that.  They're just checking for performance, continuity, leakage, gas etc.  Noise and microphonics are best tested in situ in an amp.  Gently tap each valve with a pencil or similar and listen for the result.  Volume controls would need to be up a little for any valves prior to them in the circuit, e.g. you won't hear a noisy/microphonic V1 with the channel volume down.
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  • ICBMICBM Frets: 31553
    edited August 13
    A valve tester will give you information about the performance characteristics and show up some basic faults, but it can’t actually tell you whether a valve will be any good in an amp.

    The simplest way is to put it in the amp and listen to it. If you’re familiar with the sound of the amp then a valve with poor gain, bad tone or noise should be obvious - and tapping it should show up microphonics. Trying it in V1 will usually be the most sensitive.

    There are almost no preamp valve faults which can damage an amp (unlike with power valves) so this is safe.
    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone."
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  • Glaw343Glaw343 Frets: 4
    thanks a lot, I'll pick up some new preamp valves and swap my current ones in one at a time for V1 to test them. thanks for the info guys :)
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  • RiftAmpsRiftAmps Frets: 1211
    Glaw343 said:
    I bought an Orange Vt1000 valve tester a few years ago, and it has been invaluable to me 
    would that tell you if a valve is microphonic?

    No valve testers I know of will do that.
    My Centrad 751 has a headphone socket that allows you to listen for microphony when the valve is under test conditions
    Rift Amplification
    Handwired Guitar Amplifiers
    Brackley, Northamptonshire
    www.riftamps.co.uk
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  • springheadspringhead Frets: 91
    Blimey, never heard of that one!
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  • jpfampsjpfamps Frets: 1364

    As ICBM says, unless a defective valve will damage your amp, then the best test is does is work in the amp.

    This is especially true in my experience for microphonic pre-amp valves, especially in high gain amps.


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  • DJH83004DJH83004 Frets: 166
    RiftAmps said:
    Glaw343 said:
    I bought an Orange Vt1000 valve tester a few years ago, and it has been invaluable to me 
    would that tell you if a valve is microphonic?

    No valve testers I know of will do that.
    My Centrad 751 has a headphone socket that allows you to listen for microphony when the valve is under test conditions
    Never quite worked out how that works as the Centrad 751 is essentially just an emissions tester and doesn't measure valve characteristics, so the valve is not operating as an amplifier (though I admit, they  do look cool!)
    I think as previously stated put the valve under test in the V1 position of a known amp and undertake a tap test.   
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