Question about testing possibly failed 6l6s

CatthanCatthan Frets: 355
Hey guys,

A bit silly but I've had 2 6l6s on my desk for a while and don't remember if they had failed or if I took them 2 out to fit 4 matched new ones..

The only way to test them is throw them in the boogie and see if they pop the fuse I guess.

Is there any danger to the amp from doing this other than wasting 2 fuses?

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Comments

  • ICBMICBM Frets: 71960
    There is a small risk they could take out the screen resistors as well.

    I wouldn't do it unless you have some sort of current limiter/failure indicator. A 100W light bulb in series with the amp will do it, although if you don't have some electrical fittings etc it's not that easy a thing to rig up… at least not safely.

    "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." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • jpfampsjpfamps Frets: 2723
    As ICBM says, I wouldn't do, as you do risk damaging the amp.


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  • CatthanCatthan Frets: 355
    Right, the grid resistors...
    Thanks guys
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  • ecc83ecc83 Frets: 1589

    Do you know anyone with an HT-20?

    If they will let you! Rig a test meter across one of the 7W cathode resistors and monitor the current as the valves warm up (don't forget you need  speaker/load plugged in AND a plug in the input!)

    If the voltage goes above about 12, the valves are probably shot but you are very unlikely to damage the amp.

    Note, the HT-20 runs at a fairly low HT (~300v IIRC) so this will not be a S or Bust test.


    Dave.

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  • ICBMICBM Frets: 71960
    ecc83 said:

    Note, the HT-20 runs at a fairly low HT (~300v IIRC) so this will not be a S or Bust test.

    And hence (with respect!) a bit pointless! That's the exact problem with using a proper valve tester to assess valves for a guitar amp too - they just don't apply the same high voltages as the valve will see in use, and a valve can easily pass as good on the tester and then fail in the amp.

    That's why I built a crude valve tester from an old dead valve PA amp, which runs at normal 500V-range plate voltages. I just fitted huge wirewound screen resistors so they will survive a bad short.

    To be accurate, the chance of damaging a Mesa with a faulty valve is very small, but it does exist - more so in older models where they still used carbon-comp screen resistors, the newer ones use wirewounds. There's not much else that's likely to go wrong. There are many other amps I would be even less inclined to try it in...

    "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." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • CatthanCatthan Frets: 355
    Thanks guys, I've had fried grid resistors in a boogie before and though nowt serious, I'm not eager to curry the amp to the tech or ship back for warranty work.
    Valves are already in the bin, not taking a chance with them
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  • ICBMICBM Frets: 71960
    Catthan said:
    Valves are already in the bin, not taking a chance with them
    That seems a bit premature. I hope they weren't anything potentially good...

    "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." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • CatthanCatthan Frets: 355
    ICBM said:
    Catthan said:
    Valves are already in the bin, not taking a chance with them
    That seems a bit premature. I hope they weren't anything potentially good...
    Well they were 2 Mesa 6l6s which as we've discussed I can't try without risking damage and can't build an apparatus to test them so what else can I do with them?
    One if not both may have been responsible for frying a resistor in my previous boogie but don't remember as it's been a while and lot of valves passed through my hands around that period.
    They are not out of the house yet
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  • ecc83ecc83 Frets: 1589

    "And hence (with respect!) a bit pointless! That's the exact problem with using a proper valve tester to assess valves for a guitar amp too - they just don't apply the same high voltages as the valve will see in use, and a valve can easily pass as good on the tester and then fail in the amp."

    Yes, well "for indication only then"!

    When we had a problem with some EL34s a few years ago, my "tester" was an Artisan 100 run at 253 mains in and heavy music running through it at full chat. The load was a pretty evil reactive jobbie and I used to fit each valve, set bias then give it a bit of grief with a small, rubber clad screwdriver handle. Some 10% of about 100 flashed mightily and died blowing HT and sometimes mains fuses. About 25 % flashed once and took out just HT fuse.

    Then the amp went back on the artist loan roundabout!

    Dave.

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  • ICBMICBM Frets: 71960
    Catthan said:
    Well they were 2 Mesa 6l6s which as we've discussed I can't try without risking damage and can't build an apparatus to test them so what else can I do with them?
    Give them to someone who can.

    If they're new(ish) Chinese or Russian ones I don't have a huge objection, but it's still a waste if you can find someone who can test them.

    If they're old US-made Sylvania STR415s then they're some of the best and most valuable 6L6s ever made and you'd be foolish to throw them away until you've proved conclusively that they're both dead.

    "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." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • CatthanCatthan Frets: 355
    They came out of a 2011 MkV so definitely not old sylvanias; Mesa 6l6 GCT str 440.
    They are expensive but from what I read they are not the ones boogie Mk old-timers favour..
    I'd have to drive a bit outa town for a tech to test them or ship them off somewhere and I'm sceptical as to whether it's worth the hassle tbh as chances are they are broken and that's probably why they stayed on my desk and not in my stash. 
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