Before I pop these tubes in an amp..?

Discussion in 'Glowing Bottle Tube Amp Forum' started by MrGibbly, Jun 30, 2019.

  1. MrGibbly

    MrGibbly Tele-Afflicted

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    I recently came across a box of old vacuum tubes from among my grandfather's things. They range from 7N7s, to all kinds of other tubes for TVs and radios. Only 5 were usable in guitar amplifiers. All are USA made RCA tubes from what I can tell.
    • 5U4GB
    • 6V6G
    • 6V6GT
    • VT-107-A
    • 5Y3GT
    I don't have or have access to a tube tester. I've visually inspected all the tubes and the 5Y3GT is probably bad (rattles and the base is loose). The others check out. No white inside around/at the getter; no other discoloration. No rattles. I've cleaned them up and cleaned the pins.

    What else can I do before I plop them in a known-good circuit and see what happens?
     
  2. Wally

    Wally Telefied Ad Free Member

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    FYI, 7N7’s were used in some guitar amps. I have two Masco amps that use them. Masco 15’s, iirc. There are numerous tubes that are not the ‘usual’ tubes used in Fenders that were used in guitar amps and are of interest.
    I am hesitant to install unknown tubes in an amp circuit without using a tester to check for shorts. Ommv....but I have had a tester for so long that I am accustomed to doing simple precautionary tests.
     
  3. King Fan

    King Fan Friend of Leo's

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    "Rattles" isn't usually good, but a base can sometimes be loose without breaking the vacuum. Brent Jesse's helpful tube site includes this good advice:

    "NEVER pull an octal tube out of the socket by the glass, as this will often break the adhesive bond. A slightly loose base will not affect the operation, sound, or lifespan of the tube. If you accidently break a base loose, you can re-bond the base with a thin bead of gel-type super glue or marine varnish. Allow the adhesive to cure for 24 hours before using the tube."​
     
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  4. MrGibbly

    MrGibbly Tele-Afflicted

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    Is there a way to test for shorts by using a multimeter across certain pins? I don’t want to injure myself or an amp, but I also don’t want to give up what may be perfectly good USA-made tunes...
     
  5. jimbo735

    jimbo735 Tele-Afflicted

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    I would try locally there's gota be someone there thats a tube nerd that would check them.As mentioned not wise to just stick them in there.
     
  6. dan40

    dan40 Tele-Afflicted

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    The only thing a meter would help with is to tell if the filament still has continuity. These would be the only pins on a power tube that should show continuity. None of the other pins are connected to each other inside of the tube so they should read open if there are no shorts.
     
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  7. schmee

    schmee Poster Extraordinaire

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    re: rattle. Some old tubes are loose on the base. It may be fine.
    The thing about old tubes that aren't in their packaging, (and many that are!) is they were more than likely removed and replaced from something. So they may very well be noisy, low reading or bad. Just hard to say.
    If you try the rectifiers, watch your amp and power tubes closely... a bad rectifier can take out a power tube or a transformer fast.
     
  8. magic smoke

    magic smoke Tele-Meister

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    I have tried mystery tubes in untested circuits before. Current limiters are cheaper to build than transformers....
     
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  9. tele_savales

    tele_savales Tele-Meister

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    AWESOME! I mean, really sorry man, that sucks.
     
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  10. DavidP

    DavidP Friend of Leo's

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    Yeah, a simple lightbulb current limiter would provide some insurance from catastrophic failure.
    Relatively easy DIY project -- just Google it and you'll quickly find parts & build info.
     
  11. archetype

    archetype Fiend of Leo's

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    That's how you test for shorts using an Ohm meter. Test pairs of pins that shouldn't have continuity. If any element is shorted to another it will show continuity.
     
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  12. magic smoke

    magic smoke Tele-Meister

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    @tele_savales No big deal... The current limiter saved a transformer, and now I know which tube contains the purple lightning. Swapped it out and the amp fired up and sang . All is good
     
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  13. Milspec

    Milspec Friend of Leo's

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    I have the 7N7's in a mid-40's Epiphone amplifier. Actually a pretty common tube for amps in the '40's to mid-'50's.
     
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