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Thread: Tube Tests and Specs

  1. #1
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    Tube Tests and Specs

    Hello,

    Can someone please confirm the tests that should be done on NOS preamp and power tubes? My understanding is preamp tubes should be tested for mutual conductance measured in micro ohms, gain, shorts, leaks, plate current and microphonics. Power tubes should be tested for transconductance, plate current (at 400+ volts), shorts, leaks and microphonics. How close should the plate current values be when trying to match EL34's?

    Is there a source for the specs for EL34's, 6V6GT, 6L6GC, 12AX7, 12AX7A and 12AT7's?

    Thanks very much.

  2. #2
    Supporting Member Jazz P Bass's Avatar
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    Tube Data Sheets


  3. #3
    Noodle of Reality Steve Conner's Avatar
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    Put them in your amp and rock out?
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  4. #4
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    Best tests for tubes is always in the circuit they're to be utilized in...

  5. #5
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    Put them in your amp and rock out?
    Best tests for tubes is always in the circuit they're to be utilized in...
    +1

    I know what your looking for is a way to buy tubes (NOS or otherwise that can be very $$$) that will guarantee their suitability... Good luck with that. Buy from a reputable seller and you SHOULD get usable product... He said. Recently I bought some preamp tubes from a reputable seller and one out of four had a bad heater imbalance that caused hum noise. But overall I've had pretty good experiences with reputable sellers. I guess my disclaimer is that the game is played a little loose IME. You need to be clear on your criteria. If the seller offers tubes with "low microphonics" at an upcharge and that's what you need, then you have to pay for it to be sure of what your getting. AND, as stated already, the only way to know how a tube SOUNDS is to listen to it in the circuit. Tube reviews are always subjective and often highly biased (pun intended, but true).

    Chuck

  6. #6
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    Best tests for tubes is always in the circuit they're to be utilized in...
    [2]


    and add:
    Tube testers are fascinating pieces of hardware, but are overkill in one way: they are made to test *zillions* of tubes, extinct eons ago and, if surviving, not much related to our work.
    On the other side, sometimes they don't test exactly what we need at the moment.
    Really, 99% of our work rests on some 12A*7, EL84, EL34, 6V6 or 6L6 or some of their sisters.
    I have sitting on my bench an old nondescript chassis where there's something vaguely similar to an AX84 creation, with a twin triode noval socket preamp, a SE power amp with a noval and an octal socket, with the pertinent pins parallelled, and a -10 to -60 V bias pot.
    I can plug there most any tube I want (from the list mentioned), actually listen to (and measure) its noise and hum, and by tapping it with a Bic pen (no endorsement implied) have a practical idea of its microphonism, also measure whatever bias it needs to run, say, 30mA , which gets written on a little sticker, etc.
    I can also measure real gain easily.
    I forgot: no NFB.
    Sound?: I find that highly subjective, I let the end user to pick whatever he finds best.
    The good part of such a little rig is consistency, everybody gets tested on the same amp, which is never sold.
    Anyway the little bugger does sound good, as most minimalist amps do, and many want it "as-is".
    Maybe it's true that beauty lies on the eye of the beholder.

  7. #7
    Senior Member Old Tele man's Avatar
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    ...an analogy: "...we listen to SOUNDS not NUMBERS..."

    ...well, at least with analog, since digital is nothing but zero's and one's numbers (with some time-constant delays between pulses)!
    ...and the Devil said: "...yes, but it's a DRY heat!"

  8. #8
    Supporting Member Jazz P Bass's Avatar
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    Digital

    Quote Originally Posted by Old Tele man View Post
    ...an analogy: "...we listen to SOUNDS not NUMBERS..."

    ...well, at least with analog, since digital is nothing but zero's and one's numbers (with some time-constant delays between pulses)!
    And interleaving, parity check & error correction.

  9. #9
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    Quote Originally Posted by Mayfair View Post
    Hello,

    Can someone please confirm the tests that should be done on NOS preamp and power tubes? My understanding is preamp tubes should be tested for mutual conductance measured in micro ohms, gain, shorts, leaks, plate current and microphonics. Power tubes should be tested for transconductance, plate current (at 400+ volts), shorts, leaks and microphonics. How close should the plate current values be when trying to match EL34's?

    Is there a source for the specs for EL34's, 6V6GT, 6L6GC, 12AX7, 12AX7A and 12AT7's?

    Thanks very much.
    The important one is testing for shorts. Unless you like using screen resistors as vapor producing test devices.

  10. #10
    Senior Member ThermionicScott's Avatar
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    Quote Originally Posted by Prairie Dawg View Post
    The important one is testing for shorts. Unless you like using screen resistors as vapor producing test devices.
    Yep. I'd much rather find out a tube has a short (or is leaky) in the tester rather than by frying something in my amp!

  11. #11
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    Quote Originally Posted by ThermionicScott View Post
    Yep. I'd much rather find out a tube has a short (or is leaky) in the tester rather than by frying something in my amp!
    Well, you can go for the olfactory carbon comp aroma or the metal film light show.

  12. #12
    Senior Member ThermionicScott's Avatar
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    Quote Originally Posted by Prairie Dawg View Post
    Well, you can go for the olfactory carbon comp aroma or the metal film light show.
    Well, I once turned a 10K carbon comp into a slightly-cracked 11K in my 5F2A by thinking that 7591s and 6V6s were pin-compatible -- oops! I'm not sure if the tube was shorted out before or during that operation, but it taught me to check the tubes in a tester first.

    - Scott

  13. #13
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    When I was a kid growing up, my Dad would take me to the electronics store where they had this big tube tester set up for anyone to use. Dad was into building Heath Kit radios and stuff. Very cool to listen to the BBC back in 1968 (as an 8 year old kid).

    We won't get into the horror that was him trying to teach me morse code...

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