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Thread: Testing amp tubes ?

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  1. #1
    Join Date
    Aug 2008
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    Default Testing amp tubes ?

    I've read a number of posts about testing amp / power tubes and have gathered that the only way to test is in the amp the tube is used in .
    I have not had an amp that did this in my shack , but have heard and repaired some that did blow resistors and other components with bad / shorted tubes .
    So what I have done [ so far 811 & 572B ] is use my TV-7U/D tester for shorts before putting into amps for more testing .
    I've also read that you can put 1 tube in a 2 hole amp and test for output one at a time ?
    But I question if the imbalance with just one tube can be an issue ?

  2. #2

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    Depends on the amp design. Some amps have filaments wired in series, in which case obviously that approach won't work.

    Otherwise, it might work obviously at reduced power and the plate tank circuit may not tune properly; but for just "idling" the tube to see if it fails with B+ applied, that should work fine.

    Does the tester provide 2750Vdc for the anode? If it doesn't may not be much of a test, since when tubes arc internally, they often won't arc at reduced voltage, only at "full" voltage. I've seen lots of 572Bs that work fine at 1800V and fail at their normal operating voltage, which is around 2700V.
    What if soy milk is just regular milk introducing itself in Spanish?

  3. #3
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    Default

    The amp I finished putting back together ( trade from a from a friend ) is a Yaesu FL 2100B .
    Trying to get some tubes in it to able to sell .
    And to get some tubes to work on other amps .

  4. #4

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    Most 2-4 tube amps will work just fine with one for testing on 20M at reduced drive power. My SS driver rig for the amp repair bench rarely needs the built in tuner to provide that power.

    Any tube can be DC tested outside an amp, just build a test jig and instead of using 2500VDC or more just use 900-1000V and a variable bias supply that will allow some serious plate current. Use the manufacturers tube charts to plot the performance curves. If they equal then likely they will be OK at HF RF.
    Running anodes red at that low voltage also permits regettering at a level that will not sustain an internal gas arc. Fuse everything in case there is a dead short that wont show on a DVM.
    Run air cooling as needed.

    Carl

  5. #5
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    Default

    That brings another subject that I do not have a clear understanding yet .
    Biasing for mismatched tubes and or biasing in general , and you mention variable , what can be used to vary the bias ?
    With getting some tube as new or matched becoming harder , the need for matching a variety of mixed used pulls .
    And building a power supply just for testing amp tubes is one of those projects that is on the list
    Thanks John

  6. #6

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    Biasing for mismatched tubes and or biasing in general , and you mention variable , what can be used to vary the bias ?
    Any DC supply in the proper range and capable of negative or positive voltages. I use Variacs to adjust filament, and plate voltages, and the bias is an old HP lab supply of 0-50V. Triodes, tetrodes and pentodes are strapped grounded grid.

    Carl

  7. #7

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    Carl, do triodes need to be ran like that? I thought they didn't have getters per se, but that just getting the anode hot, it acted as a getter itself to get gasses out of the tube if they haven't been ran for a while... or did I answer my own question?

  8. #8
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    Carl , not sure what your saying , adding another power supply and a variac to an amp ?
    Because the question was about using mismatched tubes in a amp.
    I am guessing that because of amplifying RF , that there could be issues with a wire wound vairac ?
    I am not remembering any kind of variable resistor that is not wire round , or meant for RF , but it would seem that a device like that would have made ?

  9. #9

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    Quote Originally Posted by KE6IIJ View Post
    Carl, do triodes need to be ran like that? I thought they didn't have getters per se, but that just getting the anode hot, it acted as a getter itself to get gasses out of the tube if they haven't been ran for a while... or did I answer my own question?

    Yes and Yes.

    The getter material is deposited on the anode of many/most glass transmitting tubes irregardless of the grid count. This includes metal and graphite anodes.
    Those with the seperate getter and the shiny deposits on the glass are easily seen.

    Carl

  10. #10

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    Carl , not sure what your saying , adding another power supply and a variac to an amp ?
    Because the question was about using mismatched tubes in a amp.
    I am discussing building a seperate tube testing and regettering jig and mismatched tubes wasnt brought up earlier.

    Mismatched tubes can often be somewhat forced to share equal current by adding a 33 Ohm 1-2W carbon or carbon film resistor in the grid lead. Heath did that with the SB-200 and it also adds an extra level of parasitic suppression.
    However in most other amps you would have to be willing to unground the grid, add an up to 33 Ohm resistor to ground, bypass with a .01 1KV disc and ~220pf silver mica and experiment with resistor values for equal idle plate currents.

    IMO it is not worth the effort unless there is a marked visable difference in anode color which may also indicate one tube is at the point of worn out low emission.

    Carl

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