Help tube experts, are these tubes a "matched" set?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Gear-Junkie, Mar 2, 2012.

  1. Gear-Junkie

    Gear-Junkie Supporting Member

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    Are two 6V6 tubes that read 88 and 92 on a Hickok TU-7 considered a matched set? Can this tester even match tubes or do I need something else? Thanks
     
  2. ski_fast

    ski_fast Silver Supporting Member

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    Yes those are close as far as general mutual conductance. I would not call those a matched set as everybody with a TV-7 does. They can and will be off as much as 25 mA in their current pull. You need a Maxi Matcher to test that. Or an amp with the proper test gear.
     
  3. Gear-Junkie

    Gear-Junkie Supporting Member

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    would it be better to simply just use a bias probe (one that plugs into an amp between the tube and the amp socket to match the mA draw of each tube to within say 5 or 10% of each other) to match power tubes?
     
  4. Prairie Dawg

    Prairie Dawg Member

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    That would work for you. It's a little time consuming but if you're only doing 1 set or trying to find a set for your amp it's not worth worrying about. The maxi matcher is overrated in my opinion, because it tests only at 325 and 400vP and has a number of fixed bias voltages. 400 is not the rated voltage for the 6L6GC or EL34 and some amps like the Ampeg SVT run the plates pretty hot. If you're not testing at operating voltages you're not getting the job done. Which is why I built a tube matcher that I can run up to 650v and match test 4 6550s at the same time. It;s based on the PriceLess Amps schematic with a few differences. Here's a link to the site. http://www.priceamp.com/
     
  5. Gear-Junkie

    Gear-Junkie Supporting Member

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    So I guess the Hickok TU-7 cant really match tubes then?
     
  6. ski_fast

    ski_fast Silver Supporting Member

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    No, a TV-7 cannot match tubes for current pull which is the most important test for a push pull amp. You might get lucky and find a pair that is somewhat close.

    And yes, the bias probe is how you would measure the match.
     
  7. Gear-Junkie

    Gear-Junkie Supporting Member

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    OK Thanks, I kinda thought it so but wanted to make sure. With a Bias Probe how close should the current draw be to be considered matched? 5, 10, 15 mA?
    [​IMG]
     
  8. V846

    V846 Member

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    cool matcher, I have a simpler version but really should add a separate heater supply..



    David
     
  9. cap47

    cap47 Member

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    4 - 5 ma. is considered matched.
     
  10. clumpster

    clumpster Supporting Member

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    5% is considered matched.
     
  11. Prairie Dawg

    Prairie Dawg Member

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    On mine I used a filament transformer from an organ that was running about forty 12AU7s for the heater supply. With four 6550s it holds steady at 6.3 volts. The power transformer came out of my stash of Thordarson TV transformers. As it had no bias tap I had to pick off one HV winding and customize it ala Marshall. Also include some 82 or 100 ohm plate load resistors to stop oscillation when testing 6550s at higher voltages. Here's a rundown on it. http://judyboxamp.blogspot.com/2011/05/tales-from-technical-fringe-building.html
     
  12. Blue Strat

    Blue Strat Member

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    So, where's the schematic? ;)

    The Maxi is far from perfect, but what other commercially available item under $1000 allows you to check 4 tubes at a time with decent results?

    Fact is, if you check the tubes at multiple points (different plate voltages and grid settings) and get similar results, between 2 or 4 tubes, you can "extrapolate" to see what will happen at higher plate voltages. It's called multi point matching.
     
  13. Prairie Dawg

    Prairie Dawg Member

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    My needs were pretty specific. I didn't feel like spending the $600 it takes to buy a Maxi when I had the parts to build a matcher to my particular requirements. I figure, nothing ventured nothing gained, right?:beer
     
  14. Blue Strat

    Blue Strat Member

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    No doubt. Personally, I didn't have $1500 worth of spare time to build something that might not equal the Maxi Matcher. :D
     
  15. Patrick620

    Patrick620 Member

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    I'm glad this thread came up. I have been using a DIY socket type probe but now I know what to do with this 20# Hammond organ PS I have had sitting around for ages.
     
  16. Blue Strat

    Blue Strat Member

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    Yep, percentages mean nothing. Biasing isn't an exact science. 5mA is ALWAYS close enough. 10mA can even work ok in tubes that are normally biased in the 30's or higher.

    NO ONE can hear a 5mA mismatch in a BLIND test. ;)
     
  17. Gear-Junkie

    Gear-Junkie Supporting Member

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    so even a 10mA difference is OK, thats good to know. I dont want to reduce the life of the tubes by having them too far apart.
     
  18. Blue Strat

    Blue Strat Member

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    People never used to worry about biasing or matching. Was that good? Not necessarily. It's always best to know as much information as possible in case something goes awry. Today we're obsessive/compulsive about a lot of things.
     
  19. cap47

    cap47 Member

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    What is 5%? That is why I used 4-5 ma. and I have also seen advertised this way for matching of power tubes! 5% of what has to be determined.
     
  20. Gear-Junkie

    Gear-Junkie Supporting Member

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    Excellent point, I was wondering the same thing. And regarding the original post, 88 and 92 are only 4 numbers apart but what do those numbers represent? If its not ma then I guess their not matched.
     

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