Getting Tubes Tested - Matching?

Discussion in '"Boat Anchor" & Classic Equipment' started by N0WTR, Jan 9, 2019.

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  1. N0WTR

    N0WTR Ham Member QRZ Page

    Finished the National NCL-2000 rebuild and I would like to get the 2 Burle 8122's that came with it (poss. original) tested to see if they are any good. I also have 2 spare sets of Burle 8122's that I was able to find - Both sellers claimed they are matched sets... but who knows.

    Is there a Ham or other outfit that has a the capability to test these tubes and be able to say yes they are good and matching or not?

    Called RF Parts and they said no and had no recommendations.

  2. N0WTR

    N0WTR Ham Member QRZ Page

  3. VE3EKJ

    VE3EKJ Ham Member QRZ Page

    You might already know this but there is a common misconception out there that matching tubes concerns the emission levels, which is all the typical tester can measure.
    That just isn't so!

    The whole reason for matching a pair or pairs of tubes is to ensure that one tube doesn't hog all the effort in driving the load. You might think you have two 50 watt tubes but if
    one is "lazy" and the other too enthusiastic you could have one fail when striving to run 100 watts.

    The normal practice is to match for idle current. This requires measuring the idling plate current of the tube for a given bias voltage on the grid. This is done with guitar and hifi tube
    amplifiers as a matter of course. A tube audio amp and an RF power amp are very, very similar - mostly by having an output transformer instead of a tank circuit to feed an antenna
    instead of a speaker.

    I've never seen a tube tester that can measure idle current. There are specialized units out there that can do the job. Rarely does a serviceman need one except for matching power tubes.
    Mostly he is looking for weak or dead receiving tubes. Also, during the Golden Years even power tubes were made close to the databook specs and any differences in idle current were not all that serious, except in rare and specialized cases.

    The good news is that you don't need an expensive idle current tester if you can do some technical work on your own. For 6146s, 807s, 6L6s and sweep tubes just look at the cathode connection to see if it goes straight to ground. If it does, open the connection and put it through a 1 ohm resistor of a watt or two rating. If you're really anal retentive about it you could bypass the resistors with a .01 cap or higher but I doubt if it will matter. Anyhow, you will now have a safe test point to measure the voltage drop from the idle current. With the rig just idling but not asked to produce some output power you can measure that current in millivolts. I usually use some jumper alligator clip wires to connect to my metre probes. Ohms Law says that with a 1 ohm resistor the reading in millivolts will be the same as the value for current - 50 mv for 50 ma. for example.

    Again, make sure you know what you are doing or else get some competent help. Also, make sure the cathodes of any pair of output tubes are not strapped together BEFORE they go to ground, giving you a combined reading for the two tubes and not a value for each of them.

    Don't worry if there is a difference of only a few ma. between your tubes but sometimes you might be talking 10 ma. or even more. That would be a good reason to try a few tubes if you've got extras or trade with a friend in order to get a more closely matched pair.

    Wild Bill VE3EKJ
  4. K9AXN

    K9AXN Premium Subscriber QRZ Page

    First meter all of the pins for shorts, then to check the idle 100ma point. Simply remove one tube and block the open hole, attach a meter to the bias supply, and power it up in low power mode. Key it up and set the bias to 100mA and record the bias voltage then do the same for the other tubes.

    Check the tubes for dents. If there are any, ,toss the tube. If the tubes have been sitting unused for a number of years don't be surprised if one should flash over and trip the overvoltage relay; then try it again. It is quite common for a 8122 to flash over once then be as good as new. The power supply was designed to protect the circuitry and tube in the event of a flash.

    We have matched them for years but have retired the test setup. The 8122 and other beam type power tubes seldom track throughout the emission range even though they may have matching idle bias numbers. We tested at 100mA, 500mA and for critical folks zero bias for 10us. The 10us test was retired as unnecessary.

    If all else fails you. I have an inventory of matched 8122's in all four quadrants of the operational range.

    Couple of questions. Have you examined the band switch especially the 80 and 40 meter contacts used for the plate padder caps, they are a weak point.

    That is a fine example of a vintage amp and if you have other concerns KM1H is a reliable authority regarding the amp.

    Regards Jim
    KB4QAA likes this.
  5. W5INC

    W5INC XML Subscriber QRZ Page

    Mr. Carl has also substituted some current Russian tubes into the 2000 amp also, which sure beats paying the price for new 8122 tubes these days. His mods to the amp for the Russian tube project is on the web and maybe here on the Zed also.

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