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Thread: Question about matching tubes

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    Member MikeH's Avatar
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    Question about matching tubes

    I have recently come across several NOS 6V6 tubes still in cartons. Of course, they are not matched pairs. Here's my question:

    Can I use a biasing socket (the sort that interrupts the connection on pin 8 to measure current) and plug that into a single ended amp (say a blackface vibro champ- because that's what I got) to measure the current on each tube to see if any are reasonable matches? I'm not looking for any specific tube data, just wether or not any of these tubes would make a good pair for my deluxe, but the champ obviously uses a different hissing scheme.

    Thanks!

    (I realize there are varying opinions on what constitutes a "reasonable match" and I realize I could also plug them into the deluxe and measure the current on each one but I don't want to have to adjust the bias pot constantly to make sure I'm not damaging tubes, etc.. It would just we way simpler if I could use the champ)

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    Old Timer Leo_Gnardo's Avatar
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    Since you plan to use those 6V6's in your Deluxe, it only makes sense to use the Deluxe to test them with your biasing socket. Champs & other SE amps generally let the output tube run at maximum current and that's certainly no reliable indicator of how they will behave in a fixed bias situation.

    If it's at all possible, before you plug 'em into your Deluxe, run those 6V6's thru a tube tester to make sure they're basically working, not shorted or otherwise compromised.

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    As the Champ is cathode bias, I think that most tubes that aren't too far off 6V6 bogie should settle to a closely similar current, so my guess is 'no, or at least not very well'.
    But what you could do is measure the cathode voltage of each tube and look for close matches.
    But the downside is that the Champ's screen grid voltage is probably a lot lower than that of a DR, and the DR's idle point should be a lot different to that of a Champ anyway.
    Such significant differences in their idle operating points may make the Champ useless for this.
    Probably better to use the DR, eg set the bias safely cool and check each 6V6 for cathode current at that same bias, one tube at a time.

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    Lifetime Member Enzo's Avatar
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    I will write slowly because you are in Ann Arbor...


    Remember that when Fender et al built amps, they didn't match tubes, they just grabbed two from the bulk box and put them in the sockets.

    I'd use the Deluxe too. Pick one socket and try each tube in it. The amp won't care that the other tube is missing during the tests.

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    Member MikeH's Avatar
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    Quote Originally Posted by Enzo View Post
    I will write slowly because you are in Ann Arbor...


    Remember that when Fender et al built amps, they didn't match tubes, they just grabbed two from the bulk box and put them in the sockets.

    I'd use the Deluxe too. Pick one socket and try each tube in it. The amp won't care that the other tube is missing during the tests.
    Ahhh... So I can do this with just one tube installed?

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    Lifetime Member Enzo's Avatar
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    You could do two at a time, but be aware the two sides of the amp won't be identical, well, unless it is a random coincidence.

    In fact that might be interesting. Stick two tubes in the amp and measure them, then swap sockets and remeasure. How close are the tubes readings socket to socket?

    But if all you want is a tube current reading for comparison, yes, you can do one at a time. And that way every tube has the same conditions.

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    Quote Originally Posted by Enzo View Post
    I will write slowly because you are in Ann Arbor...

    Remember that when Fender et al built amps, they didn't match tubes, they just grabbed two from the bulk box and put them in the sockets...
    Might it be feasible that they bought selected tubes, ie that would idle within a reasonable range, thereby avoiding tubes too far from bogie?

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    Lifetime Member Enzo's Avatar
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    They wouldn't bother, that would raise the cost of the tubes considerably.

    The amp doesn;t care if the tubes are matched or not. The amp can't tell the difference between mismatched tubes and a symmetrical wave signal and asymmetrical signals in a matched output stage.

    It is amp owners that feel the need for matched tubes.

    I do buy them matched, as it tends to reduce hum in the output stage - at least potentially. Also, when I buy matched tubes, I know they have been burned in to some extent and thus are tested. Bulk tubes are cheaper, but it is luck of the draw. When Fender buys 100,000 tubes, if they get the occasional bad one, they can pitch it and grab another. When I buy a half dozen tubes, I don;t want any duds.

    I don't know which Deluxe you have, but look at the drawings for the popular 5E3. On the layout drawing upper right, see the notes?

    It says "values shown + or - 20%". That is how tight Fender considered their specs. Not very tight at all.

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    Bent Member Chuck H's Avatar
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    BUT... Fender still bought tubes from more consistent and reliable sources. So, for example, a "batch" would likely have similar enough performance. I don't think that's nearly the case with a "lot" of NOS tubes. Who knows how or why each one got into the "lot"? Who wouldn't trouble to match up NOS 6V6's if possible? That's the first way to increase their value in the market. There's every possibility that untested or unmatched tubes are in a "lot" because they are the ones that didn't have a mate when matching was done. In other words, I wouldn't consider grabbing two tubes from the tube bin at Fender in 1964 to be the same as using two "unmatched" antique tubes from a small lot without provenance in 2018.

    But I buy tubes this way too. That's why I know you need a selection process before using them. I test them one at a time in a push/pull circuit to cull duds and match close-ish pairs. IMHE when you buy a "lot" of "unmatched" tubes there are always some duds. Maybe they even read fine at idle, but do something hinky when you put the screws to them.

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    Lifetime Member Enzo's Avatar
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    They may have been more consistent then, but they still varied all over the map. If I need an example, I can point out the many times someone played a four power tube amp with only three in it and loved it. The amp didn't care about the 2/1 mismatch.
    I wouldn't consider grabbing two tubes from the tube bin at Fender in 1964 to be the same as using two "unmatched" antique tubes from a small lot without provenance in 2018.
    Sure, but now we are wandering afield. My initial point with Fender and random tubes was they didn't worry about it then, and it is not really a concern now. it only matters to the person buying tubes, not the amp.

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