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In the Forum: Audio For Dummies ™
In the Thread: Get, or made up a tube tester, it’s necessary
Post Subject: pike with a double twist (6c33 dive)Posted by hagtech on: 1/8/2007
Sadly I have never spent the time to try and correlate the sonics of a tube with the trace characteristics.  Ok, sometimes it is obvious.  A flat line is a dead tube.  Normally I measure and compare both sections of dual triodes.  I need them matched in my circuits.  Very well matched for balanced operation, reasonably matched for channels (for single-ended circuits I split the halves between channels (another long subject)).  I grade each tube.  Not so good tubes hit the unused bin (I throw away quite a few).

Not well matched tubes often show the characteristic of one side having a funny shape.  It's like the curves are more linear, start stronger near zero bias, but then stay flat and weaken at higher currents.  At least compared to the good tube.  I assume the funny shape would correlate to bad sound.  But like I said, I never spent the time to listen.  I just chuck 'em. 

Another test is to watch the tube as it warms up.  You don't want to push this test too hard as it can lead to cathode stripping.  As the heater warms up, I apply plate voltage.  The curves come up but max out at the available electron limit - the curves have an "S" shape.  The tops go flat.  This flat continues to rise as the cathode reaches operating levels.  Eventually I cannot see it anymore.  It all happens in ten seconds.  Now, I have noticed that one old and very used tube I had would not be strong and held onto the "S" shape.  A real good indication there was very little emission left.

Having done this for many years, I would guess that you can pick out a bad one, but not pick out a good one.  Does that make sense?  A bad tube will be obvious.  But two good tubes will measure fine and operate fine, but not guarantee sonics.  One may sound way better than the other.  In circuit they will have the same electrical performance, same risetime, same distortion, same gain.  But to the ear they can be world's apart.

estimated gain is not the same

Well, I've only done two amplifier designs.  One PPT, the other SET.  I ran the math based on average curves, not specific tubes.  So I don't think I can really answer the question.  Not only that, I think I made a mistake on the PP calculations.  The gains came out right, but not the output impedance.  The first OPTs I used had the specified turns ratio.  But the transient performance sucked.  I plopped in a very similar tranny (or so I thought) with a lower turns ratio and now everything came together.  Huge difference.  But the output impedance was as low as I had calculated for the other tranny.  So I dunno. 


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