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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: they're herePosted by hagtech on: 1/12/2007
Tubes came today. 

solve the problem of what we should test for

I don't think you can get one tester to do everything.  Not only does one need the basic parameters such as transconductance, but you have shorts, gas, leakage, microphonics, secondary emission, etc.  All kinds of relevant stuff to check.  The question is what to check for.  I think the way to answer this is with experience.  And RAM is way ahead of most people in this regard.  Has he explained what the 'factor' consists of?

With my limited experience, I would suggest looking at grid current.  Measure it versus bias at different operating points.  What you will find interesting is that there is a point where it passes through zero.  That's usually where I set my operating point (it's more of a locus of points).  I modified a VacuTrace adapter card and added a 10k resistor on the grid, then used a DVM to measure voltage across it. 

For me, I use the tester to weed out bad tubes, then match and grade the good ones.  Those with funny shaped curves get lost.  My amplifiers ship with these modern production tubes that I screen.  Can't remember the last time I had a DOA.  In fact, maybe never.  But hey, I'm not really a big player like Lamm.  It's ok for me to ship with Sovtek or similar new stock tubes because customers almost always roll them.  No reason for me to put money into something they replace.  I guess the point I want to make is that tested tubes should be 99% acceptable.  So what I buy from the factory gives me about a 30% reject rate.  Same as you.  So it's about 70% ok coming in, 99% going out.


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