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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: Not on the new tubePosted by hagtech on: 1/25/2007
I really do not understand what conclusion you make by shutting the tube down.


I make no conclusion.  It's just an interesting thing to watch.  A good experiment.  I was hoping it would be a clue for everyone here.

Since the 6c33 can drive way more current than my machine can trace, I decided to run 1/2 the tube at a time.  I wired in a switch on the adapter card to select between the pins 12 heater and the pins 76 heater.  The hysteresis shown is seen only on one side of the "old" tube.  It is not seen on the "new" tube.  I can only assume the tube in question has developed a problem over time.  The video starts showing the hysteresis (that may not even be the correct word for the anomaly) and ends up showing the good side.  What does it mean?  I dunno.

But I can theorize how this might manifest itself.  In order for the loop to occur, something in the tube has to change pretty drastically.  And in a hurry.  We're talking at audio frequencies.  Maybe mid-bass region.  I don't know if it is a thermal issue, emission,  grid current, or whatever.  But I am NOT driving this tube hard.  My guess is that the distortion would be very audible.  IMD for sure.  It also only shows up when grid voltages are more negative.  That is, more a function of grid drive than plate current.

On the other hand, when the heater is shut down, the loop immediately shows up on all the curves.  That could indicate it is an emission issue. 

jh

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