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Help on Telefunken tubes

Hi, My problem is this: I have a tele tube that has most of the label wiped off. I need to know if it's a ax7 or au7. I have other ax7's but this tube in question looks just like the others. It is a smooth plate and I don't know if au7's come in ribbed and smooth plates. One thing I did notice is that the little pins at the top of my ax7's look the same. The tube in question does not.Could that be an indication? Any help would be a life saver. Thanks in advance, Bret (BTW I don't have a tube tester)
south43
4 responses Add your response
south43 OP
386 posts
 
Thanks Albert for the advice. The tele in question turned out to be a au7. Good listening!
swampwalker's avatar
swampwalker
5,138 posts
 
Marty- terrific hint. many thanks.
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albertporter's avatar
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albertporter
4,949 posts
 
Telefunken 12AU7 and 12AX7 come in both smooth and ribbed plates. I just looked at several samples of each from my collection and the plate length and overall construction appears to be almost identical between these two Telefunken tube types.

The only way I can see is by the pin out to the ring getter. To check this, turn both tubes upside down, and look for the gap in the (otherwise) solid row of pins.

With this gap turned to 12:00 O'clock position, if the first pin CLOCKWISE supplies the getter, this is a 12AX7.

If the ring getter is connected a few pins further clockwise, this is a 12AU7. Hope this helps.
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