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Thanks Steve -

For a very well put reply.

As you describe, testing GM won't tell you poop about how well matched a pair of tubes will be in a real amplifier. As one example I can show you lots of pairs of 6L6GC's or 5881's that match perfectly for GM and even plate current on a Hickok 539C or TV7 and are way out of balance in a Heathkit W3. Just because tubes are well balanced at 150v in a tester doesn't tell you squat about how they'll act at 400v. Put'em on a bench supply with regulated screen, plate and bias voltages at the actual operating point and watch the plate currents - you'll see why!

If you're willing to take the time, perhaps the most realistic tube tester in the world (other than the actual equipment the tube will be operating in) is a bench supply with current meters.

I'd be suspicious that the KT77's are fine, but poorly matched at the operating point actually used. I'd also be suspicious that the buyer's tester may have an oscillation issue. It would help to know what sort of equipment the buyer (and seller) are using, particularly for testing. The real answer is to curve trace out the KT77's.

Lastly, this underscores the value of purchasing rare tubes from a good dealer. Much of the Ebay stuff is questionable imho. I can't laugh hard enough when I see folks buy untested 845's at $500 each. Gosh, If I was dishonest I'd make a killing on all of the gassy 845's and WE211's I use as display pieces. Sure, I know they're useless - but easy to sell "as is" and I can honestly say that my tube tester doesn't test these types :)





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  • Thanks Steve - - Gary Kaufman 05/8/0507:59:06 05/8/05 (0)


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