Question for Mark Gilmore

OTL Asylum

OTL, Output Transformerless Amplifier User Group.

While I was updating Travis Franklin's two MA1 MKII amps in February, he ordered 24 Sovtek 6AS7's from New Sensor. Before we decided I should update the amps, he had been trying used and NOS tubes from other sources. The dramatic failures became so frequent it was clear that the amps were in desperate need of at least the "resistor mod". After the update, Travis wanted to start with a full set of new 6AS7's.
Your "Care and Feeding . . ." article suggests current matching to within 20 to 25 percent from tube to tube and, by inference, within both sections of each tube. We spent an evening swapping each new tube in and out of a monitored socket in one amp and recording the measurements. The test results can be viewed at http://home.swbell.net/mtb3/. Given the 25 percent matching goal within a current range of about 50 to 100mA there appears to be only 13 or 14 out of the 24 tubes that are useable. With more to choose from a "stereo" set could of course be selected.
Although you stated in your "Care and Feeding . . ." article that emission matching is useless Travis ordered matched pairs hoping it would help. The tubes in fact have small labels on the end of each box, each with two numbers as in "40/40" denoting, presumably, each section's rank. My emission tester confirms the "matching" but as you can see from the current measurements, some tubes differ by as much as 50 percent and would not be a good choice.
On to the questions. What is the chance that New Sensor would take back the undesirable tubes in exchange for others that may also fail to "measure up"? Do you have any suggested pleadings that might sway the New Sensor RMA person? Should Travis return all 24 Sovteks and purchase Svetlanas? Should more hundreds of dollars be forked over to chase down a boatload of tubes from which to weed? How undesirable would it be to have one amplifier with "low current" tubes (correctly biased, of course) and one amplifier with "high current" tubes?

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Topic - Question for Mark Gilmore - Marty Bronstad 12:55:31 03/03/00 (9)

Re: Question for Mark Gilmore - Marty 09:44:52 03/04/00 (5)

I agree... - Kevin 12:29:02 03/04/00 (4)

Yow! - Dan 17:19:16 03/04/00 (3)

Re: Yow! - Kevin 13:25:20 03/05/00 (2)

Excellent response! - Dan 15:27:02 03/05/00 (1)

Re: Excellent response! - Kevin Covi 13:35:29 03/06/00 (0)

Re: Question for Mark Gilmore - mark 13:53:32 03/03/00 (2)

Ralph - new business? - Dan 14:56:05 03/03/00 (1)

Re: Ralph - new business? - Tre' 00:44:51 03/04/00 (0)