Good Tube Selection for Atma Sphere S30?

12-14-11: Atmasphere
In practice, tubes with leakage can buzz ...

Ralph, assuming you are referring to heater-to-cathode leakage, how many megohms of leakage resistance would you consider to be the threshold of acceptability for a 6SN7, as measured say on a vintage Hickok tester?

Also, based on your experience is it expectable that a tube showing marginal leakage (perhaps 2 to 5 megohms, for example) during its early hours of use will improve over time?

Thanks! Best regards,
-- Al

Thanks, Ralph. Excellent info.

What prompted my question is that I am using a matched quad of a highly regarded and expensive current manufacture 6SN7, of Chinese (not Russian) origin, which in some sections of some of the tubes measure as low as 3 or 4 megohms. I won't mention the name, but it is reminiscent of a famous Italian actress :-)

That results in a very slight hum with my VAC amp and 97db speakers. The tubes sound wonderful otherwise, btw. Some vintage CBS tubes I have, which sound poor in comparison, have no measurable leakage and do not produce any hum.

Thanks again. Best regards,
-- Al