Amps Atma-Sphere M-60 Mono blocks OTL design

Hi Bob,

I’m not sure that even the ancient drugstore-type tube testers some of us remember from our childhoods measured heater-to-cathode leakage. Most of the better professionally oriented testers from those days did, however. (I have a vintage Hickok 800a).

For most users I suppose the only practical alternative is to see how the tubes perform in the particular application, and hope for the best. And given the many positive experiences that have been reported with Sophia tubes, including the 6SN7, I suppose the odds of success are good. Although note that in some of the cases I referred to the hum problems didn’t arise until the tubes had been in use for some period of time. And of course I have no way of knowing if the problems that I and a couple of others reported represented atypical examples of the particular tube type, or were due to a combination of tube characteristics that are more typical combined with sensitivity of the particular component designs to those characteristics.

Note also that as I said in the thread I linked to Ralph has indicated in the past that he rejects tubes having heater-to-cathode resistances of less than 10 megohms (i.e., 10 million ohms). And for the 8 Sophia 6SN7s that were in my possession (4 originally purchased; 4 replacements provided under the warranty) ...

The heater-to-cathode leakage in my Sophia’s, as measured on my vintage Hickok tube tester, was as low as 3 or 4 megohms on some sections of some of the tubes, and was in the 5 to 10 megohm area on most of the others.

... Also, I’ll mention that on my Hickok 800a tester all of the vintage 6SN7 tubes I have measure well above 10 megohms, and in many cases are high enough to be unmeasurable.

FWIW, I personally don’t envision Sophia tubes as being in my future, despite all of the glowing praise they have received from others.

Best regards,
-- Al

Thanks Ozzy and Almarg.
You guys are the best.
Bob

So far we've yet to hear about any hum problems with the Sophia or TJ Music tube. Al's report is the first I've seen.

We made a circuit change in our amps regarding how our filament circuit is handled about 12 years ago. That really reduced the hum issues caused by cathode/filament leakage; every now and then we run into a bad tube in the field causing a problem but its nothing like it was before we made that change. So I suspect much relies on the circuit design.

FWIW in our preamps the leakage would be a non-issue.

Ralph, I am still awed by what your amp and preamp can do.
After growing up on tubes, I never thought I would return to them, but, you did it.
Bob