A cautionary note about the Sophia 6SN7: See the following thread from 2012, in which I and another member both reported that it caused hum problems in our VAC Renaissance amplifiers, which I determined was due to excessive heater-to-cathode leakage.
https://forum.audiogon.com/discussions/vac-30-30-sophia-6sn7-s/As I said in that thread:
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.
I recall that Ralph (Atmasphere) commented some time ago in another thread that he rejects any tube measuring less than 10 megohms.
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.
What I had purchased was a matched quad of Sophia’s "grade A" version of the tube ($100 per tube), with their optional one year warranty ($18 per tube at that time, which I see they’ve increased to $20). To update the history I reported in the other thread, I eventually had Sophia exchange the four tubes for a new matched quad, under the warranty. The replacement tubes worked well for a few months, even though their heater-to-cathode leakage did not measure a great deal better than on the first set. After a few months, though, the replacement tubes developed the same hum problem, and I retired them to my tube collection.
If I had speakers that were significantly less efficient (mine are 97.5 db/1 watt/1 meter), or if I had used the tubes in a different amplifier or other component, it seems quite conceivable that I would not have observed this problem.
Best regards,
-- Al