Query the seller or "Uncle" Kevin at Upscale Audio. Question: Any indication that the tubes are from the same lot/batch? Or were they made at different times?
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Some of my my 12au7's glow a little more than others. If they sound good,then I would not be concerned at all... Happy listening!
Tube Life from... http://www.primaluna-usa.com/tube-issues/
"If you see one small signal tube glowing a little brighter than the other, that is nothing to worry about. "The part that glows is the heater", which is similar to a filament inside a light bulb. So it either lights up or it doesn’t. If it looks dimmer, that’s because it’s slid up into a sleeve called a cathode, and during assembly the heater may stick up and out of the top a little more or a little less. This has zero effect on performance."
@aolmrd1241 Hey Mark. Your resonator is a rarity! The designer/engineer told me that he was waiting to sell out the remaining stock of the Mark 2 versions before marketing the Mark 3. I haven't seen them for sale yet. Timing! I got lucky since he just received the Mark 3 back from the fabricator and was excited to share the results. My room is just too small to run two units at the same time. Enjoy!
Hi Jon. I’m using these M8136’s in my BellaMax preamp. It’s a hybrid design of Musica Bella and Purity Audio by Bill Baker of Response Audio. I’ve got about 6 pairs of 12AU7’s from different manufacturers. This is the only pair that displays different brightness levels. Having observed a 5687 driver tube glow & blow on my amp, I was concerned. Glad to hear more confirmation that it’s not an issue. I just purchased a ’previously owned’ Conrad Johnson D/A-2B dac. Really, really nice!