Mullard M8136

Last year, I purchased a ‘matched’ pair of NOS M8136’s with the Mullard logo. They’ve been sitting un-tested in a drawer. Three days ago, I put them in my preamp. They sound very nice. The glow at the bottom of one tube is much brighter than the glow on bottom of the other. There is no tube noise or arc’ing at all. The vocals are dead-centered. I switched tube positions in the preamp - and the vocals remain centered. After playing several tracks, it’s difficult to tell if there are any differences between the channels. There could possibly be a tad more volume coming from the channel that has the tube with the brighter glow. If so, it’s not much. Certainly not enough for the balance to be lopsided. Perhaps, this bright glow is normal for the M8136 – and maybe the other one is actually dimmer than it should be. I don’t know. Yet, this glow is brighter than all my other 12au7 tubes. Unable to return them since I waited so long. Any opinions if this tube should be removed before any problems arise. Though, they do sound quite nice.

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?
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...

"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   Thx for posting that pertinent info.  Good to know.  Last night, I turned off the lighting in the listening room - and the one tube is WAY brighter than the other.  But, there is no distortion, noise or arc'ing - or any issues.  Actually, the music sounds kind of great.   All is good.
Good to hear Arnie. By the way,still digging on the Chartres Mark 3...
@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!
@steakster Hey Arnie I was hoping to check your system out but it's not posted, what are you running those Mullards in? I've also had tubes glow brighter than others in a matched quad with zero issues or ill effects.
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!
Arnie nice and I believe that’s a tubed dac, running an AN Signature Dac-3 also tubed so I am sure it sounds lovely and rich. And I've read a bit about Musica Bella looks like well made interesting gear. Glad you’re enjoying everything!