I suspect that Nagra test and match these tubes to standards significantly above what you're likely to acquire off the shelf.
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Nagra burns-in THEN critically grades the tubes to a super fine degree (color coding the tube base red,green or black depending on noise level).
Sure you can roll your own, and have had many owners successfully, and to rave reviews, use NOS tubes. The PL-L has just 3 tubes.
A typical Nagra tube kit may contain three different brands, but usually Sovtek and EI.
Nagra introduced a cheaper "Dealer" tube kit that is just a cardboard box with the three tubes, selling for much cheaper than the alternative. That is a deluxe kit with Nagra gloves, wrenches and tubes, enclosed in a nice clamshell case with foam cutouts for a bit more.
Disclosure: I used to be their US Service Manager.
I've had a tube get noisy on my PL-L, I've order the tube kit b/c of the comments above on testing/specs/matching Nagra does.
On the side, I have tried a few other 12ax7's (The TJ Full Music 12ax7's are pretty nice-more detail/bottom end...brings the image forward somewhat). You do get a gain here or there, but seem to loose something here or there also. The stock tubes seem to be just about perfectly balanced on all parameters.
I think the Nagra tube kit is about $250. So if you buy a few 12ax7's doesn't take long to burn through $250 in NOS tubes...and be underwater in comparison...
Let us know what you decide...
I am sure there are many people who would be happy with the stock tubes which a manufacturer selects for quality out of larger lots. However the tubes the manufacturer must choose from are only those that are currently available in large enough numbers to permit a reliable supply chain. The sound of those tubes are still limited by the inherent quality of the materials and processes that go into making them.
Many old tubes are indeed noisy or have other faults but there are still quite a few around that simply sound better than the stock tube does no matter how carefully selected. I love my old tubes and yes, I got some that aren't good, but I got many that simply blow current production tubes out of the water.
I must say that I am thankful that great strides are being made in current production. I have been very impressed with the current Gold Lion tubes in particular. Indeed some manufacturers are using the GL tubes, but I remain amazed by how many products come with the cheapest tubes around. I wonder if there isn't some silly misguided penny pinching going on in some cases.
How do you know if your tubes need to be changed? How many hours?I looked inside my PL-L because is thought there was a usage gauge but I did not see one. I just googled and I do not think that this was in my model but apparently was in older models.
It could be my imagination but I do think the sound is degrading. Not as liquid and maybe a little harsh at times. I do not have a lot of experience with tubes so curious of thoughts.
Do yourself a a real favor and get your selected very low noise and well matched tubes from Jim McShane over at audio asylum. Very fair prices, great service and advice. I have bought mainly new production but also a small number of NOS My experiences with him have been really positive.
P.S. I am not a shill, just very pleased with this dealer.