Manufacturers' warning not to change tubes

It is possible that the tube gear in question could be tough on tubes. I've seen companies, like Audible Illusions, recommend NOT to use NOS tubes because their design is tough for older tubes. They don't like folks spending hundreds of dollars and having tubes die within a couple of months. Many new tubes may sound a tad grainier, but are built more ruggedly. So it's possible that the tube designer is just trying to warn folks the their designs may be more treacherous on tubes.

I totally agree with Jmcgrogan2. Let me also add that manufacturers like ARC 'voice' their products with a particular type or types of tubes that are not NOS. They do this so that their products sounds consistently good from unit to unit. This is also true of output tubes where specs can vary widely with NOS tubes. Sometimes using NOS tubes creates more problems then it solves.

Many companies sell replacement tubes at a very elevated price. They want to sell you these rather than you buying tubes elsewhere. I do see the warrenty issue if you put in some bad tubes that cause the failure of the amp they should not have to honor their warrenty.
Alan

We have no problem with customers installing output tubes that they got from elsewhere.

People often ask us which 6SN7s sound best in our amps and we tell them what information we have from the field as well.

However, people want to roll tubes in our preamps too. In the line stage we have no problem with that, but in the phono section its common that that NOS tubes will simply not meet our specs for noise. That results in a noisy phono stage, so we recommend that customers get low noise tubes from us so the noise floor will meet spec. I'd have no problem with them getting the tubes elsewhere if we thought that they would have a chance of getting quiet tubes! If you have an all-tube phono section capable of LOMC operation, you have to have hand-picked tubes, and most of the NOS tubes that were quiet enough were sold off the shelves a good 30-40 years ago.

Being a manufacturer of tube preamplifier, I can say that we have come across issues with many NOS tubes. Many consumers have contacted us with issues including poor channel balance or poor sound. My first comment is always to install the tubes that came with the unit and report back. In every situation, it was the NOS tube.

With our circuits treating tubes very kindly, we do not have a problem with consumers experimenting with tubes but do recommend they buy from a reliable source that offers refunds or exchanges.

Its unfortunate that today's tubes simply do not have the same sound as many NOS tubes and it is a real gamble when paying top $$ for those golden treasures only to have them go bad within a couple months.

Know your tube dealer and you'll be fine.