Here are some thoughts on tube replacement in amps in general, picking tubes, and tube replacement and bias in TheAirtightGarage amps.
I am just speaking "off the cuff", nothing formal or written in stone, just experiences I have had with tubes and tubes in amplifiers I have built or modified.
There is no perfect or greatest tube. I know many people who would disagree with this, saying that the RCA Black Plate 6L6 was the greatest 6L6 ever, or that the GE 6550A was the best 6550A ever, and so on and so fourth. Let me say the following: it always depends on what gear you are using it in, how that gear was designed, what you are driving that gear with, and what that gear is driving. I have literally heard "the best" tubes in amps costing thousands upon thousands of dollars, and due to my curiosity, have swapped them out with "lesser" tubes to get different and sometimes much better results. It always depends, and it is always a matter of personal taste. If someone tells you that tube X is the best tube for amp Y, that is what they think, with their ears and their setup. It may not be the best for you.
So, you say, how do I choose a good tube, when I do not want to spend hundreds of dollars purchasing all available makes and then testing myself? Good question. Some ideas on that are this: ask around, but keeping in mind the previous paragraph, ask others with similar (or the same) amps what guitar they are using, what effects (if any) they are using, what kinds of pickups, etc. This can make a huge difference in sound, and changing out the tubes will again make another difference. My real point of the matter is that there is no best tube, decide what you would like to hear out of your amp, then read what others who are after the same thing use (in the same amp wouuld be nice).
Do not discount the cheap stuff, well, not all the time anyway. Short story: I used to own a very high end tube audio amplifier, I purchased it used right after it had been back from the factory (my friend had a cap job and had it re-tubed, then sold it due to finances). After about two years, I needed to change the tubes. I called the factory for their opinion, and they recommended a tube company known to me to be pretty bottom of the line. I thought they were nuts, so I ordered the top-of-the-line tubes, matched. I installed them and set the bias. It sounded really bad, compared to what was in it previously. I could not believe it. I double checked the bias and let them burn in. I then set the bias again after about 100 hours. No difference, these new tubes really sounded dead compared to the old ones (which were house labeled from the company). I then decided to use the suggestion from the factory and put the cheapies in... they sounded great. My lifeless soundstage was now wide open and had depth. It was freakish, my friends were tired of me telling the story, but it does bring up an interesting notation, that is, some tubes are much better in some circuits than others, regardless of who made them.
Yes, the tubes of old are better, most of the time. There are some exceptions to this, namely if you want more power or more distortion or both. I am not saying no good tubes are made today, but they tend to be more microphonic, more colored, or have more noise than vintage stuff. That goes without saying that they did have tubes made long ago that were not that good either, but I digress. The aforementioned RCA black plate 6L6 (in my opinion) does sound better in my amp than all others I have tried to date, but if I want all out power with a little increased microphonics, the RCA is not my first choice. Given the cost of a NOS pair that are truly unused in the box, the RCA is not my first, second or third choice either. But since I am on the topic of vintage tubes, let me say this, in the preamp section it is well worth it to have a small stash of vintage tubes that work well in the amp in choice locations. In many cases you do not need to fill the amp with vintage tubes to get a really good sound, but only in critical locations. I have read a lot recently about people making a huge deal out of putting high end vintage tubes in the phase splitter location. Well, O.K., I agree that the chance of it sounding worse is slim at best, but what kind of improvement are we talking about here? I have seen guitarists spend fifty dollars on a vintage NOS Mullard for the phase inverter (a very fine tube), but I take a peek at their amp and they are using the worst tubes you can get in the preamp section. I think their money would have been much better spent experimenting with tubes in the front end, epsecially when they mention that the tube change did not "clean up" the amp like they thought it would. Vintage tubes are great, they were typically built better, with better materials, etc, but keep in mind what you are after and where you are putting them in the circuit. Is it really going to matter given your other tubes? It is worth it for the price?
Driver tube changes in TheAirtightGarage amps. TheAirtightGarage amps come stock with a 12ay7 in the first input tube location on the Betsy, Emily and the Tonebuster. This tube will yield the most dramatic sound change in the amp compared to changing any other tube(s). I use a JAN 6072A black plate in this location, and from time to time (depending on the customers wants and needs) will use a NOS General Electric grey plate 12ay7. I find the Raytheon or RCA tubes to be very nice, yet not as silky smooth as the JAN 6072A. Any good NOS tube will pretty nice in this location, and swapping from one manufacturer to another will give slightly different results. Like most other amps, you can in fact swap out this tube with one that has higer gain, but be warned, this will make the most dramatic difference in the sound of this amplifier. If you change to a 12at7 for instance, the overall tone of the amp will be heavily impacted. I am talking about a major change, not a minor one (like swapping a GE for an RCA). Some people like this, but most who have heard a real 12ay7 in there do not like to replace it with a 12at7/5751/12ax7/7025. You may like this, and given that most people have replacements for the other two 12ax7 tubes in the amp, it is not a costly test. The second tube in the Betsy and Tonebuster amps will yield a more subtle change than swapping the first tube. A lot of vintage tubes sound very simillar in this location, particularly the RCA style 12ax7 (long grey plate). I personally prefer a NOS RCA 7025 in this position, but they are very hard to get reliably. Incredibly, the Sovtek 12ax7LPS is nice in this position for a current production tube, as is the EH. I find the JJ and EI Elite tubes to be too upfront with a lack of depth in this location. Again, it is a subtle change, and I would recommend the GE or RCA for a vintage tube and the SED or Sovtek for a new production. In the phase splitter location, tube swapping is even more subtle in the overall impact on the tone, and swapping from a good modern 12ax7 to a NOS is very mild in tonal change. I go for reliability in this location, and just pick what has the least amount of negative impact on the tone here. Try a GE greyplate or a SED, they are reliable and sound good. In the second driver stage of the Emily we see the same picture. Not as heavy an impact on the sound as the first tube location, but a subtle change. Depending on the customer I use a Sovtek for attack and high gain, a SED for straight clean sounds or if I need even more warm tone out of it I will use a NOS RCA or GE, particularly the RCA 7025 or a GE "5 star". In either amp, for all out warm vintage tone (some people do not like this much, so watch out) try a Mullard, Valvo or Mazda 12ax7 in the second tube socket. You may want to try a Telefunken, but oddly, I find them too smooth, just barely lacking in attack and harmonics compared to the others.
Bias I am not even going to go into biasing technique or the theoretical argument on where to measure for the bias current or which method is better or not. There are pages and pages of argument and rebuttal on the net and in books on the issue of bias, where to check bias, how to check bias, and how all of them are wrong. To be blunt, I prefer to "stand on the shoulders of giants" and use many other methods developed over the years, and compare them. I take an average of about five methods, compare them to what I do and see what kind of error I get. The truth be told, this paragraph is really only here to let you know that yes, there are many different ways to check bias, there are many bias points for different tubes, and setting the bias should be done by a competent tech when you change tubes that are not exactly at the same current as those you are replacing.
Bias settings for TheAirtightGarage amps. Here are the standard settings for the bias on the Betsy and the Tonebuster. First, a small notation on the output tubes that come with them. In the Betsy, unless the customer has wanted otherwise or told us what they are looking for and it warrants a change of output tube, it is loaded with Sovtek 6L6WXT+ tubes in the output. Think they suck? Well, so do many guitar players. I had a guitarist jamming on the Betsy and he was telling me how great it sounded and how buttery it was and he asked what tubes were in it. When I said Sovtek I thought I was going to need to call for medical assistance. He could not belive it. He had to go to the back of the amp and check for himself, as he thought I was kidding, and then he still could not belive it. Well, to be honest, when I built the first one I could not belive it either. But after testing about ten different 6L6 tubes in there, they were the ones that fit the bill... this goes along with the first paragraph of these thoughts. The Tonebuster is loaded with JAN Phillips 5881 tubes. Due to the upgraded parts most tubes are too harsh in this amp. The aforementioned Sovteks are way too "upfront" and in your face, without the smooth as water sound they had in the Betsy. The SED tubes are nice, but suffer the same problem as the Sovteks. Basically, the same goes for the JJ, Sovtek 5881, etc. This may seem strange at first, but it makes sense due to the circuit construction and part selection.
O.K., so after a breakin period and a good listening (read jamming) test, the Betsy is setup for 35ma with the Sovteks and the Tonebuster is setup for 34ma with the JAN 5881 tubes. I note the following, if you use other tubes in the Betsy, go up a hair on the bias with SED/JJ tubes (say to 36ma), and if you use other tubes in the Tonebuster you way want to slide the bias up a hair also. The Emily amplifier uses fixed bias, and I suggest JJ 6V6 tubes, or if you want to go all out, get some General Electric NOS tubes. Well, that is enough ranting and raving for now on tubes. If you need further suggestions on tubes in any of TheAirtightGarage amplifiers, just drop me some email or call. I have used every vintage and new tube in them, and can make suggestions based on your playing style and setup.