New Tube Color Code Question

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May 12, 2024
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I've read several of the threads on this forum and elsewhere about the tube color codes, but I haven't seen my questions addressed yet. I have a new Triple Crown TC-50 head that was delivered with EL-34 tubes marked STR 447 BLU (I assume this is blue). I have read that some of you have measured voltages of the various color code tubes, so in theory there are differences. But Mesa's official position is that the tone differences between different colors is not significant and the colors are just for matching. I don't know that these allegedly small differences will mean anything to me until I've been using my amp for a while.

My questions:
1. I want to switch to 6L6 tubes. Do all tubes marked BLU have the same voltage characteristics? If not, what color code 6L6 has similar voltage characteristics to a blue EL-34? Or should I even care?
2. Is there a chart somewhere explaining the meaning of STR 447? I read one thread that mentioned that this indicates country of origin, among other things. It would be nice to know where my tubes are coming from, and whether that makes a quality difference.

Thanks for any help!
Following up on my original question. After much research both here and elsewhere, I ordered a set of tubes from Guitar Center. They are not returnable, so I had them delivered to the nearest GC store and examined them before I walked out of the store with them. They are Mesa STR-445 6L6 GCs coded yellow.

From my research, the yellows should add a bit of crunch to the sound, which is exactly what I was looking for. The reds are similar in power output. The picture in their ad was for reds, so I would have taken those as well. The EL34s that came in the amp are grays. I never did determine if 6L6s and EL34s with the same matching code have similar power output, but that is now a moot point.

The 445 part of the number indicates a country of origin of Slovakia, which was my preference for personal reasons. I would have taken a Chinese or Russian version if I couldn't find Slovakian tubes, but I got lucky on my first try.

Installation and testing hopefully later this week, hopefully.

Spinaltap11, you directed me to a page on the Mesa web site and asked if that was helpful. I had seen it before I posted my question. But it is clear from posts online that there are differences between the different colors that you can find via instrumented testing, and some feel there are discernable differences in the sound. But check out the info on the back of the package re: Mesa's testing, especially the reference to their "Robotube" testing machine. No doubt their procedures result in a good match, but it seems like they don't care about any of the differences that others have found through independent testing or actual use.


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You cannot go by the color code alone. If you are sticking with the same tube that came in the amp then you may hear some differences. There is not much of a difference between the green, gray and blue EL34 of the same tube in the TC50.

When it comes to the wider selection of the 6L6 tubes, they all have a different characteristic to them even if they have the same color code. STR441, STR443 are the Russian made 6L6. They do not sound the same, different tube. STR445 is the JJ 6L6 tube. Sounds more like a 6CA7 but is a 6L6. STR440, STR448 are Chinese, STR440 is out of production but they can still be found. The tubes with the red base are specifically made for TAD by PSVANE. I have tried the STR446 tube (EL34) and it sounds close to the STR447 but not as harsh. Trick with the STR447, give it some time and it will settle down a bit and sound good.

The alternate is the 5881 STR425 (russian). If you want more old school and do not mind a cut in power, the 6V6 (STR417) sounds really good in the TC50. Did not care for those in the TC100 though.
Reading between the lines, I believe Mesa's stance is that since the tone comes from the preamp section in their designs, having the power tubes within a wide acceptable range is fine.

That being said each color code bumps the tube rating about 5-6mA, and depending on the spec of the amp it is possible to have colors that are so cold they get into crossover distortion or so hot they can fail prematurely. This requires testing, though I have seen on modern Mesas they're tending to run them colder for longevity and was even told with the Badlander that crossover distortion from running ~14 mA was part of the design..

Therefore the measurement parts is real, but the tone part is personal preference.

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