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Post by andy on Sept 25, 2009 9:25:20 GMT -5
I am looking to re-tube my little Vox Night Train and am considering substituting at least one 12aX7 with a lower gain variant, such as 12aT7 or 12aU7.
This seems a fairly commonplace change to make, but is it really sensible to use different spec tubes in a circuit, or are they generally interchangeable, as a rule?
Hope you all appreciate the full use of the US terminology in this post!
Cheers, Andy
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Post by sumgai on Sept 25, 2009 11:28:04 GMT -5
andy, Saying that tubes are "generally interchangable as a rule" is too broad a brush stroke. I know what you meant, but taken out of context, a person reading that might get bad ideas. As least, bad as far as tube life is concerned.....  The guitar world uses amps almost exclusively designed around the dual-triode, a very popular trade-off between space, power consumed (for the heaters), and signal amplification factor (the gain). To be sure, there are many oddball tubes used in pre-amp designs, but can you name one? Neither can I, if I don't pull out my handy-dandy GrooveTubes Tubes and Amps Manual. In fact, just about everyone has tried it at least once or twice, even Fender. (Look up the SuperChamp by Rivera.) But when the cows have been put back in the barn for the night, they all come back to that ubiquitous little devil, the 12AX7. So you can, in theory, substitute any member of that family (and there are dozens of 'em, literally!), and realize a change in how the amp works, and thus a change in your tone. Are there any limits, or gotchas? Well, rather than go into a lengthy discussion, and by-the-by, repeating what's been better-said by other websites, why don't we just consult the oracl google: Google Results for 12AX7 substitutionJust about every link you'll find there is good. Sure, some are too snooty, and some are too skimpy, but several of 'em are just right - take your pick!  But if you do have further questions, don't hesitate to ask.  Oh, and you have to have javascript enabled in your browser for this to work. Nothing personal, my fellow Nut, just wanted to try out a new toy I tripped across awhile ago. ;D HTH sumgai |
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Post by andy on Sept 26, 2009 6:04:51 GMT -5
;D ;D ;D I thought it was just Windows, but now Google is giving me back-chat, too. Those two shouldn't be hanging out together so much. I sat up the night before searching with every combination of valve types, preamp, and phase inverter. Somehow I skipped the simple stages of using words directly related to what I wanted to know. Thanks for nudging me in the right direction, Sumgai. Those links are all very handy, and as always, the indecision and polar opinions have made me think that the issue is best left to the experts! If one guy says it is fine and a gateway to tonal bliss, another says it will blow your amp up, I think I'll just look for some low gain 12aX7s. There is at least one company which can select valves for gain, so, as might well become my catchphrase, I think I'll just keep it simple! ...And safe.  |
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Post by sumgai on Sept 26, 2009 15:22:01 GMT -5
andy, Well, by the very definition of "gain factor", you'll have a difficult time of finding any "low gain" 12AX7's.  And for those who cry "Foul! You can't do that!", I'd have to ask, 'May I see the little man behind the curtain, please?' Meaning that either they have an agenda (sell you something you didn't want and don't need) or else when Gawd was passing out brains, they thought she said "Trains", so they said "No thanks, we're staying right here". The main factor is, if a certain amount of gain is desired, you select and plug in a certain tube type, that's the bottom line. What's affected by changing the gain? Only one thing - the amount of voltage used to drive the next amplifier stage. That's it, really!! Oh sure, if you want, you can do strange things to that output voltage, but in the long run, the general difference between the dual-triode tube types is how much they amplify a signal, compared to each other. What happens to the signal at the next amplifier stage, if it's lower or higher than expected (designed for)? Now that's the magic question! The answer is "Not much". Having too much signal is an obvious "watch out" - distortion becomes more likely. Too little signal, and you'll simply turn up the amp's Volume knob a bit. How's all this change the tone? Answer: "It doesn't". At least, not enough to talk about. One can hook up all the test equipment at his disposal, and perchance observe a very slight difference in the waveform, but can one hear that difference, when all is said and done? In most cases, it's doubtful. So why even contemplate this mod? Answer: "The brownie points". You get GuitarNutz brownie points if you change anything in your amp, even just the tubes. ;D More to the point, the only practical reason to reduce a tube's gain is to moderate a slightly perceived harshness in one's tone - it might be just on the ragged edge of distorting too much, and reducing the gain might smooth it out enough to make a difference. Some folks call that tone, and I'll not dispute them on it. They paid the piper, they can call whatever tune they want.  HTH sumgai |
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Post by sumgai on Sept 26, 2009 15:37:44 GMT -5
Andy, I forgot to address this part of your post. I think I'll just look for some low gain 12aX7s. All tubes have a tolerance range in their ratings. If a tube is designed to have an amplification factor (gain factor) of 100, then it might actually have only 90, or even 85, or it might go up, like to 110 or even higher. The bottom line here is, searching for a "low end of the range" 12AX7 is like saying "I want a thinner 0.010in gauge string". It might happen, tolerances being what they are for every manufactured thing, but what are the chances of sucess in searching for such an item?  About the same between the two, I'd imagine. A 5751 has a gain factor of about 70, roughly two-thirds of what you have now. That should be a significant enough change, for experimental purposes. Go lower if you want, like I said above, no harm will result. There is at least one company which can select valves for gain. I'll bet that by now, you can pretty much guess what I've got to say about that, eh? The site that's selling the selected tubes has a disclaimer that reads "We can't tell you how your new tubes will sound in your equipment, nor can anyone else tell you either." That should be warning enough right there to tell you that you are about to embark on an experiment that might get, shall we say, costly. That is, unless the site takes back whatever it sells you, even if it's for nothing more than you don't like it. Things to ponder.... HTH sumgai |
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Post by andy on Sept 26, 2009 18:11:25 GMT -5
Sterling words indeed. I hear them all. Well, I won't turn down a point or two. They can be exchanged for cash, can't they? But the main reason for all this is just a sideline really- the amp ships with the bog standard Sovtek valves which are OEM in just about any production amp. Perhaps there is nothing to be gained from a change, but I'm willing to blow a birthday present on finding out! Thing is, while I'm at it, I'd like to push the onset of breakup a bit further round the dial- not essential, but if there is an option while I have the lid off, I'd look into it. In fact, both of us being solid-staters for the main part lately, the reason I picked this amp was to reduce the background noise I was experiencing with my Frontman, and to give myself the option of plugging into different cabs, both for the sake of recording. The tube-ness is just a side issue anyway. Just for your perusal, here is the site best known here for gain-grading, or as they say, drive rating valves. They seem very willing to support their claims, so I'm sure they won't mind a bit of advertising here. www.watfordvalves.com/aboutus.aspI've read a great deal about the benefits of certain brands over others on t'web, but I know how we guitarists fall for the ol' snake oil tricks time and time again. I don't think of myself as above it!  I'll look into those 5751's too- price allowing. |
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Post by sumgai on Sept 26, 2009 18:44:09 GMT -5
andy, As you're looking around for alternatives, don't overlook the alternate labels for these tube types. An ECC82 is the Euro equivalent of a 12AU7, a little bit a lower gain than the 5751. Sometimes these other labeling systems can yield tubes that are "just right". Plus, because they aren't as widely known, they may be available at a lower cost. Emphasis on the 'maybe'....  HTH sumgai |
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But if you do have further questions, don't hesitate to ask. 
About the same between the two, I'd imagine.
