Johnny dB Posted December 5, 2007 Share Posted December 5, 2007 So I picked up a pair of MC-30 locally; and I was doing some tube rolling. The Telefunkens that were in the 12AU7 spot have no markings on them besides the telefunken symbol. I was wondering if there is a way to tell a 12AX7 from a 12AU7? They do have numbers underneath where the diamond is. I was also wondering how do you bias the tubes on these 30's? Quote Link to comment Share on other sites More sharing options...
Groomlakearea51 Posted December 5, 2007 Share Posted December 5, 2007 HarryO and NOSValves on the forum come to mind, and there are also a number of other very competent tube specialists on the forum. There is a cross-reference list, I believe, at the valleytube.com site. BTW: what are the numbers? They may be the ECC numbers that they used in Europe instead of the US designations. You can also look at tubedepot.com and type in 12AX7 and 12AU7 and see the ECC numbers that come up as variants. Quote Link to comment Share on other sites More sharing options...
BadDad Posted December 5, 2007 Share Posted December 5, 2007 I will second the opinion on HarryO. He knows his tubes and has been very helpful to me. Great guy. Quote Link to comment Share on other sites More sharing options...
Johnny dB Posted December 5, 2007 Author Share Posted December 5, 2007 Nope, not the ECC numbers. I wish they still had those. It's the numbers on the bottom between the pins above or in the middle of the diamond. I think it's a 28 (I'm at work right now and the tubes are at home). They're probably build numbers or for dating purposes or something else. Quote Link to comment Share on other sites More sharing options...
dp1954 Posted December 6, 2007 Share Posted December 6, 2007 The mc30's are self biasing so nothing to worry about there. If they haven't already been rebuilt you might want to consider that. I own a pair and it made a world of difference in the sound. Very nice amps. Dave Quote Link to comment Share on other sites More sharing options...
Coytee Posted December 6, 2007 Share Posted December 6, 2007 Regarding what Dave said about having them tinkered on... there are many who can/will do it. Don't know if you are aware of Terry DeWick or not but he's here in Knoxville and highly regarded by many. (I don't even know if your 30's are in need of tinkering so this is just a "fyi" thing) http://mcintoshaudio.com/ (click on DeWick repairs) To give you an idea about Terry... I've owned various Mac stuff, including some 30's. When I got my 2102 I was wondering if I could swap out their KT88's for some KT-90's. I got real brainy & called McIntosh the company. Got their tech department and asked the guy if I could do that. He ended up scratching his head... he then asked if I'd said I lived in Knoxville (which I had done in the early part of our chat) He asked me if I knew Terry. I said I did and I'd been to his house/shop a number of times. Then, and remember, this is the tech guy AT McIntosh, the very place that BUILT the darn thing... he says "why don't you call Terry, I'm sure he'll know the answer to your question better than I will" HUH?? Terry is a factory authorized service shop but I was still flabbergasted that the tech guy at the factory didn't seem to know the answer and would send me to someone else. That was all the 'verification' I needed regarding Terry's abilities and their respect of them. Aside from the McIntosh endorsement, Terry is just about as nice of person you might ever get the pleasure of knowing (outside of Amy [A]) Quote Link to comment Share on other sites More sharing options...
seti Posted December 6, 2007 Share Posted December 6, 2007 Aren't thy physically different? Try posting some pics. Quote Link to comment Share on other sites More sharing options...
dp1954 Posted December 6, 2007 Share Posted December 6, 2007 I second what coytee said. Terry rebuilt mine and I couldn't be happier. Amps like these deserve a proper rebuild. Quote Link to comment Share on other sites More sharing options...
joshnich Posted December 6, 2007 Share Posted December 6, 2007 AIf you are thinking of having your amps rebuilt, do yourself a favor and do a forum search for David H's (audible nectar) thread on his rebuild journey with his Mc30s. I think it would be worth your while. Craig NOSvalves is also an excellent resourse for the rebuilting options on these amps. Josh Quote Link to comment Share on other sites More sharing options...
Chicago_Pete Posted December 6, 2007 Share Posted December 6, 2007 Craig rebuilt mine and they have never sounded better. Top notch! Quote Link to comment Share on other sites More sharing options...
Johnny dB Posted December 6, 2007 Author Share Posted December 6, 2007 Thanks guys for the responses. I've already spoken with Craig regarding these, since he's done all my rebuilds so far. I'm not sure a rebuild is needed just yet, as these sound pretty darn good as is. As far as the 12AU7 question, I found a pair of RCA cleartops that came with my Foreplay so I'm using those. I still would like to know exactly what these telefunkens are. They look exactly like the 12AX7. If I get my camera back I'll post some pics. Quote Link to comment Share on other sites More sharing options...
Moderators Travis In Austin Posted December 6, 2007 Moderators Share Posted December 6, 2007 Johnny, The teles look different on the inside, i.e., the guts are different. The problem is you don't have controls that you can compare them to. Do you have a camera that will allow you to take photos of what you have? If you can post them, we can tell you which is which. Travis Quote Link to comment Share on other sites More sharing options...
NOSValves Posted December 6, 2007 Share Posted December 6, 2007 John, The visual difference between a Tele 12AX7 and 12AU7 is very small but its there. Man is it this going to be hard to explain in written form though. You do have a Tele 12AX7 to compare out of one of your Scott amps don't you? Give me a call and I'll just guide you on finding the difference. A magnifying glass would make it easier. Yup its that hard to see. Craig Quote Link to comment Share on other sites More sharing options...
Dflip Posted December 6, 2007 Share Posted December 6, 2007 Craig does very good work, he rebuilt my MC-30's and I do like the sound. Mullards in the 12 AU7 spot work quite well as they add a nice warmth. The Telefunkens work great in the the 12 AX7 tube locations. The other locations, find as many blackplates as you can. Quote Link to comment Share on other sites More sharing options...
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