Mac 275 questions........

Really?!? I didn't realize they are using EH! When did they start using them? My amplifier is 3 years old, and I was under the impression they used different ones. That's good to know. I have a 2275, and was about to replace stock tubes with a set of EH KT-88 based on a set of rave reviews I have read. I guess the safest bet is to get the reissued GL. Most people seem to be happy with them. By the way, I completely agree with your assessment of the NOS TFK 12AX7's. I bought them for my 2275, and the sound improved considerably. It's much cleaner, detailed, and airy.

Bosrt, I pasted the following from my AK post:

I was curious why some folks didn't find tube changes had much effect with Mac amps and others like myself (and friends) were astounded at the improvement. I'm a natural detective, so I couldn't just 'let it go' After asking everybody lots of questions, I realized that most, if not all of the people who were 'underwhelmed' had changed their power tubes first (or only!)

I changed my 12AX7's first. Not because someone with prior experience had given me a hot tip. But because one night right after I got my amp, a (vintage gear) friend came by to hear the new toy, and brought some old Telefunken 12AX7's 'just for fun' and we were both blown away! Only later I read where Ron C. (I think) said that in order of sonic importance, it would be the AX's, AT's, and last the KT88's.

You will definitely hear subtle sonic differences in power tubes if you change the little ones first. But the biggest difference between power tubes will be in power (duh!) and the EH's (when they work) are the weakest. They have transconductance values around 4500-5000. The Pentas and (I'm assuming) the reissued Gold Lions as well, are much stronger 6000 +/_. The strongest, are the NOS Gold Lion, Gold Monarch, and GEC KT88's which, when new, are 10,000-12,000.

McIntosh also declares that it doesn't matter that much to have matching pairs of power tubes (or even the little signal/driver tubes.) Strictly speaking, that's true. And just to prove it, they make no provision for adjusting the bias if they don't match! However, if you're concerned with tube life (power tubes), or with precise channel matching (signal tubes) then you'd like to have tubes that match. No, they don't have to match within 5%, or even 10%. I'd say 20% or better is reasonable. Except if you're using the single ended inputs, I'd recommend that V-1 should have its two internal triodes matching within 10% or better.

Nsgarch,

I just checked the tubes that I changed in my McIntosh Ma-2275, and they were the AT's...changing them made a big difference. Now, I'd like to see if changing the four 12AX7A can make a difference too. Telefunken never made 12AX7A. They used to make 12AX7's instead. Do you know if I can use those to replace the 12AX7A's? If not, do you have any recommendation in terms of what 12AX7A's I might use? Thanks, Giovanni

I got into it last nite and had some success.

I preferred Mullards in the V2 12Ax7. Nice bass and some cleanish warmth in the mids. Highs nice. A Brimar was too bright. The other 12Ax7 got RCA 5751s. All these were more open and detailed than the original tubes. Also brighter.

For the AT7s, Brimars were good and RCAs good. The AT7s were substituted with what I could find on hand. Didn't work on rolling here much.

I also found as I replaced more small tubes the instrument solidity and location stabilized with a guitar intro that I know well. Cibelle/cibelle 2nd track.

B

12AX7A's are simply 12AX7's with a filament center tap that allows them to be run in series or parallel. Not an issue with the Mac amp.