MC275 Change 12AU7 to 6CG7- Is It Sacrelidge?

[AA6U]

I came into possession of a classic McIntosh MC275 that still had all of the original capacitors and sounded kind of tired, so I refurbished it by rebuilding the power supply and changing out all of the capacitors with audio-grade polypropylene ones. It sounded good, but not as good as I had expected (output tubes are RCA 6550).

In the past, I tested changing the 12AU7s in a pair of MC30s to 6CG7s as recommended in other posts. I did one amp and then compared it to the other, and the results were astounding- clearer sound, especially in the upper midrange. I changed the other MC30 and never looked back. I thought I'd try the mod in the MC275, so I changed one channel and compared them. Again, a significant improvement in the upper midrange, so I changed the other channel.

Has anyone else done this in the MC275? It's easily reversible. I am very pleased with the improvement in sound and would like to hear thoughts from others on this change.

I know there are purists who insist that the original can't be improved so this would be considered sacrilege, but I could hear a distinct improvement with the change.

 

[jeanmarie]

How was it done? Can it be done to MC 75s.

 

[AA6U]

All you have to do is change the filament wiring, that's why it is so easy to implement and reverse if desired. Yes, you could do it to the MC75.

 

[damacman]

You may also prefer the KT88 output tubes to the 6550s ...

 

[AA6U]

I have a set that I haven't tried yet. I'll audition them and see how they sound. The guy I bought the MC275 from dealt with McIntosh extensively (he used to sell them in the 1960s), and he said that they recommend 6550s in the old version.

 

[AA6U]

By the way, I received a question from someone who wanted to know how to change the wiring to use a 6CG7. Attached is a rough drawing showing the stock connections and the modified wiring for the 6CG7. The changes involve the filament only.

 

[Mike Gibson]

I have an original 275 and have tried all sorts of power tubes. Tung-Sol 6550 reissues, original Gold Lions= KT-88's, Penta Labs KT-88's, EH KT-88's, original Tung-Sol 6550's and Gold Lion reissue KT-88's. The gang here bought me the Gold Lion reissues and signal tubes and it's my favorite combination. I haven't found any aftermarket 12AZ7's so those are still stock. The amp was rebuilt around 5 years ago and usually runs the horns on my Altecs. It provides the "shimmer" I like. No ringing, shrieking or anything harsh. It was my experience so far that the small signal tubes provided the biggest sonic difference. But I'm old and my hearing is probably suspect.

 

[AA6U]

I'm using an assortment of American tubes including 12AZ7s (some people substitute 12AT7s for the 12AZ7s) and 6550 for outputs. I'm going to try some American KT-88s that the previous owner gave me when I bought it to see how they sound. I'm happy with the 6550s, but I'm curious. However, I still think my MC30s sound somewhat better than the MC275 (a little fuller and better bottom end).

 

[AA6U]

I also found an improvement by using a GE 5157 in place of a Mullard 12AX7. I read somewhere that a 5157 tube sounds better than a 12AX7. It did in my case.

 

[Mike Gibson]

I tried "Dullards" in mine and gave them away. Something several people besides me have noticed is the the EH small tubes sound as good or better than most NOS vintage tubes. They'd still be in my 275 if I hadn't been gifted the Gold Lion reissues.

 

[AA6U]

I would agree on the EH small tubes. I did a refurbishing job on a Mac C11 that had six Telefunkens and a two RCA 12AX7s, and it sounded great. The guy asked me to replace all of them with EH tubes so he could keep the Telefunkens as spares. The EH tubes sounded just as good as the Telefunkens.

 

[jlovda]

McIntosh must have had an incredibly ignorant staff of amplifier engineers in the 50's and 60's to keep choosing the wrong tube for every situation.

 

[AA6U]

I read somewhere that McIntosh used 12AU7s because they generally did the job, were inexpensive and, because the heater pin configuration is identical to the other driver tubes, it was faster and easier to wire during production, thus cutting costs. 6CG7 tubes were more expensive at the time. Also, it seems like the 12AU7 was the "go to" tube for that function during those days thus they used it as their standard for amp design.

 

[jlovda]

I read somewhere that McIntosh used 12AU7s because they generally did the job, were inexpensive and, because the heater pin configuration is identical to the other driver tubes, it was faster and easier to wire during production, thus cutting costs. 6CG7 tubes were more expensive at the time. Also, it seems like the 12AU7 was the "go to" tube for that function during those days thus they used it as their standard for amp design.

So there's a Chevy hiding under that chrome Ferrari skin. The wiring differences seem trivial. I will hypothesize that they couldn't measure any difference in distortion, frequency response, noise floor, etc. to be worth the trouble and I am sure Gordon Gow or someone at his level was the final arbiter of sound. Are we talking about NOS tubes from the 60's or current production? I guess this falls under "tube rolling" which I as an SS person cannot relate to.

 

[AA6U]

I'm using tubes from the 60's and 70's.

 

[apachef1]

can this mod be done on an Mc-225? 12AX7 > 12AU7 > 12BH7 > 12AU7 > 12BH7
*******************************************12AX7 > 6CG7 > 12BH7 > 6CG7 > 12BH7

Or does this cause a problem.

Ed

 

[AA6U]

No problems with it on other models. I've also done it to a McIntosh MC-30 and MC-225 as well as a Heathkit W5M with no problems. It made the least difference on the MC-225 and the biggest improvement on the MC-30. I'm just starting to rebuild an MC-240 and plan to do it to that too.

 

[c_dk]

I would suspect minor frequency response aberrations that push certain listner's buttons......but where is the "magic" in that.

I also would suspect that the 12au7s were tighter to spec on the bell curve.....Mac always bought parts at the top of the bell curve.

When you would visit the factory back in the day you could witness them checking each batch for consistency.

DOB's frustration with finding tubes that met their spec is why the clinics quit replacing tubes.

 

[AA6U]

Perhaps. However, 12AU7 was the "go to" tube for that purpose during that period. Interestingly, Audio Research used the 6CG7 in their D70 power amp, so it must have some "magic." All I can say is that I did a direct A/B comparison and found the 6CG7 sounded better than the 12AU7 to me. I remember reading a post somewhere that someone who did the same change measured the amp's distortion and it was less with the 6CG7. For me, how it sounds is what matters, and the change is easily reversible so no permanent damage is done to the amp.

 

[c_dk]

While I do not doubt that changing the tubes which do have different internal resistances, capacitance, and inductive loadings, will alter the sound what would be interesting is how and where is the spectral balance altered.

I know it would really blow up the fun if it truly was just a expensive EQ but inquiring minds should really want to know.