2. amps-preamps
  3. 1175279050

Tube rolling, Macintosh MC275 Mk IV

But first, I just got the following email from a friend with a new Mk IV (driving Vantages) who had just tried a few NOS 12AX and 12AT tubes I mailed him (he knows who he is ;--)
WOW, I was not prepared for what I was hearing, the first thing that came to my mind was"snap" the music came at me with an energy that was not there before. I soon realize that the music was a lot more dynamic. Meaning there was more treble energy yet still maintain a quality that was silky smooth. Yes there was more detail and nuances that was not previous heard. I wanted to go through my favorite recordings to hear what I have been missing. There is further separation between instruments with ample amounts of air that only enhances the listening experience. In other words I like it, I will listen more tonight and tell you about the improvements. I can see why tube rolling can be fun
I've done some extensive listening/rolling, and have included friends and neighbors in the project -- some with even better hearing than I. Here are my/our conclusions from tubes (including the stock tubes) actually auditioned so far. All twin triode tubes (AX's and AT's) had matched triodes. All pairs of tubes (AT's and KT88's) were matched as well:

12AX7
RCA -- 1957 smoothplate, blackplate, longplate (new, tested over 100%)
Amperex -- Orange Globe, made in Harleen Holland (tested 95%)
Telefunken -- Smoothplate, greyplate (tested 90%)
Mullard -- not included (yet)
12AT7
RCA -- 1957 ribbed, greyplate (new, tested over 100%)
Amperex -- not included (yet)
Telefunken -- ribbed, greyplate (new, tested over 100%)
Mullard -- tried, but not fully A-B'd
KT88
Penta Labs -- KT88SC, new matched quad, (tested over 100%)
GE UK -- NOS: GEC, Genalex, Gold Lion/Monarch (not yet, maybe one day soon if I'm lucky)

Tube groups were rotated in all possible (at this time) combinations, using the Diane Schure/Count Basie Orch. GRP-LP, Track: "Travelin' Light". (system link below.) It was fun. Tedious, but fun! There were some combos that were very close sonically, but the unanimous, winning configuration (after many, many "let me hear that agains") was:

12AX7: RCA -- 1957 smoothplate, blackplate, longplate
12AT7: Telefunken -- ribbed, greyplate
KT-88: Penta Labs -- KT88SC, solid plate, 3-getter

Better than a MC275 MKIV, with stock tubes, out of the box? You have absolutely NO IDEA !! If you want to give it a try, this will save you some time and money; at the very least, give you an excellent place to start ;--)
.
128x128nsgarch
mrtennis
3,295 posts
 
changes in gain structure can sometimes affect the performance of a stereo system. i have been told that 12at7 and 12ax7 tubes were interchanged, by "accident" with no harm to the tubes or amplifier. it is possible that using 12ay7 might provide a preferred presentation than a 12ax7.

our hobby is based upon personal preference. "playing around" with 12 volt tubes may be a way of attaining a "sound" that is desired.
nsgarch's avatar
nsgarch OP
2,524 posts
 
"our hobby is based upon personal preference. "playing around" with 12 volt tubes may be a way of attaining a "sound" that is desired."

I would think, in this case anyway, the "personal preference" part would come when selecting the amplifier in the first place. If one has to go so far afield as to substitute 12V filament tubes for 6V to achieve certain results, maybe one should consider a different amp? But, to each, his own . . . . ;--)
nsgarch's avatar
nsgarch OP
2,524 posts
 
Recently, I've been using some 5751 tubes instead of the 12AX7's (see Joe's Tube Lore: http://www.nickdangerous.com/misc/mghead/joestubelore.htm )
I thought I'd give some 5751 tubes a spin. Joe basically dislikes ALL 12AX7's and considers the 5751's far superior. The 5751 is a military/premium version of the 12AX7. It (generally) has 3 mica spacers instead of two with copper reinforcing rods and closely matched triodes. It also has slightly lower gain than a standard 12AX7, but not enough to be noticeable IMO. I tried three of Joe's top choices: The RCA, GE, and his favorite, the Sylvania Gold Brand (note, the old, 1950-1960 3-mica Sylvania Green Label is the same tube, and cheaper) and found the RCA to have a slight edge in the MC275 over the Sylvania. The GE was a close third IMO. But these things are a function of the equipment you are putting them in. However, I do agree with Joe that the 5751's are better than the two 12AX7's I liked best (Telefunken and RCA black/long plate.) They're not as expensive (yet!) as the NOS 12AX7's, so you might want to give them a try.

OK, separate (but somewhat related) subject: Substituting 12V filament tubes for 6v filament tubes. For instance the 5965 for the 12AT7's. This is NOT the same as subbing 5751's for 12AX7's! The 5965 has a larger filament current draw and could ruin power supplies if they're not designed to provide higher filament current. Be careful, I think the MC275 PS could be damaged!
More to discover