Mac MC240

We have 4 MC240s among our friends. The bugle boy nor Amperex (both made in Holland) makes no difference. If you want the best 12AU7s then go with Amperex 7316 tubes.
Cleaner without loosing the nice sounding. Even the Telefunken 12AU7s are nice too.
Reason why we didn't like the RCA 6L6GC bk plate vs say RCA or GE 7027A is the lower freq control. The RCA 6L6GC bk plate sounded very loose and lack of control for lower frequency response. The 7027As are much tighter and more dynamic and it is not thin sounding.
Yes, I'd designed and made my own silver cable after went threw over a dozen speaker cables and 50 interconnect RCA cables.
Just a hobbiest not a businessman.

Hope this helps.

Thanks again Chang. (By the way, are you the Chang of the "Chang" filter?) I'll have a look in my tubes drawer and see whethe rI have the 7316 (most probably not), then I'll take your advise to try the Telef diamond bottom 12au7. Really.....I have tried the RCA bk plate, but with another amp, they didn't sound as you said and I really enjoyed the low. It could be the 240 is "sweet" and "thick" itself and if using the bk plate, it will be toooooo fat and as exactly as what you said. For the 7027A, is it a driect replacement of 6l6gc (plug and play)? I'll look for them.

Thanks again.

Yes the 7027A are direct replacement in the case of MC240.
Telefunken 12AU7s are more smooth and refined but less dynamic and punchy when compared to Amperex 12AU7s.

I have GE grey plate for the 12BH7, Sylvania is one pair of of 12AX7, the other is Tungsram, the 12 AU7's are RCA clear tops, and the output tubes 6L6GC are mundane made in USSR tubes...

I would say that I didnt hear that much difference rolling these signal tubes...I will try the latest Svetlana C wing for the output tubes..

I have MC40's.

The stereo vintage Mac amps are much more desirable. Both transformer's winding had to have same number of turns in order to make the match pair.
Since I don't have MC40 ( only heard the MC30s), I can't tell you how it sound. It is older than the MC240s. Aside from that, all vintage gear's condition can vary a lot depends on how much life left in the capacitor and the accuracy of the old carbon composition resistors.

There is a big difference between the Russian/China 6L6GCs vs the NOS 7027a and RCA bk plate 6L6GCs. The clear top RCA 12Au7 sounded warm and muddy. The top end roll off so much and the bass is merky. The GE grey plate 12BH7 also lack of clarity and fullness of the RCA bk plate 12BH7.
Definitely prefer the mullar 12AX7 long plate over everything else.
Again, just our observation in our 4 totally different systems with same results. Speakers range from Rogers LS3/5A 15 ohm, Proc 2.5, B&W 805s, ATC SCM10, platnum solos, Joseph audio RM-22si, Paragon regents... Preamp tested was ARC SP-11 MKII (3 different units) Counterpoint SA5000 (3 different units) and 2 different Marantz 7 preamps.

Again, opinions are subjective to our setups.