Mac MC240

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.

I am kinda new at the tubes..but its funny, i find the amps as they are now, to be tremendous powerhouses throughout the freq range..again ,maybe ignorance is bliss in my case. Just cant spring for NOS tubes really.

Also,I double checked, and I dont have the RCA clear tops (differemt amps)...they are Sylvania also.

When i bought my amps, a McTech brought them up to specs. they are driving Tannoy System 12 DMTs thru a Berning TF-10 preamp/

Dear friends,
Thanks for all the inputs. I definitely agree with Chang's comment on the clear top RCA 12au7. I have a matched pair. It is good but all depend on which preamp you used. I don't think I have the long plate of the Mullard 12ax7 (or I may, just somewhere in my galaxy!), but I have a short plate.

Two more questions: 1. It is on the position of the power tubes. Say, the one nearest to the front panel (with the knobs and speaker wire screws) is #1 and #4 will be the one near the back. Which pair is for the right channel and which is for the left channel? If I have two matched pair (not matched quad), how should I align them? 2. What is the first 12ax7 (usually with a protection cap) used for? I can see the rest six tubes are for the right and left channels, just not sure what is the 1st one used for.

Thanks again and have a nice weekend.

1. For the output tubes, the one in front of the left transformer is left channel ( The center transformer is power transformer) the vise versa. V8 and V9 are left and V10 and V11 are right.
Since MC240 are auto-bias, it is recommend that match quad is used. If not match quad then match pair should be 2nd choice. Honestly, I couldn't tell the difference even when tubes are not matched.

2. The first 12AX7 is the input tube for both channels.

Thanks Chang.