Mac MC240

Thanks Chang.

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.

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.

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/

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 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.

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.

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.

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 Chang. For the Amperex 12au7, do I need the bugle boy or just amperex from USA (or Holland) is good? For the power tubes, I'll stick with my RCA 6L6GC black plate. Did you do the silver cable yourself? Or actually you have some for sale?

Thanks again.

1. Hi/Lo voltage 115/117 switch
2. Mullard 12AX7/ Amperex 12AU7/ RCA 12BH7 bk plate/ US brand 7027A instead of 6L6GC not Russian tubes
3. Home brew pure silver cable/ NBS Master speaker cable