Modified MC240 thoughts

[ChefIvo]

Hi all, long story short, I recently acquired a MC240. I have never came across one of these before. I paid the huge sum of $100. I did not tested it( garage sale) and took it right away to my friend who's repairing audio electronics. After we looked at the amp we saw that it uses different preamp tubes than the original. 5965 instead of 12bh7, 6bk7b instead of 12au7 and 12at7 instead of 12ax7. We plugged it in and we were shocked. This amp had the biggest sound I've ever heard. Even my friend said that he never heard anything like this before.
Very dynamic, great bass, smooth and detailed highs. We took the bottom of and found that it has been recapped with some high quality parts. Somebody used a military grade silver wires and caps that costs around $40 each (12 or 13 as I remember). I enjoyed the amp for a month and then decided to put it all stock. To use the original 12au7 we just had to re wire the tube socket. we put all the tubes specified for it and turned it on. What a difference. The MC240 sounded mediocre at best compared to before. Still better than almost everything I heard before, but the magic was gone. I know it will hurt the value but for now I will convert it back and enjoy it.
BTW we used a Sony CDP-707es, technics sl-1600mkii with Denon moving coil. 1994 MB QUART 2000 speakers (28hz-35khz frequency response) and KLIPSCH KLF-20
Here are few pictures of the amp.
Thank you

 

[BillWojo]

Can you post a schematic of the modification? It would be very interesting to see what they did.
By the way, you stole that for a hundred bucks! Congrats!

BillWojo

 

[ChefIvo]

Can you post a schematic of the modification? It would be very interesting to see what they did.
By the way, you stole that for a hundred bucks! Congrats!

BillWojo

Bill I'll take the cover off tomorrow and take pictures

Also the Balance pot looks like it has been changed and now it's like some type of loudness control. It doesn't not make it louder like normal loudness control, just makes the sound bigger. I forgot to mention I'm using the twin inputs.

 

[ChefIvo]

More photos

 

[arts]

Good grief,there's all kinds of ''stuff'' going on in there.I'll second wanting to see a schematic....

 

[drtool]

 

[c_dk]

I'd like to see a proper bench test. With the known response variables between the various tubes swapped, well who knows what the frequency response curve now looks like.

 

[dewickt]

I bet the THD, IMD, and S/N look bad, with bass boost and a wild response curve, this is a candidate for gutting and rebuild.
Always makes me wonder why they sold it if the mods made it sound so good...

 

[c_dk]

I can't help but think a set of 40s and a turbo whale tail would sure make that Porsche Speedster go fast......

I am certain it does sound different.....

I really do hate to be such a wet blanket and it may have a bunch of "special" parts, that said the workmanship is just well.....?

 

[nj pheonix]

Sacrilege !

 

[ChefIvo]

I bet the THD, IMD, and S/N look bad, with bass boost and a wild response curve, this is a candidate for gutting and rebuild.
Always makes me wonder why they sold it if the mods made it sound so good...

Hi Terry, the guy who created that monster passed away a couple of years ago. I never met him. His wife was cleaning the shed. That's how I found it. She did not sell it because it sounds bad. She did not has any idea what is it.
I know rebuilding is a good idea, but I want to do a comparison with another mc240 before I take that step.
There's a sticker on it that say that the mods are done in 1978, so whoever built it was enjoying for almost 40years

 

[c_dk]

Doesn't your local tech have a proper test bench. These particular amps have been tested 1000s of times at Mac clinics all over the U.S. The base line measurements are well known.

 

[ChefIvo]

Doesn't your local tech have a proper test bench. These particular amps have been tested 1000s of times at Mac clinics all over the U.S. The base line measurements are well known.

There is another guy who is certified McIntosh tech I will ask him to do some testing

 

[ChefIvo]

So we did a bench test.
Frequency Response under 10hz-50khz
46watts at 8ohm
THD 0.08%
I took a video and will try to post it

 

[damacman]

I once had two pair of MC60s that had been been reworked similarly by some mad man in Florida. Best sounding tube amps I've yet heard! Man did they sing.

 

[arts]

Unfortunately,most ''mods'',especially the more radical examples,rarely have any schematics,notes or explanations as to why they were done or what they were hoping to achieve by implementing them. Nor will you ever find any reference to before and after test results.Still,I gotta admit I'm awfully curious about this one

 

[Mike Gibson]

It seems to me that since you are the one who has to live with it and you love the sound that would be the deciding factor for me. If you go to sell it you can always put it back to the original topology. Just my uninformed .002.

 

[c_dk]

How linear is the response......+ and- dB from 20 to 20 k at 8 or 9 frequencies between those extremes.

Since the orginal amps were +- 1/2 dB from 20 to 20 k we would expect to see deviations from that linear response to achieve the sound difference you describe.

 

[ChefIvo]

How linear is the response......+ and- dB from 20 to 20 k at 8 or 9 frequencies between those extremes.

Since the orginal amps were +- 1/2 dB from 20 to 20 k we would expect to see deviations from that linear response to achieve the sound difference you describe.

The tech guy(official McIntosh repair shop)
Said that everything is within +- 1db
He asked me to take videos so you can see the results. I just don't know how to upload them

 

[mnmmt]

If you love the sound and the tests show it is operating properly, I wouldn't change a thing. Just run it and enjoy it! And, if you are so inclined, map out a schematic so the rest of us can have a look at how it is set up to achieve this.