My boss Jack has had a pair of old dusty and slightly pitted MC-30's laying in his office for several years. The story was that they belonged to his father who purchase them new sometime in the golden age of Hi-Fi, and they had been languishing at the home of one of his other sons for years. Jack came across them and rescued the pair thinking that someday he would put them to use driving a pair of Renkus-Heinz commercial 16ohm speakers we've had in our warehouse, also collecting dust. Well, he finally asked me if I could do something with them.. duh!
I got them home (heavy buggers!) and got one up on the bench, opened it up and found that some repairman from the distant past had done some meatball surgery that had to be corrected. One of the amps power supply section was really messed up by someone - the rectifier feeds a 30uf cap first, then a 150 ohm power resistor to an 80uf section forming a CRC filter. But what I found was actually a 10K power resistor feeding the second stage filter (which is B+ for the output stage). I don't see how this could have even worked! Plus, there was a 12au7 instead of a 12ax7 in the driver cathode follower stage. Also, the power cord had long since disintegrated and there were some burnt wires (why did they use 22 gauge wires from the rectifier to the filter cap??), but overall things looked pretty good.
First things first, get rid of all the 45 year old electrolytics! Except for the main filter caps, which measured good with the cap meter (all 3 sections), there were dried up bias supply caps and input stage cathode bypasses that had to go. Also, the paper/wax coupling caps were suspect; I've heard that they tend to get leaky- though some audiophiles like the way they sound. I don't. I like detailed music reproduction, thank you. These caps would be good for a guitar amp where you're going for a colored sound, not Hi-Fi (now days there are very expensive 'boutique' paper in oil capacitors available, I'm not talking about these). Most of the other coupling caps are some sort of plastic encapsulated film, not sure about them. I left them in for now.
Since Jack is not an audiophile I didn't use any exotic parts; rather than an 'upgrade' this project is more of a resurrection. I used what I had on hand. I replaced the caps listed above with standard modern electrolytics, replaced the output stage coupling caps with poly film (.47uf @ 630V) and installed grounded power cords (I cut the IEC ends off of some stock power cords that come with AV equipment). I also put in some 330uf @ 450V Panasonic snap-in caps that I've had laying around since the '80's (these were in the first iteration of the ESA 66-100 which have long since been scrapped for parts). These I paralleled with the 80uf main B+ caps, and bypassed them with some .1uf 450V poly's.
I do see alot of room for improvement, though. Like replace all the old coupling caps with decent film caps like Solen or better, do more power supply bypassing for the input and inverter stages and get rid of all the unnecessary input wiring and hardware. The input RCA's could use an upgrade, and a heavier barrier strip so you can connect some 'real' speaker cables would be nice.
The design of the amp is interesting. There's the obvious 'Unity Coupled' output transformers which have a cathode winding from one tube in phase with the plate winding of the other tube. There is the 'bootstrapping' of the driver; the B+ for the driver load resistor is derived from the plate winding of the same phase tube giving a positive feedback to the driver tube while delivering negative feedback to the output tube. And there's the 12ax7 cathode follower between the driver and output stages, direct coupled to the input grids and therefore having the -45V bias voltage on their cathodes. This tube is also bootstrapped; the positive feedback is necessary to derive the very high AC drive voltage required for the output stage (remember the cathodes are in the transformer winding causing negative feedback, lowering the overall gain of the stage). The input stage is pretty standard and uses 1/2 of a 12ax7 direct coupled to a 12au7 configured as a cathode coupled phase inverter. The McIntosh design was way ahead of its time and the performance is reported to be excellent. All in all, alot of circuitry for 30 watts!
Ok, so after all the circuit work, I cleaned up the chassis and tubes, plugged them into AC, and checked out the voltages. One amp measured spot-on, the other had some funkyness in the output stage. One output tube was in cutoff and it's bias voltage measured -220v! Some visual inspection revealed that I had inadvertently cut a 220K cathode resistor for one of the followers, which supplies bias voltage to the grid of the 6L6. Repairing this error fixed it right up!
Alright! I'm too impatient to put these things on the scope and do the requisite battery of tests, I just want to plug them in and play some tunes! They sound pretty fine, I must say. They're a little dark and the bass is not as tight as I'm used to (the 66-100's are highly resolving and have excellent, tight bass). But overall, they have that tube magic. I bet new output tubes and performing the rest of the upgrades as listed above would bring them into better focus, but my boss just wants them to work and I think that they'll do just fine with the high-sensitivity horn loaded Renkus-Heinz speakers he's going to drive for his office system.
Just put these babys on the test bench. Amplifiers terminated into 8 ohm resistive dummy loads.
Square wave response is good for 1Khz, 10Khz is a bit rounded off on the leading edges, suggesting a bit of high-freq. rolloff.
Freq. Response is flat from about 30hz out to about 15Khz, and there's a bit of droop on either extreme, as viewed on an o-scope.
Power is 45 watts before visible clipping on the amp with the better output tubes. The one w/ the weaker set puts out about 25 watts before clip. Think I'll recommend new tubes to Jack.
No distortion measurements made at this time.
I had fun restoring these amplifiers. They are very well made, well laid out and easy to work on. I'm sure Jack will enjoy listening to them as much as I have!