MC30 first rebuild, old versus new circuit which is better?

CDinMB

Active Member
Hi, new to the forum and McIntosh stereo. I have loved the sound of these amps since I was a kid and finally bought them. I am now finding my self rebuilding them almost immediately after purchase and have a question before I go any further.

Is there a preferred circuit design among the running changes for the MC30?

Here is why I ask. I am rebuilding the two MC30s due to one developing a loud pop on start up. This is my first time working on Mac amps and they are my personal units I want to keep forever, so I want to build them right. I decided to replace all the circuit components not just the power capacitors. I am not an amp design expert but I can follow a circuit and layout. As I dig into my amp's circuit and study the schematics I find my circuit doesn't appear anywhere in documentation, so I am assuming it is an older circuit. I am faced with the choice of replacing existing parts as is or rebuilding to the new circuit design in the docs. Replacing what exists is easier but is there a reason to change it? Since the number and value of the capacitors are different, I assume this will change how the valves behave and have some sonic impact. I have experience with old Fender tube guitar amps I own and I know changing component values can change tone. I have searched and read for a few days here on the forum and not found any discussion of the +/- of either circuit. Some pictures show people have units with the same 6 cap design in the signal path I have versus 7 in the newer circuit but I haven't found a discussion of the difference.

So for the Mac MC30 experts looking at my amp's circuit, should I rebuild it as is, or move to the newer circuit and cap values shown in the schematics?

IMG_2898.JPGIMG_4186.JPG IMG_1972.JPG

Thanks for input, I am really at a cross roads and don't want to do this twice.

TIA
Carmen
 
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since you can follow a schematic, why not post original and mod and let the tube wizards
explain, then you can make an intelligent decision based on fact.

newer is not necessarily better. it may be a circuit condition that needs attention.

if you don't do this and upgrade to the new mods, anything going wrong puts everyone
back at square one with the question why.

do document each step of the way and for each mod. that way you can retrace your steps.

recently I helped some "re-capping" problems that had nothing to do with "recapping"
IOW the problem was separate from the recapping and recapping was blamed because
it took the most time or it was coincidental to the refresh that included recapping or it
was something else not stated that was the problem.

since you have two, do fix the one with the "pop" then when they are identical, at
least in operation, then change one to the new mods and compare. that way you
have a control sample.
 
Truth be told, especially if you aren't an very well versed expert, I would not concern myself over which specific version of the circuit you have. I was in a similar place with my MC60s; my particular variation wasn't exactly represented by any circuit diagram I could find. My amps had a couple subtle differences, too.

My recommendation is: Take a lot of pictures and restore YOUR amps. Replace things exactly with new components in the exact same places. Believe me, this will be challenge enough. Then, if anything goes wrong, you aren't in the weeds wondering if it was a component, your modification to the circuit, a tube, or otherwise. Trying to change a circuit at the same time is asking for trouble, and confuses any troubleshooting.

Furthermore, if they are working and you can't find an exact schematic, take a set of resistance and (carefully) voltage measurements for a basis of comparison. This I did not do, and I sorely wished I had, because the service manuals close to my actual schematic did not have a full set. Worked out OK but could have been a problem.

Good luck!
 
Both units are the same. One had rect caps replaced before I bought it but the circuit is the same. I also have an MC40 which I am using while the other 30 is apart.


IMG_2179.JPG IMG_2178.JPG

I think I found a schematic for my units but I haven't traced the circuit completely yet. Capacitor count and values are consistent. Willl do that tomorrow.


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Can't really tell, but that looks like the schematic for a much later unit. Send me a PM w/your email and I'll send you all of the different schematics that I have from very first unit to the last (I think). What are the transformer #'s on yours?
 
First, thanks for the all help from x3workshop! I now have schematics and a direction. Here is my fitting of caps. Still missing one 8uf, on order. Decided to stay with the original circuit.

A question about the Jupiter HT cryogenic caps in this chassis. They are supposed to be ok for higher temp of 70c/158F. They were highly rated and reasonably priced .047uf caps. Does anyone know if they will hold up or am I headed for a wax melt down and catastrophic failure?

IMG_2950.JPG

Carmen
 
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I do not think a 70C rated part would be wise.

I'm not exactly sure how hot these run inside, at steady state while operating, but just from waving your hand over the chassis I think it could well be close to 100C (boiling point of water). When I did my MC60 restoration, I used a 3D printed bracket; that was made with ABS. ABS has an extrusion temp of 230C but a glass transition point of 105C. So far my brackets are doing just fine, and I've had the amps on continuously for periods in excess of 24 hours a couple times when forgetting to switch off the system. Take from that what you may.
 
Thinking about this, I thought why guess? I'll measure it. K type thermocouple accurate to +/-1.8F set between the two capacitors in question in my operating MC30. I let it warm up at idle and once stable I added music at half gain which is more than I ever listen to on a sustained basis. I am in the middle of the test ATM and will post the graph. I can say it took 45 minutes to stabilize at idle. Starting room temp of 73F and stabilized at 123F with no load.

IMG_2951.JPG
Thermocouple between the two caps running through the I/O pin opening to the outside.

IMG_2952.JPG

So far 15 min of sustained load and the temp has gone up to 126F.

IMG_2187.JPG
 
I also was in the same boat (to a degree) with MC60s.
I had two different versions. I rebuilt each to its original design. At the time i asked Dave Gillespie why mac changed them and IIRC he said it made it easier on the driver circuit. I'm sure i can't tell them apart sonically. Good luck with your project.
 
Well 4+ hours in, running steady state with music at 1/2 gain, not really able to sit in the room at that volume and worried about the neighbors! It was a worthy test though as it stabilized at 137F/58.3C. That should be well within the 158F/70C max sustained temp for the Jupiter HTs. Glad I don't have to trash them and replace them with my other choice, Rike Q-Caps. Although the Rikes sound like excellent caps, that would be a few week delay. I may try them on my other unit and decide which I like better.

Here is the data. Actually it is still fluctuating between 136/137 on the readout for the last two readings, so I called it 137.

IMG_2190.PNG
Hope this helps someone else someday. Time to start mounting resistors!
 
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First, thanks for the all help from x3workshop! I now have schematics and a direction. Here is my fitting of caps. Still missing one 8uf, on order. Decided to stay with the original circuit.

A question about the Jupiter HT cryogenic caps in this chassis. They are supposed to be ok for higher temp of 70c/158F. They were highly rated and reasonably priced .047uf caps. Does anyone know if they will hold up or am I headed for a wax melt down and catastrophic failure?

View attachment 916558

Carmen

Carmen,

How do those Mundorf changed the sound of MC30? I have Silver Gold in Oil in my preamp, love them, but they did sound little brighter than stock caps.
 
I am still waiting for a couple of parts and will have it running this weekend. I will report back how it compares to the other 30 that has had three caps replaced (shown in the thermocouple picture above) and the 40 which is all original, I think. These are pushing LaScalas so I don't need brightness, I was shooting for detail in the mid range and control of the bottom end. I liked the sound of this amp in the high and mids before the rebuild, it was sonically the same to the other 30 albeit with more volume, but certainly softer and rounder through the range than the 40. It was not brittle at all, which is great on the high end, but I do prefer the 40 for bass. Logically I think any one of these amps is not going to give me the softness at the top and control at the bottom and bi-amps on the LaScala Ind are in my future at some point. That would mean finding another 40 and more rebuilding. I am just happy in denial ATM, hoping to be proven wrong.
 
Some believe (Mundorf included) that those caps need a hundred or so hours to burn in.
My personal experience is the most change happens in the first 30 hours or so. Not looking to reopen the do caps break in dispute. I never found that was the case with polypro films. YMMV, If you disagree that's fine. I found it to be the case so i thought I'd share. If your experience is anything like mine, you'll find they smooth out and probably won't be as bright in the end. Please let us know
 
Finished soldering and need to trace the circuit and check all the connections. Not the prettiest, but hopefully it will sound great. The only change from the original circuit is the 9.1K resistor as suggested here in other posts. Originally there was a 4.7k so I have one to drop in if heater isn't at 6.3v or bias is way out.

IMG_4274.JPG

Probably should have gone with electrolytics and saved some space.

IMG_4272.JPG
If anyone sees any obvious mistakes that my eyes aren't please speak up. Thanks!
C
 
Finished soldering and need to trace the circuit and check all the connections. Not the prettiest, but hopefully it will sound great. The only change from the original circuit is the 9.1K resistor as suggested here in other posts. Originally there was a 4.7k so I have one to drop in if heater isn't at 6.3v or bias is way out.

View attachment 918135

Probably should have gone with electrolytics and saved some space.

View attachment 918137
If anyone sees any obvious mistakes that my eyes aren't please speak up. Thanks!
C


I used those Mundorph caps in my MC30's and am very pleased with them. I thought they were very reasonable as well.
Those Jupiter caps look nice too, but they were a bit pricier. Was thinking of trying them as coupling caps in my Fisher SA300.
 
Well, I have it up and running and it sounds awesome, better than the other MC30. Better bass and clearer mids, not harsh. I have a few questions about voltages about my primary tap choice. I moved the tap to 125v since my wall voltage usually sits at 120 and in the summer FPL likes to juice it 121+. So on the variac I have the voltage sitting at 120.1v measured by the same DVM I used for all other readings.

Since I moved the tap to 125v all my readings are a bit low of standard but heater is a bit high. I can leave it or move the tap back to 117 measure the voltages there. My assumption is that would increase my plate voltage closer to spec but I would need to reduce my heater volts. The other change I made was 9.1k resister in place of the 4.7k that was there.

Here are the results of readings.
Measured. Mac std
6L6 Plate 400. 440
Pin3 400. 440
Pin5. -39. -42
5U4 pin2. 414. 460
12AX pin1 123. 120
Pin3. 1.16 1.1
12AU pin1. 235. 240
Pin2. 123. 120
Pin3/8. 127. 129
Pin6. 244. 240
Pin7. 96. 96
12BH pin1. 328. 352
Pin3/8. 15. 18
Pin6. 324. 352

Heater 6.5v. 6.3
I know spec on the heaters is +/-10% so 5.4 to 6.9 but 5% is recommended. I am still within that. So good.

Plate voltage being lower than 440 will cause the amp have more head room, longer tube life but sound thin. I guess I will run it this way and see if I like it. It sounds great so far and I am sure it will settle as the components learn to play nice together.

I am thinking listen for a while and measure it again after a few hours?
C
 
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