A pair of MC75. my first McIntosh tube amp.

Hi, Greetings from Monterrey, Mexico.

This is my first post, but I have years been visiting this excellent forum, please let me take this opportunity to thank you all for sharing your great knowledge. I’d make a list, but I am sure I’d miss some. Without your contributions I would not have been able to restore and/or fix my others McIntosh (MC2105, Mc2125, MC7300, C28. C26) in the wonderful state these are now. Thank you everyone.

Said that, I recently bought my first McIntosh tube amplifiers, a pair of the original version MC75. They are in very good aesthetic and working conditions, excluding the tubes, all parts are original.

They are a very late series numbers (within the last 300 built in 1970) and as you can see the previous owner loved them very much.


I noticed that the line voltage switch factory has been eliminated in this version.
As recommended I plan to renew all electrolytics and all the components out of specs with modern parts to be sure they amplify safely, conserving the original sound and aesthetics.

After an exhaustive search I could find a lot of information, (not much on the MC75 specifically) but found a big gem, the great post of Rodeodave from Australia (http://www.audiokarma.org/forums/sho...d.php?t=289952
with a complete list of components and suggestions. Thank you Dave…

Ok, problems from the start.
After a good first photo shoot, I proceeded to physically compare my amp with the diagram. The power supply of these MC75 is different, to explain it better Dave post a photo of the HV PSU (460V, 380V, 170V) which it is different in my amp.

Dave MC75:


My MC75:


A cable union instead of R30, and also R29 changes from 10k to 150k.

I think these changes are for the very late series of my MC75 but I'm not sure because I can’t find more than the same diagram in all the usual places.

I will appreciate any help or comment.

Regards from Mexico.

My thread may or may not be helpful, websearch "Twin MC75 refurb project next" , my AK identification.

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Wow - congrats! You don't mess around.

Good choice! Look forward to your listening impressions vs the transistorized amps.

THANK YOU...

Yes Pio1980, there are small differences in the PS of the late series.

I found R11 and R12 anode resistors also changed for the same reason.



All parts are originals. I will not change any values of the components currently installed if they are not recommended by the experts.
But it would be helpful to have the right diagram. I've searched a lot and can not get it.

Anyone have a diagram for the late series of the MC75?

I'll post step-by-step pictures of the restoration process if there's interest.

More pics. You will notice that all Film Capacitors are Sprage 160P.





I will appreciate any help or comment.

Thank you...

acmmty said:

THANK YOU...

I'll post step-by-step pictures of the restoration process if there's interest.

Thank you...

Click to expand...

Very Interested, thank you.

A few days ago I saw in the big auctions site a pair of MI75 fully restored by the famous classic hifi shop . The MI75 is the industrial version of the MC75. The ad showed a list with all the parts replaced.

The list have differences in the values of some parts.

My question is. There is no problem if I post the list? So that you can make comments or suggestions.

Thanks

acmmty said:

A few days ago I saw in the big auctions site a pair of fully restored MI75 full restored by the famous classic hifi shop . The MI75 is the industrial version of the MC75. The ad showed a list with all the parts replaced.

The list have differences in the values of some parts.


My question is. There is no problem if I post the list? So that you can make comments or suggestions.

Thanks

Click to expand...

Please post a link or the list. :yes:

For the MC75 twins, I measured all resistors as I went and replaced only the ones that were out of tolerance with film types of adequate wattage. I replaced all electrolyticaps with similar ratings except the bias reservoir, which went 22uF as per TDW's recommendation, do mind the polarity. I replaced the selenium bias rectifier with a silicon type and rewired the terminal board to include a 220 Ohm 1/2 Watt flame proof film series dropping resistor to comp for the lower voltage loss thru the silicon rectifier. This value was determined by setting AC voltage input by variac to give a measured 6.3 VAC standard across the heater bus lines with tubes installed, and getting the bias voltages with the new rectifier to match factory spec with the resistor value selection. I replaced all bumblebee caps in critical locations where voltage leakage would be detrimental, and replaced those in the power supply with safety types, also adding one across the AC input after the line fuse for additional noise supression. The red ink labeled black capacitors were retained as per TDW's recommendation as they were not defective.
Output tubes were matched from owner's used stock for symmetrical clipping at full power under loading after all defective/deficient small signal tubes were replaced.

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I wonder if everyone realizes the MC 75 of that time had better published specs than the 275, 240, 40 and 225. Only the 3500 with its 0.15 THD had better specs, though its full power frequency response wasn't as wide.

twiiii said:

I wonder if everyone realizes the MC 75 of that time had better published specs than the 275, 240, 40 and 225. Only the 3500 with its 0.15 THD had better specs, though its full power frequency response wasn't as wide.

Click to expand...

I wouldn't say "no" if offered a pair of them. [emoji106][emoji16]

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Thanks Pio1980 for sharing your knowledge with this newbie.:thmbsp:

This is the parts list of de MI75 that I mentioned in a previous post.




I will appreciate any opinions or comment.

Ideally iirc, the driver and predriver tubes should have closely matched sections for best performance. Perhaps one of the experts will comment.

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Nobody wants to say something in relation with the parts list?

For example:

The Bigger power supply capacitors have an advantage?

The termistor CL40 Vs CL80?


P.D.
Sorry for my English, I need a lot of practice.

Here's the list of parts I used for a set of MC-75s. Quantities are for 2 amps:

Qty 4: 820mf/250v, for the doubler circuit (some will tell you these are too much for the power transformer but they're available)
Qty 2: 220mf/500v for the filters
Qty 2: 100/100mf/500v for the filters
Qty 8: Mounting clamps for above caps
Qty 6: 47 mf/500v, for the first voltage stage
Qty 4: 0.22mf/600v, Orange Drop, coupling caps
Qty 4: 0.047mf/630v Illinois Cap MPW’s, coupling caps
Qty 2: 0.47mf/630v Illinois Cap MPW’s, coupling caps
Qty 6: Power supply diodes DO-41, or Hex-freds
Qty 2: CL-90 thermistors, one per amp

Then, measure the resistors and replace any that are out of spec immediately.

Once rebuilt, then set the bias voltage as called for in the Hints and Kinks Stickey for MC-30s and MC-225s. All the vintage Mac amps use the same arrangement.

Once set up and all the voltage reading are in spec, run the amps for a few weeks and then check all the resistors again. Odds are that more of them may have drifted from the heat-up/cool-down cycles. I've seen this on a number of the 60s vintage amps. Swap them, of course.

Then, if the operating voltages are back in spec, go back to listening for a few months and then test the resistors again.

Hopefully, that's all the resistors that need swapping.

Hope that helps.

The amps I redid now reside in the capable hands of Analog Addict. They were fun projects.

Cheers,

David

Hi guys, thanks for the feedback.

After desoldering I measured all resistors.



Red - Out of spec
Orange - In the limit
Yellow - In spec but drift
Green - Good

What do you think?

Change all parts as Rodeodave did
http://www.audiokarma.org/forums/showpost.php?p=5184926&postcount=41

Or replace any that are out of spec as you recommend me.

What type of parts would you choose and where?

Thanks...

Angela Instruments is supposed to have Allen Bradley resistors in stock, otherwise you can contact Jim McShane for most recommended parts. Some may prefer to replace twist lock elyticaps with new twist lock caps, which are available.

Hi. I am still here.

Sorry for the long silence.
I apologize for the delay in completing this project.
I had some medical issues that came up shortly after my last post.
Thank God I am fully recovered.

Pio1980 Thanks for the information and for reminding me of this outstanding project. I sincerely appreciate you taking the time …

Other delay causes:

- Difference in value between an original vs. restored amplifier
- Metal film resistors can affect the original sound.

I've thought about this for a while.
I'm fan of mcintosh since the late 70s and I only sell to buy something better.
I buy these amplifiers to use them and not for display as museum pieces. Well a bit from my friends lol...

I think the effect of the inductance of the metal film resistors may be important in the radio frequency or maybe in guitar amplifiers (NO NFB). With the use of good components the working voltage of the resistors is more important.

Finally my decision is to continue with the restoration with modern parts.
For the signal I plan to use Vishay RN and CMF (same with diferent standards) and CPF and MOX in the power supply.
I plan to restore one amp first and then make A-B testing with the other amp.

Due I live in Mexico, I want to buy all parts from mouser, very good service, free shipping on orders over $40.00 and I receive my order in just 2 days.

So my current proposed list is:

Capacitors:
Label, uF/WV, Mfr. #, uF/VDC, Tolerance, Temp., Type, Manufacturer.

C2 33p 598-CM05ED330GO3 33p/500 2% 125 °C Mica High-Reliability Dipped Capacitors CDE

C3 100/3 UKZ1E101MPM 100/25 20% 85 °C Premium Grade Type For Audio NICHICON

C4, C7, C8. 0.22/600 598-224MPW630K 0.22/630 VDC 10% 85 °C AXIAL Illinois Capacitor

C5, C6. 0.047/600 715P47356LA3 0.047/600 5% 85 °C Orange Drop CDE

C9/C10 12/250 TVA1505 12/250 20% 85 °C Axial Lead, Low leakage current TVA ATOM®

C11, C12. Ceramic. stay

C13/C14 250/250 ELXS3B1VSN331MP40S 300/315 20% 105 C General long life, Snap In United Chemi-Con

C15a /C16a 100/500 ELXS501VSN221MR45S 250/500 20% 105 °C General long life UCC

* C15b+C16b 30/450 UCS2E101MHD 100/250 20% 105 °C 10000 16 25 7.5 High ripple, Long Life NICHICON

C17 12/250 UCS2E220MPD1TD 22/250 20% 105 °C 10000 12.5 20 High ripple, Long Life NICHICON

* C15b and C16b are in parallel in the late series of MC75. Replace with one 100uf/250v. (diagram says 60vdc.)

Resistors:
Label, Ohms, Power Rating, Mfr. #, Tolerance, Watts, Temp. Coef., Voltage Rating, Temp., Noise, Dia., Leght, Type, Grade.

R1, R15, R16.220K 0.5W RN60D2203FB14 1.0% 0.25 100 300 - 55 C to + 175 C 40DB 0.64 10.8 MF Military

R2, R8 100K.0.5W RN60D1003FB14 1.0% 0.25 100 300 - 55 C to + 155 C 40DB 0.64 10.8 MF Military

R4 22K 0.5W RN60D2202FB14 1.0% 0.25 100 300 - 65 C to + 175 C 40DB 0.64 10.8 MF Military

R5 2.7K 0.5W RN60D2701FB14 1.0% 0.25 100 300 - 55 C to + 155 C 40DB 0.64 10.8 MF Military

R6 3.3K 0.5W RN60D3301FB14 1.0% 0.25 100 300 - 55 C to + 155 C 40DB 0.64 10.8 MF Military

R7 68 0.5W RN60D68R0FB14 1.0% 0.25 100 300 - 55 C to + 155 C 40DB 0.64 10.8 MF Military

R9, R13, R14 2.2M 0.5W RN65C2214FB14 1.0% 0.5 50 300 - 55 C to + 155 C 40DB 0.64 14.27 MF Military

R10, R23, R24 18K 1W CMF6018K000FHBF 1.0% .5/1 w 50 450 V - 55 C to + 175 C 0.64 8.74 Industrial

R11 1W CPF251K000FKE14 1.0% 2W 100 350 V 0.81 8.74 MF VISHAY

R12 56K 1W CPF256K000FKE14 1.0% 2W 100 350 0.81 8.74 MF VISHAY Ind. Pwr.

R17 2.7K 1W CPF22K7000FKE14 1.0% 2W 100 350 - 65 C to + 230 C 0.81 10.8 Industrial Power, Precision, Flameproof

R18, R21 12K 2W CPF212K000FKE14 1.0% 2W 100 350 V - 65 C to + 230 C 0.81 10.8 Industrial Power,

R19 R20 1M 0.5W RN60D1004FB14 1.0% 0.25W 100 300 - 55 C to + 155 C 40DB 0.64 10.8 MF Military

R22 R25 18K 0.5W CMF6018K000FHBF 1.0% 0.5W/1W 50 500 - 55 C to + 175 C 0.1 0.64 10.8 MF Industrial, Prec

R26 R27 220 5W CP0005220R0JE14 5.0% 5W 300 1000 - 65 C to + 275 C 0.91 22.22 WW VISHAY 25W X 5Seg

R29 150K 1W MO2C154J 5.0% 2W 200 500 - 55 C to + 200 C 0.8 18 MOX KOA

R31 100K 1W MO2CT631R104J 5.0% 2W 200 500 - 55 C to + 200 C 0.8 18 MOX KOA SPEER

R32 100K 0.5W RN65D1003FB14 1.0% 0,5W 100 350 0.64 17.45 MF VISHAY

R33 62K 0.5W RN65D6192FB14 1.0% 0.5 W 100 350 -55°C to +175°C 0.64 14.27 MF VISHAY

Diodes:
SRI, SR2 UF5408
SR3 UF4007 (plus a dropping resistor)

I would appreciate your opinions and advice.
Thank you...

Hi there

After the caps arrived I rebuilt the power supply in one MC75.
The other amp is waiting for the A -B testing.
I replaced all electrolyticaps, the out of spec resistors and the selenium bias rectifier plus a dropping resistor to restore the -150V bias across C17.
One can capacitor is leaking but looks good on my RC meter. For security reasons it would be better to rebuild both power supplies before the A-B Testing.

Some pics:









Bringing the unit up on a variac, I get no hum and the test signals looks fine in my oscilloscope and distortion analyzer.

For today I'm quite satisfied with the results.

I think your choices are good, careful work should give good long term service.