How far should I go with the Vintage Mcintosh MC275 Resto?

[kelossus]

Looking for a little guidance with an upcoming MC275 MK1 restoration.

So to my question.....How far do I go with the rebuild? Cosmetically it will come up fine ( it's never going to be a museum piece) but the transformer covers have a little pitting on top. I don' think I am going to go all out and get them resprayed. The chassis will come up good, just some very light surface rust/dirt which you can rub off with your finger. So cosmetics, I can live it.

Parts replacements.....I am going to replace the PSU section with the deluxe kit available on Ebay, CE capacitors etc...This a no brainer but I am also considering replacing the bumblebees and possibly the carbon resistors. Even if they test good I am considering replacements. Is this a bad idea? I want this amp to be good to go for another 50 years and given I am not a tech and require paying someone to do the work I am not all enthused to take it back in 5 years time when something goes bang.

 

[muon*]

Part replacements can be tricky, as it can inevitably change the sound and character or the amp., carefully chosen it can work well, but if not...

[muon*]

You could try replacing the carbon composition resistors with Allen Bradley Carbon Composition Resistors, but have to buy a fair few to match values, or do the baking of them as some do to remove moisture they have absorbed over time. Some values are hard to find these days too.

 

An alternative might be Amtrans Carbon Film Resistors.

 

Bubble Bee's are not attainable anymore i don't think, and the ones that they say are reproductions are not. If we are talking about coupling and decoupling caps you could go for a copper foil type.

 

That's the approach I'd take anyway.

Edited July 12, 2021 by muon*

[muon*]

The original carbon composition resistors might even be Allen Bradley.

[Willmax]

Wow Chris, that is an exciting and interesting project to undertake. I look forward to reading your experience on getting this done. I'll be watching with interest for the love of vintage Hi-Fi gear. 

Edited July 12, 2021 by Willmax

[kelossus]

5 hours ago, muon* said:

You could try replacing the carbon composition resistors with Allen Bradley Carbon Composition Resistors, but have to buy a fair few to match values, or do the baking of them as some do to remove moisture they have absorbed over time. Some values are hard to find these days too.

 

An alternative might be Amtrans Carbon Film Resistors.

 

Bubble Bee's are not attainable anymore i don't think, and the ones that they say are reproductions are not. If we are talking about coupling and decoupling caps you could go for a copper foil type.

 

That's the approach I'd take anyway.

 

Jupiter actually make repro bumblebees. How close they are to original I don't know but I would say Jupiter are amongst the best of the exotic capacitors so they have sparked my interest. If I were to replace all the bumble bees I would use a combination of the Jupiter Bumblebee and the Red Astor caps. For whatever reason the Jupiter Bumblebees aren't made in 0.22uf so I have to resort to the Red Astors.

 

https://www.hificollective.co.uk/catalog/jupiter-bumblebee-capacitors.html

Edited July 12, 2021 by kelossus

[kelossus]

1 hour ago, Willmax said:

Wow Chris, that is an exciting and interesting project to undertake. I look forward to reading your experience on getting this done. I'll be watching with interest for the love of vintage gear. 

Yes mate I am excited also. I am thinking of a complete system overhaul and retiring my Sovereigns until I can afford a bigger house. A return to horns and tubes is on the table thus far.

 

Once this gets underway I'll post pictures as the resto progresses.

[muon*]

2 hours ago, kelossus said:

 

Jupiter actually make repro bumblebees. How close they are to original I don't know but I would say Jupiter are amongst the best of the exotic capacitors so they have sparked my interest. If I were to replace all the bumble bees I would use a combination of the Jupiter Bumblebee and the Red Astor caps. For whatever reason the Jupiter Bumblebees aren't made in 0.22uf so I have to resort to the Red Astors.

 

https://www.hificollective.co.uk/catalog/jupiter-bumblebee-capacitors.html

The original Sprague? 'Bubble Bee' was Paper in Oil, the later ones were either Mylar or aluminum? and oil and were nicknamed Black beauties.

 

As I understand it *shrug*

 

Did know Jupiter had those ones.

 

Edit: I notice the Jupiter BB's are 400v, just make sure whatever you use is suitable voltage in a given position.

Edited July 12, 2021 by muon*

[Gryffles]

@kelossus you might consider the Arizona caps as they make a Mylar oil. Can’t remember which colour but you’ll figure it out.

[Bunno77]

Didn't think you were too keen on the 275 sound

[kelossus]

38 minutes ago, Bunno77 said:

Didn't think you were too keen on the 275 sound

Nah I'm not a big fan of Mcintosh SS amps but having owned a MC275 MV in the past I have nothing but fond memories. A chance to own an original was hard to pass up. You would think the original is going to sound different to the reissue anyway.

 

I got my K2's back and so am exploring tube options again.

[kelossus]

8 hours ago, muon* said:

The original carbon composition resistors might even be Allen Bradley.

Yes they are. The tech doesn't want to change them or the bumblebees.

Edited July 12, 2021 by kelossus

[muon*]

37 minutes ago, kelossus said:

Yes they are. The tech doesn't want to change them or the bumblebees.

I read on some forums that the original BB's can leak over the long term *shrug*

[kelossus]

16 minutes ago, muon* said:

I read on some forums that the original BB's can leak over the long term *shrug*

Yeah I should re-state. He is of the camp, If they test fine they should be left alone.

Edited July 12, 2021 by kelossus

[xlr8or]

The Mk1 has a cult following status that holds its resale value particularly well compared to the later versions. I see very few listed and from the ones that come up their price tag keeps increasing each time. The thing that is going most in value in the Mk1 is the heavy iron use compared to later versions.

 

Aesthetically, I wouldn't bother restoring anything on the outside unless some part of the external chassis has a mechanical fault such as a switch, socket etc. Inside, I would perform a complete overhaul of the caps, resistors and tube sockets to bring everything back to original working condition and within specification.

 

On a side note if some of the original power tubes are also included wowza you've got a great amp on your hands. I've read that these things came with original Genelax GL KT88's.

 

The only mod I would make is changing the bias from self to fixed with external adjustment bias points.

 

Have you any photos to share?

[kelossus]

1 hour ago, xlr8or said:

The Mk1 has a cult following status that holds its resale value particularly well compared to the later versions. I see very few listed and from the ones that come up their price tag keeps increasing each time. The thing that is going most in value in the Mk1 is the heavy iron use compared to later versions.

 

Aesthetically, I wouldn't bother restoring anything on the outside unless some part of the external chassis has a mechanical fault such as a switch, socket etc. Inside, I would perform a complete overhaul of the caps, resistors and tube sockets to bring everything back to original working condition and within specification.

 

On a side note if some of the original power tubes are also included wowza you've got a great amp on your hands. I've read that these things came with original Genelax GL KT88's.

 

The only mod I would make is changing the bias from self to fixed with external adjustment bias points.

 

Have you any photos to share?

All the small noval tubes are definitely original, the output valves are GE 6550A's. I am unsure if these were ever used in the MK1 so likely replacements.

 

The amp is at the techs so I can't take any further photos. The polished chassis is in pretty good nick with the lettering almost perfectly in tact. As mentioned a light surface rust is seen on the chassis beneath the tube cage and the transformers have that pitting clearly seen on the tops. The surface rust nearly wipes off clean with your finger so there might be something that could remove it while taking care to keep the lettering.

 

Edited July 12, 2021 by kelossus

[xlr8or]

Fantastic stuff. Further mods to consider are replacing the speaker terminals with modern variants. If you run out of space here you can simply ask the tech to use one set and have a switch that flicks through the various taps. Also, remove the fitted power cable and install an IEC socket instead. The RCA connectors may also need to be upgraded. Check all the pots and if they need to be changed. One optional consideration to think over is it worth also replacing the 117v 60Hz power trafo with a 240v 50Hz version to avoid using a step down transformer.

[BLAH BLAH]

Go the full bells and whistles on refurb...

[MattyW]

On 7/12/2021 at 8:30 PM, Gryffles said:

@kelossus you might consider the Arizona caps as they make a Mylar oil. Can’t remember which colour but you’ll figure it out.

 

I'm a big fan of the Arizona Cactus Blue caps. Lovely vintage tone bettering any most caps I've tried..... Prefer them to Jupiter, though not over Duelund.

 

Yes agree with @muon* Amtrans Carbon Film Resistors are truly excellent 

 

If it's a unit you plan to keep.... Well I'd go all out on it. Carefully lift the labels on the transformer covers and have them resprayed then affix them once more. Remove the branding and respray the sides.... I wouldn't touch the front and back though. Maybe a coat of clear to ensure the print stays in place over time. There's just something special about old gear restored to mint condition..... Have to situate them where you can see them when you do this to derive full visual enjoyment from them. 

 

No matter what you do you'll enjoy these. Your systems always sound great 

Edited July 13, 2021 by MattyW

[BLAH BLAH]

Oh yes now we're talking @MattyW...totally agree being the owner of some vintage audio gear my good self...there's just summit special about old audio gear restored to close as mint condition... 

[kelossus]

Ok, since you talked me into it, its getting a complete overhaul. Amtrans resistors and the Arizona blue caps.

 

I don't think I'll make any cosmetic mods like speaker terminals, iec etc...Maybe replace the power cable, that's all.

Edited July 14, 2021 by kelossus

[xlr8or]

8 minutes ago, kelossus said:

I don't think I'll make any cosmetic mods like speaker terminals, iec etc...Maybe replace the power cable, that's all.

 

Those older style speaker terminals only accept thin gauge bare wire or mini spades - a real PITA to use. You may want to reconsider that comestic option only.

[kelossus]

6 minutes ago, xlr8or said:

 

Those older style speaker terminals only accept thin gauge bare wire or mini spades - a real PITA to use. You may want to reconsider that comestic option only.

I have those small brass adapters for older amps.

[Willmax]

20 hours ago, kelossus said:

I don't think I'll make any cosmetic mods like speaker terminals, iec etc.

Good move I think re keeping the external appearance as original as possible. The electronics need to be brought up to spec most certainly. 

[kelossus]

The MC275 is on the techs bench today. As crazy as he thinks I am, I have committed to replacing every capacitor and resistor inside.

 

All the original valves tested perfect and I am considering a quad of GEC KT88's to top this all off. I still don't know if the GE 6550As in it are original.

Edited July 19, 2021 by kelossus