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Old 2nd March 2013, 05:47 AM   #1
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Default McIntosh MC-275 buy or build an auto-bias tube amp?

Hi guys, I need your help. After 3 Ĺ years my divorce is almost over and I want to celebrate. Instead of buying a house, new SUV or finding a forty something female companion, I decided to upgrade my crappy stereo system (Technics SA1000, a pair of Dynaco MKIVís, Technics rack system ST9030, SH9020 etc and assorted old school stuff). I came across a deal for a new in the box McIntosh MC-275 25th Anniversary edition amplifier that looks really tempting. The other option is to build an auto-biasing tube amp. My dream system has always been a pair of 6L6 monoblocks driving tweaked Altec VOTT A7ís. Yes, I know I live in the past.

She got the tools so I will have some issues fabricating the chassis and cabinet. I did manage to get my test equipment and my soldering stuff out. I have an apartment so I am working off of the kitchen table.

I looked for auto-biasing amps but didnít find anything that really stood out. I listen to jazz, blues, old easy listening and heavy metal (Devil Wears Prada and Mastadon) and have a pair of Technics SB-6000ís that still sound half decent.

What is going to give me the most bang for the buck? Buy the turnkey or build?
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Old 2nd March 2013, 06:50 AM   #2
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IMO, you get more for your money, when you build.

Spec's for the SB-6000s are 6 Ω/93 dB. Let's derate a bit for impedance curve dips. Call the speakers 4 Ω/91 dB. Even for "head banger" stuff, 30 or so WPC should be plenty. As you mentioned 6L6s, how about Mullard style circuitry and Russian 6П3С-E (6p3s-e) "finals"? Edcor's CXPP60-MS-6.6K is a "budget" possibility for the O/P trafos. With SOAF not a factor, a large, sturdy, commercial kitchen aluminum baking pan will do nicely for the chassis. Finish the chassis with "hammerite" in a color you like.
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Old 2nd March 2013, 10:01 AM   #3
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Three and a half years of divorce? I can't begin to imagine what that is like because that to me is a long time without listening to music through a nice amp without distraction.

So you want to figure if you want to build or buy an amp and you have a kitchen table, new girlfriend, and live in an apartment/condo/small space. Well, you came to the right place because I, like many others here, have an opinion for you to consider.

An MC275 is a pretty impressive piece of hardware, with clean lines, classic McIntosh tube amp shape, low distortion, pumping out 75 W alone into two speakers and 150 W as a monoblock. That my good man, being output through your speakers is ear bleeding levels for your neighbours and you might get a sternly written letter sent to you by the property management company if you turn it up. It is probably going to run north of $2,000 if in excellent condition with new tubes. That is a good chunk of money.

Three and a half years of drama, you can probably do without more drama (re: sternly written letter mentioned up there) because you want to listen to jazz, old easy listening (Gerry Raferty doing Baker Street or Down The Line), and your (I can't say I have ever heard of them before, but I'll definitely look them up), devil music of Mastadon Wearing Prada.

With all that said, here are three suggestions for some DIY amps depending on the size of your kitchen table, time you have for hobbies, current set of speakers, and skill level.

Scott17 here makes a well respected kit. Not 6L6 tubes, but a KT88 amp based on Alex Gendrano / Mikael Abdellah design. It runs single-ended for a pleasant sound and when you want extra power, flick of a switch and you go ultralinear. If you lurk here at diyaudio, then you might have seen him, but he has changed his old avatar to a new one (I liked the old one better of a satellite dish array).

KT88 SE Basic or Master Kit

You haven't the space or tools, but with the Master Kit you have everything (including chassis, tubes, sometimes paint colours, chokes, instructions, and transformers) you need. You supply the tools, solder, different coloured wire, and kitchen table. Contact Scott17 for more or read the whole thread and see what others have to say about his kit.

Another member here said he listens to this type of amp in SE when he is alone to listen to music and flips the switch to UL when he has a party.

Which leads me to my next suggestion of a very highly respected vendor here, George of tubelab.com . Read the posts in the Vendor Forums here and then read up on his website.

Tubelab - diyAudio

Tubelab Home

This man has some super stress tested boards with what sounds to be very flexible layouts. For example.... You decide you want the SSE (Simple Single Ended KT88 amp) with no bias adjustments? This is it! Want the ability to go Cathode feedback? SURE! SE versus UL? THERE'S SWITCH FOR THAT! Tube rectification versus solid state? THERE'S SWITCH FOR THAT! Roll your own tubes (6L6, KT88, EL34, and others I think)? SURE! CLC or CRC power? YOU CAN MAKE IT SWITCHABLE! Want big motor run caps? THERE IS AN OPTION FOR THAT TOO!

And (are you sitting down?) you can do all this on a PCB and mount components either side so you can figure out on the chassis you want and how you want it to look.

The boards help out diy builders with limited space a lot because you can mount components, solder, and put them away for dinner then bring them back out a few hours or days later to work on. And this provides the most flexibility depending on how much switching you want in there and which way you want to build it. There is a lot of posts and you can see a lot of pictures of creative builds. You get the boards and you can probably find a Mouser or DigiKey order list for the BOM and the order the iron from Edcor. The chassis is up to you, but there are enough drawings out there for drilling a Hammond steel box or to getting a top plate CNC routered for yourself with the related files.

The other cool thing is that George has some experiments you have to see at

The 833A SE Amp Prototype

His boards can take a beating and his designs and builds are rock solid. The Simple SE again is low erwattage than the McIntosh ( but you can't easily build a McIntosh either) and a max of 10 to 12 watts in UL tube sound with options of going SE and rectification options.


BUT if you want loud enough that you can't hear how awesome this reply was, then you will have to go push-pull. And keeping the kitchen table in mind, the try this one last suggestion: the Bob Latino tubes4hifi ST70 kit remake and redesign based on the legendary Dynaco ST70. This an overbuilt well thought out and well thought of kit which lets you go triode mode to pentode mode on the fly so you can listen to which setup you like. I am reservations about doing that but others have and it was designed to be switchable, so sure, why not?

tubes4hifi amplifier KITs page

The ST70 kit comes complete with thick heavy duty chassis and all the iron you can lift. Then three diy options -- no tubes (supply your own), with EL34, or lastly with KT88 tubes. Heck, you can even buy it fully assembled with tubes included if you wanted.

Then there are the other options of attenuator, cap upgrade, and I think that is it. You can easily bias it with a single solitary voltmeter through the front and done!

And there are other PP amps and monoblocks for sale also. From reading the forums connected to the kit, people have successfully rolled some small (6L6) to pretty big (KT88) tubes in the ST70, various different tubes (KT66, KT77, 6CA7, 6L6, EL34, KT88, KT120, and a host of drivers tubes too!) and some HUGE (KT120) tubes through all of his kits.



There. Three options. Three different build types from complete to the nuts SE kit to bare PCB SE build and order parts build, to a full PP kit with stainless chassis to an full SE kit with walnut chassis.

You decide on yor pain threshold, space constraints, your own sill level, time you have for this, and how kuch you want to spend. All of these will sound very peasant through your speakers and any future speakers you might have.

Good luck. And enjoy.


Disclaimer:

I don't own any of these kits or have built any of them. I have researched them to the very last little bits though, contacted the designers when I had a specific question, or just bumped into them in the forums. Last year i was in the same boat as you (no, not divorced with a girlfriend, but trying to decide on a tube amp to build) as I was going to pick one based on time, space, skill, and wife. I was about to pull the trigger, but ended up going with a Class A Nelson Pass Mini-Aleph that is essentially 85% done (never seen better soldering in my life by another diy'er) because of the opportunity came up.

After that amp is done, then I am going to be back at looking at one of those three up there I am going to order because that is number two on a four project list of mine. I am pretty much sure which one, but one project at a time...

Last edited by overtheairbroadcast; 2nd March 2013 at 10:04 AM. Reason: Spelling.
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Old 2nd March 2013, 01:31 PM   #4
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the best part about building your own is the pride of doing it yourself. Of course owning a nice piece of McIntosh bling is nice too
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Old 2nd March 2013, 02:31 PM   #5
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25th anniversary? Must be a typo. The MC275 was introduced in 1961.

Edit: Here are some links to the various versions of the MC275:

MC-275 (Original "I") MKI
MC-275 Gordon Gow MKII
MC-275 Reissue ("III") MKIII
MC-275 Gold Reissue MKIII gold
MC-275 Reissue ("IV") MKIV
MKV
McIntosh MC275 50th Anniversary Limited Edition MKVI
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Last edited by Rodeodave; 2nd March 2013 at 02:42 PM.
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Old 2nd March 2013, 02:58 PM   #6
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Maybe he's thinking of the Mk ii Commemorative Badge Edition re-issue (not 25th anniversary either, just a badge) or the Stainless Mk iii... or the 50th anniversary possibly.

Of course at these current prices, I would rather build a time machine and travel back to the past and buy me a whole bunch of tubes and amps (McIntosh, and The Fisher, plus a tube tester) at the Radio Shack and bring them back with me to present (yeah, yeah, I know a time machine isn't cheap either, but look at those prices from 1964!). That way I could say that it is one owner, the caps are like new, and gently used.

1964 Radio Shack Catalog

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Old 2nd March 2013, 09:04 PM   #7
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First to Rodeodave and overtheairbroadcast, it was a typo. It is the 50th edition not the 25th, my bad. It was a bit late when I made the post last night.

The MC is a great deal for ~5K so I guess the budget for the amp project would be in the 3K to 6K range. I thought I would use my Technics SU9070 as the preamp and the SB6000 for the speakers for now. I want to do one piece at a time.

I have been using an Audioengine D1 and Sennheiser HD380 headphones to listen to CD’s and FLAC files on the computer to get by so I am not completely deprived. Not ideal but acceptable until I get the rest of the gear out of storage.

This may sound contradictory that I want power but I really don’t crank the levels anymore. Tinnitus has taken its toll and I try to protect what hearing I have left. The Technics SA1000 is over 300WRMS per channel but it has never been above 20% the entire time I owned it. I have always believed that it doesn’t have to be loud to be good but it better be clean. I like having extra headroom and don’t like the transient peaks clipping (in the unlikely event I can hear it anymore ). Based on this, I am leaning to the push-pull output stage. I am thinking that the SE’s won’t have enough power.

As to the actual building, leaving it on the table isn’t a problem. I have discovered the great joy of putting something down, coming back a week later and it is still just where I left it. I never knew how good the simple pleasures could be.

I have more research to do before I waste more of your time. I thought about refurbishing the Dynaco MKIVs I already own. Most of the owner comments thought that even after they heavily modified the amp it still wasn’t where they wanted it to be. I really like the P-P project that George at TubeLab is working on. The P-P Universal Driver Board pushing a pair of 300’s sounds intriguing. I need to call a few local machine shops about chassis fabrication. I need to read more about the ST120.

Most of my life I had to settle and I am tired of it. I have a hodgepodge of stereo gear that was good in its day but this is now, not the 70s. Music is about the only thing I really care about anymore and I want something that sounds better than what I have and looks the part. I guess I just want to do something for me, spoil myself a bit and start my new life with a clean sheet.

Thanks Guys.
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Old 2nd March 2013, 09:33 PM   #8
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Mark,

Get the gain structure right when you build the amp and you will not need a preamp for line level sources, like CDPs and tuners. You will need to build a preamp for phono, should you choose to use that source.

Clean power is important. However, the fact remains that tube amps clip more gracefully than SS. Compression sets in, before hard clipping occurs. So, you don't need quite as much reserve power, when tubes are employed.

BTW, the budget you mentioned will easily cover the costs of acquisition and overhaul of a Harman/Kardon Citation II. A "Deuce" refurbished ala Jim McShane will more than hold its own against ANYTHING (past or present). You get 60+ glorious WPC continuous and 3+ dB. of dynamic headroom. The only speakers I would not mate to a Cit. 2 are the big Thiels, with their outrageous impedance curve dips down to 1 Ω.
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Old 2nd March 2013, 10:14 PM   #9
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Unless you go for some original in the box unused GEC KT88 tubes and gold pin unused NOS Philips or Mullard 12AX7, I think you would be able to keep any of the amps I suggested as a project within the $1,500 mark. Even the ST-120 wired and including tubes will come in under that pre-tax if I recall correctly.

But if you have that kind of funds to spend, then audio is a very good outlet and an excellent outlet for relaxing in the build process.

I wouldn't want to run any amp to clip which would shake some walls already, but tube distortion is nicer to the ears than SS clipping.

Above all, enjoy.
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Old 2nd March 2013, 10:20 PM   #10
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Phat Daddy,
If you really intend to purchase Altec A7's I wouldn't think at some point after you move you are going to want to use a small SE amplifier. At some point you are going to want to turn those babies up and you are going to go into massive distortion with only a few watts, don't just look at the rated efficiency of the horn and compression driver as the bass speaker on those horns is going to be what sets your power requirements. I have a set of Altec Barcelona speakers which is a close cousin to the A7 but used an air suspension 15" for much lower bass than you can get from the A7 but without the midrange horn output of an A7. I used an old McIntosh MA6100 to drive them and I think that is rated at 90 watts per channel though it is SS and not tubes. I sold my fire bottle Mac's a long time ago, just didn't want to deal with tubes and bias settings and such and the hum that went with them. I highly doubt that you could ever get an A7 through the apartment door in the first place that is why I said when you move! If you really love tubes look for a real amplifier with a push pull output or something will some balls, SE is for listening to all horn loaded systems with sub-woofers or no real bass to speak of. If you really like the sound of a McIntosh a SE amp is not going to be the same type of sound, but perhaps something by Manley or one of the other tube amps would suit you better. You could build your own version of many of these old amplifiers and there are many threads here dealing with tube amps. The real cost seems to be in the transformers more than anything else. If you have the coin then build what you really want, not something that is going to let you down by not meeting your expectations. I know the sound you are looking for, I grew up with that and it isn't the same as a small speaker with a SE amplifier.
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