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 Post subject: Mcintosh MC30 Restoration
PostPosted: Mar Sun 14, 2010 1:51 am 
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I finally have most all of the parts to start the restoration on my pair of MC30 Mono-blocks. They are in typical as found condition with a lot of chassis corrosion, but otherwise in good shape. A fellow on E bay has started selling fantastic reproduction chassis, so I pickup up a pair, and started on them. I figured I'd post my progress as I go along.
Image This is how they looked initially

Here is an old chassis alongside one of the new ones, I've already fitted NOS Cinch and Amphenol sockets with semi-tubular rivets as original.
Image

Image

Transformers sanded, primed and painted with gloss rustoleum
I still have to reproduce the tags or buy reproductions
Image

Transformers bolted back onto the chassis -
Image

Underneath view -
Image

I still need to order some capacitors, I think I'm going to go with the Solens for anything in the signal path, that's 5 in each chassis unless someone has a better recommendation.

I've also been thinking about adding some sort of B+ delay to these since NOS 5881's or 7581's aren't cheap. Any suggestions? I'm looking for something simple I can hide under the chassis.

When I get the rest of the parts, I'll post some more pictures.

Mark


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 Post subject:
PostPosted: Mar Sun 14, 2010 2:57 am 
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Nice looking chassis, did they come with the lettering? The riveted sockets are a nice touch of authenticity. I would not worry about a B+ delay if you are using a tube rectifier. I know it wouldn't be original but the 5AR4 would be an improvement over the 5U4.


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 Post subject:
PostPosted: Mar Sun 14, 2010 6:19 am 
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I agree. Use the 5AR4 and don't fret about "originality."

After all, you're using 5881's or 7581A's instead of the originally-mandated metal 1614's, which are somewhat tougher, incidentally, than 5881s or 6L6-G-GA-GB tubes (higher plate dissipation rating and all that).

In view of that, I'd stick with 7581's or 6L6GC's, in your place. 5881s in those amps may be running too close to redline.

Best of luck on your project :), Larry

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 Post subject:
PostPosted: Mar Sun 14, 2010 6:23 pm 
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Very nice job on your restoration. Keep the photos coming. As far as coupling capacitors, I personally use Sprague 715P or 716P, Illonois or Mallory 150's, They sound good to me and are better than the coupling capacitors used back when the MC30 was built.

Sal

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 Post subject:
PostPosted: Mar Sun 14, 2010 7:15 pm 
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Location: North Eastern Ohio
Hello everyone,
Thanks for all of the nice comments!

Lou, the chassis did come with the lettering on them, silk screened.

I agree with you guys on the 5AR4, because it has separate cathodes the warm up time will be on par with the rest of the tubes vs. a filamentary cathode, I have a ton of 5U4's but only came across one 5AR4 in my inventory. I have a few GZ32's I was thinking of using but using them might be a little close to danger as far as the ratings are concerned.

Larry, as far as the output tubes go, I wasn't going to use the 1614's because I didn't have any on hand, and I'm generally not a big fan of metal power tubes.. Maybe I've seen too many 6F6's in old radios with the paint burned off of them!

That being said, the 1614's seem to go cheap on E bay

For some reason in my mind I kept thinking that the 1614's were just a premium metal 6L6, but I think I had them confused with the 1619, the premium 6L6 with the 2.5 volt filament.
In fact, you wont find a 1614 in a regular tube manual, I only found it in my set of RCA engineering manuals in the transmitting tube section. The maximum values on it are quite different from the 6L6 family - maximum plate voltage is 550 with a plate dissipation of 25 watts, vs. the 6L6 at 360 volts and 19.5 watts.

I was hoping to use 5581's because I have a bunch of them, but this amp puts 460 volts on the plates, so they probably are out of the question.

The best tube would probably be the EL37 but I don't have enough to to both amps and I can't afford "real" ones.

The 7581A's that came in these are still functional, but they are real tired.

I've been thinking about making some adapters and running 807's - I have a bunch of NOS ones.
The transmitting tube manual hints that the 807 is essentially the glass version of the 1614 with higher ratings. I'd just have to machine some nice plate cap insulators on the lathe.

If I end up going with new tubes, I'm looking at the "JJ" brand KT66's they sell for $33.00 each at AES which is reasonable. has anyone ever used these? I think they are Russian.

As far as the coupling caps go, I was going to use the Solen's because they are only about a $1 more each then an orange drop, and they are made in Europe which is normally a plus for quality and they have super low di-electric absorption numbers like .00001

I'll post some more pic's as I progress.
Mark


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 Post subject:
PostPosted: Mar Sun 14, 2010 9:31 pm 
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I like JJs, they are made in the Slovak Republic. I just built an amp with JJ's KT88s.

You can get JJ 7027s for $41 for a pair or Russian 6N2CE for $15 a pair at http://store.triodestore.com/6l6typ5875kt.html


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 Post subject:
PostPosted: Mar Mon 15, 2010 7:41 pm 
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Somewhere in the past, I got the idea that the Mc 30 was about the same as the earlier Mac "A-116" which has similar specs and uses 6BG6 tubes, which may be more plentiful, maybe cheaper, if you run out of favorites to stuff in the outputs. (I don't know if the A-116 used the same primary Z as the Mc 30)?

Charlie


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 Post subject:
PostPosted: Mar Tue 16, 2010 6:23 pm 
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GTE Sylvania brand 6BG6GA's are supposedly identical internally to 6L6GC's, there was a guy selling adapters to use them several years ago, and had a website showing the construction of that tube.

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 Post subject:
PostPosted: Mar Tue 16, 2010 7:55 pm 
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I knew a chap selling those adaptors, Dennis. He claimed he made them himself, and I suppose he did.

I got four of them from him at the Shelbyfest back in the 1990's. They worked, but a 6BG6 atop that adaptor stands as tall as an 807 by itself. No good unless you've got physical headroom for them, and I no longer have any 6L6 amplifiers with that kind of room.

Larry

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 Post subject:
PostPosted: Mar Wed 17, 2010 12:21 am 
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[quote="BigBandsMan"]I knew a chap selling those adaptors, Dennis. He claimed he made them himself, and I suppose he did.

Does that guy STILLl have 6BG6's and adapters? That was some years ago?

Charlie


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 Post subject:
PostPosted: Mar Wed 17, 2010 12:25 am 
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I haven't heard from him in some years, Charlie. He used to come to the RARSfest here in Raleigh every year, but I haven't seen him in at least ten years now. Don't even know if he's still around.

PM pending.....sent!

:)

Larry

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 Post subject:
PostPosted: Mar Wed 17, 2010 1:43 pm 
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Lookin' gorgeous, Mark!!


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 Post subject:
PostPosted: Mar Wed 17, 2010 2:32 pm 
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Don't know if this is active, but this is the site you were talking about.

http://www.vacuumtubes.com/6BG6.html

I remember seeing this at least 6 or more years ago. Price is only slightly higher than then.


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 Post subject:
PostPosted: Mar Wed 17, 2010 2:47 pm 
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Yep! That's him, alright. SND Tube Sales.

Many thanks, Anbitet66!

:wink:

Larry

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 Post subject:
PostPosted: Mar Mon 22, 2010 4:13 am 
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One done, 1 to go!
Here's my progress tonight-
Image
I could not buy the correct value new manufacture "FP" cap, AES told be they were back-ordered 6-8 months, So I Used a pair of Sprague "Atoms" and an NOS FP cap for the 80 Uf/500v section

Here's the tag strip-
Image
The black Beauty I left there is just connected to the pre-amp octal socket that I'll never use so I left it there-
Image

It sounds really good, although I ran a pair of tired 7581's to test it - Image

Only thing I'm concerned with is that the bias power supply voltage is way high - The Photo-fact specify's 340 volts and I'm seeing 430 with zero signal. The Photo-fact seems to have some other typo's in it so I wonder if it's right.

The selenium was replaced with a diode, originally had a 3.3K resistor between the transformer and the selenium, I've got a 5.6K in there now and have only got the voltage to move down to 390..
Is there a difference if the resistor is on the AC or DC side of the diode? what am I missing?


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 Post subject:
PostPosted: Mar Mon 22, 2010 11:22 pm 
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If you are speaking of hundreds of volts positive, you are not speaking of bias voltage.

The bias voltage is what appears at the control grids, a negative voltage, usually anywhere from -20 to -40 volts, depending on output tube types and circuit.

Those large positive voltages you refer to would seem more appropriate for plate voltages. The bias supply has nothing to do with plate voltages, which derive from the B+ supply.

Assuming that you have a capacitor-input power supply filter, you can expect fairly large swings in plate voltage readings if the rectifier is coming up before the output tubes do. The voltage will soar until the output tubes begin drawing their current, and will settle into its typical working range as the tubes become completely warmed up and arrive at full current draw.

The critical thing is that the soar voltage must never exceed, or even equal, the working ratings of the filter caps.

If your circuit is wired correctly and you see plate voltages well above schematic ratings, remember that this amp was designed for line voltages substantially lower than they are in most places today, and you will likely need a bucking transformer or Variac to avoid overvolting the amp.

As for the dropping resistor installed before the replacement bias rectifier diode, it is best to place it where McIntosh originally placed it. I expect it was originally intended to make life a little easier for the original selenium diode, but there is nothing to gain by putting it on the DC side of the replacement diode.

Hope this helps, Larry

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 Post subject:
PostPosted: Mar Mon 22, 2010 11:54 pm 
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Deleted, my idea was all wrong.


Last edited by Lou deGonzague on Mar Wed 24, 2010 9:26 pm, edited 1 time in total.

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 Post subject:
PostPosted: Mar Tue 23, 2010 12:03 am 
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Hi Larry,
you know, sometimes I ask dumb questions, but I'm not an idiot.

The bias power supply is supposed to supply -340 volts to the control grid voltage divider network in the driver and output stage-
From there, the output tubes see -45

See schematic below

At the 10 Uf cap I have 429 volts there with the 3.3K resistor installed and a diode.

With a 5.6K resistor there I have 390V.

I assume this is because there is very little current draw in this circuit and the diode has such a small voltage drop compared to the selenium, I was just surprised to see 80 extra volts there. That was my whole point to the question. The voltage drop across a typical selenium is usually 20-30 volts.

I'll have to measure the current in the circuit and calculate the appropriate resistor accordingly.

The power transformer has a 125V primary tap which I am using so high line voltage has nothing to do with this issue.

My nominal line voltage is 126 volts.

Again, I appreciate any help, I post here so I can learn new things from other members, and gain insight as to why certain things happen and why but Sometimes I feel like I'm treated like an idiot.



Image


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 Post subject:
PostPosted: Mar Tue 23, 2010 12:13 am 
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Location: Seattle WA US
Suggest you check values of R24 thru R29. Old carbon composition resistors, like those in your photos, from some manufacturers tend to drift very high - as much as 50% high for some makers.

--Chuck


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 Post subject:
PostPosted: Mar Tue 23, 2010 12:34 am 
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Quote:
The bias power supply is supposed to supply -340 volts to the control grid voltage divider network in the driver and output stage-
From there, the output tubes see -45


In this instrument, the output tube bias voltage is determined
indirectly by the -340V supply, as divided by R24 and R26? (can't quite read it). The 12AX7 drivers are wired as cathode followers. The cathode follower topology has a low output impedance and a voltage gain of slightly less than one. The cathode "follows" the grid; goes wherever it goes. (Actually it's a little positive compared to the grid, after all, the 12AX7 needs bias too.) In this case, the 12AX7 grids see -340*100K/(100K+820K). This is not 45. Tilt!

Dave Wise


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