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    Building, troubleshooting and testing of these amplifiers should only be
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McIntosh MC-3500 Schematic Information

Hi Folks,
its the first time I wrote in diyAudio. And this is my inquiry . Is anyone out there who got the AC/ Audio levels values for the MC-3500 schematic. I found two schematics in the net , but they got only the DC values on it.

Thanks for looking into your files for me.

73 Wolfgang
Hi Frank,
thanks for reply . No I´m not an owner of these nice amps. These rare items employ an very interesting circuitry. I like to mess up with it and maybe I will make my own copy of it. The pinpoint of it are the output trafo. It is a tricky thing but not rocket science. The AC voltages in the circuit make the thing much more easier to calculate the OP trafo as I have no curves for the OP valves. These original OP Valves in the Amp also hard to get, but the 6P45S from Russia is quite simiar in performance and obtainable for a reasonable price. Otherwise the PL519 , the most produced Line Amp valve here in Europe for Colour TVs, is another alternative. The only differ from the heater, which is 40V/0,3Amp. The advatage is the lower heater current if you have to fire 8 valves 2,4Amp intead 20Amps at 6 V Heaters.

A friend of mine run an professial transformer manufactur. He build special Trafos for the Unversity lab and some Commercials. He promised me help if I´m in need of winding my own.

I own these beasts :
An externally hosted image should be here but it was not working when we last tested it.

Siemens eladyn 6s ela 2796 , 250W rms out. 6x EL34, 800V plate voltage. Mains input 230V/700VA, DIN rack mount, weight 37kg

regards from Hamburg

73 Wolfgang
That output TX is no small trick. I disagree that the AC voltages are required for calculating its ratio. Feed 10V( or any arbitrary, stable voltage ) at 60 Hz to the plate winding and measure voltages on all the others, including the secondaries. With voltage ratio squared times the secondary's load( from which you've derrived the voltage ratio ) you get the primary load seen by the tubes.

You'll find 5 windings with the same voltage across them( the penta-filar primary ). You'll need to multiply the previously calculated load by 4 to get the primary impedanceas you've got as much a load on cathode as on plate( 2x the voltage ratio equals 4x the impedance, yes?). However, as this amp will quickly cut off one phase, the tubes will see only a quarter of the primary impedance over most of the output power.

Ah! Why didn't you say so? The transformer impedences I can help you with. The primary has 5 windings, like Douglas said and are pentafilar wound. Each of these 5 windings has an impedence of 150 ohms end to end (ie. plate to plate, cathode to cathode, etc). The secondary has 4 windings, 2 have an impedence of 1 ohm, the other 2 have an impedence of 4 ohms, with a one ohm tap on them.

The transformer primary is wound in 10 sections with the secondaries in 8 sections interleaved between the 10 primary sections. They are wound like the original Williamson on 2 bobbins on a huge double "C" core. Each bobbin has "half" of the transformer on it; the 5 sections of one side of the primary interleaved with 4 sections of the secondary. I think, the center taps are the outer most winding on each coil (ie. the outer ends of the primaries are closest to the inner core of the bobbins with the center taps furthest away from the inner core). I hope this is understandable.

I have a pair of these transformers that I bought from a HAM radio operator. He bought a pair of these amps as surplus and only wanted the power transformers off of the amps; he scrapped the rest.

Anyway, the transformers are VERY heavy, probably about 75 pounds each. PL519's will work well if they are of good quality.

Hope this information helps...
Hi Daniel,
ok, I ask in this way because I didn´t believe that anyone has windings information about these transformers. At given power , lowest frequency and secondary load impedance you can calculate a Transformer if you know the primaries ac values.

Yes Daniel I understand your explanation for the winding scheme .Thank you very much for it. I refered to different photographies and estimated the transformer has a size of about 170x 170mm 200mm high. Thats an really amazing size .

The 2 Output windings have each half voltage . For the ac consideration it is as they are one winding and that will be only 1/4 of the impedance. Half the volts by twice the current.

In the ac equivalent circuit both valves operate parallel to the o/p transformer . So the former anode load impedance of all tube of 2,5k Ohms will be appear now on the primary as 150 Ohms.

Hi Daniel,
me again. I made a Photo of a transformer which ( I hope) is similar to what I have to wind. Is that trafo type the same sort we talk about?

Pics of the trafo:
An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

Here a table of Split cores which are available here :
TYPO3 Error

I think the U 8.2 SU114b) core is suitable for the o/p trafo do you agree?

Finally another 2 questions :
can you tell me the dc resistance of the primary and what diameters are the wire? The projekt is ongoing, the tubes are on the way , I got a suitable anode mains trafo , 2 second hand 19" rack housings. all small signal valve and lots of bits and pieces .

And many thanks for help.

73 Wolfgang
Here's the dimensions:
7.25 inches wide X 5.25 inches deep X 5.875 inches tall.
Weight is more like 50 pounds.

Resistance measurements of the primary windings: Plate winding & cathode winding for power tubes: 5.6 ohms each side of the center tap.

The other three primary windings are 21.6 ohms each side of the center tap.

I have no idea what the diameters of the wire is, but with the resistance of the windings, primary impedence, etc. you can probably calculate. Basically, 2 sizes are used in this primary: one size for the plate and cathode windings, and a smaller diameter for the other 3 primary windings.

Hope this helps...

don't want to start an argument but this Output transformer is not a dual bobbin, it uses a single bobbin just like all MC outputs. Your sections for Pri and Sec are also off target. The problem with winding this transformer is the wire that is needed to do the job without arcing over is no longer made. It is a special order and there is a minimum buy of 5000 pounds. If you substitute another closer wire you will get CORONA effect which will sound like crackling sounds during loud music passages.

Also, this transformer does not use a traditional bobbin, you have to make a mandrel, spin the wire and varnish it prior to taking the mandrel apart, then it will just barely fit in the C-core window width, just like the original. If you use a traditional plastic/nylon bobbin you will not be able to fit the correct number of turns per layer. Also, the particular dimensions for this C-core is no longer made and is a custom item as well.

So now you understand that MC outputs are difficult to make due to obtaining the correct materials without putting up a huge investment.

Best Regards

Hi Daniel,

your resistance values tell us that the wire used for the feedback, the bootstrap and the screen grid windings respectively, is about half the diameter of that for the cathode and anode windings, don't they?

Best regards

Only one wire gauge is used for the entire transformer - primary and secondary. It's a matter of paralleling the windings to get the current density needed.

I had a pair of these (043-766) till I sold them off earlier this year. I did extensive measurements and stand behind my statements. They are wound like the original Williamson specs; it's the only way I know of where 10 primary sections and 8 secondary sections can be wound and come out with an even geometery. Also, the primary plate and cathode windings are wound with a thicker guage of wire than the feedback, screen, and driver primary windings as they have lower total resistance than the other 3.

You seem like you know a lot about Mac outputs. Please enlighten us... And don't hide behind the " I would share info but am afraid of being intellectually ripped off" like you have in the past. Please share what you know; I would like to be informed.

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test amp finished

Hi Daniel,
it is a long time ago when I start this thread. But now I´m finished both output transformers in the way you did explained to me. As you said, many many wires to turn, there was a sort of a nasty task to fiddle with all these five wires layer by layer to the bobbin. To make these both transformers took a whole week . I had the advantage to use the winding equipment in a local transformer shop, while one of their stuff where on holyday.

The Cores are the C cores which I told you about. They are SE170a DIN cores made by Vacuumschmelze , Germany, the cores are nearly 40 years old and salvaged from the power supply of a huge Siemens HF Transmitter. I sent you a picture a little time ago. The bobbins are DIN EI170-2 dual chamber coil former.

pics of my outputs

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

So I´m another step further with my first approach to the amp. I build an test amp to make some measurements on the circuit. My output valves are the russian 6P45S . I aquire a lot of 30 valves for lousy 250 EUR. The Power transformers are custom made by the local transformer shop.

here are some pics of my test amp.

An externally hosted image should be here but it was not working when we last tested it.

Uploaded with ImageShack.us
An externally hosted image should be here but it was not working when we last tested it.

Uploaded with ImageShack.us

An externally hosted image should be here but it was not working when we last tested it.

Uploaded with ImageShack.us

As I said it is a test amp, so I have only one channel at the moment, but it sounds fantastic. Its has an unbeleavable dry bass from deep down below , transparent mids and crystal clear highs . No matter what kind of music it play, it has a real live atmosphere.

It delivers max. 450W rms below clipping At 350W the THD on 1kHZ is 0,01 % , at 40 Hz 0,1 % ( the signal generator has the same distortion) at 5 kHz 0,1 %.

The Power requirement ist 400W with no signal and 1 kW with full power below clipping level, at exactly 230Vac.

These russin outputs are nasty high µ types they have nearly twice the transconductance than the origininal 6LQ6. The amp has a tendency to oscillate. I used a lot of ferrite beads to keep the amp quite. So I´m still work on it.

I made a protection circuit which survey the g2 current, the g2 voltage and the main plate current, if one of these parameters exceed the limit , the circuit trip the main circuit breaker. The fan is speed controlled by a NTC sensor. I improved the g2 voltage regulator.

I spend over 300 hours to collect parts, wind the trafos, make the metall work and the wiring... and believe me I´m not finished at all.

regards from Hamburg, Germany

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Hi Wolfgang,
I had always wondered if you had ever undertook the task of making the amps; now I know! Congratulations on a great job. McIntosh used little chokes to subdue the osscilations. They put a choke at the plate cap of every output tube, maybe 2.2uh each in value.Please write again when you have the pair completed.
Thanks again for sharing...
danFrank....don't want to start an argument but this Output transformer is not a dual bobbin <snip>

I do believe this to be true !

I was told the output transformers were the magic to these amps, difficult to make and horrendously expensive. One of my 3500's started giving me issues and it did appear to be the output transformer and they were not obtainable anymore ( 1984) i later sold the pr on in 1989.


it is a long time ago when I start this thread. <snip>

Nice job wolfgang, are the 6lQ6's no longer available? lol, i remember having to buy 16 when re tubing , don't miss those days at all....:p

The Mc3500, were at their best in the mids/highs, the dry bass is exactly as i remember them, but much better than the ill defined muddy bloat exhibited by other tubes amps , even today . Fast and dynamic ..

Ancient memory , but i would put them up against the best today ..... :)
Joined 2004
Paid Member
:cop: In the future please resize and attach your pictures to your posts - just go to "advanced" and then "manage attachments" - this will result in manageable sized pictures and ones that stay with the forum over time so that someone interested in the topic can see them a few years hence. (Works better too as some pix are huge and others are tiny and don't expand when clicked)

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Keep in mind that the 8 ohm tap on this Output Transformer is actually a 9 Ohm tap, but they call it 8 ohms in the manual. Hint: When connecting a 8 ohm load to this 9 ohm tap, you drop the effective load to 108 Ohms on all 8 tubes. This now allows the power to extend higher to 350 Watts with 6LQ6 tubes.

The 470V dips to about 450V at full power and the screens are regulated at 180V but drops around 5V across the 200 Ohm screen resistor at full power output... So full power calculations are done at 450V Plate and 175V screen....of course there ia normal variance.

Also, due to the cost of the Quad coat wire they had to order in very huge quantities, this is one reason the MC outputs across the product line use the same wire gauge for everything, the other reason they used one small wire gauge for everything was for better packing density, thus more efficient wire packing, they just parallel as many wires together as needed to get the required circular mills needed...this also avoids any skin effects when using many smaller insulated wires in parallel, the secondary also was done in the same small wire gauge. All secondary section were also paralleled. When using the same wire gauge the layer to layer build is more even and smooth and not lumpy which also would waste packing space.

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as a once MI350 owner i'll chime in with some interesting trivia. the tubes used in these amps were horizontal sweep tubes commonly used in tv sets. they were very cheap and common at one point and then the cb radio guys figured out they could make cheap illegal amplifiers with them. cb amps eat tubes for lunch the way some of those guys talk and talk.

finding a complete matched set of sweep tubes for one of these amps will be near impossible as the cb crowd trolls the radio boards and swap meets looking for these same tubes.

for those that own any of these incredible amps, if given the chance to purchase a complete matched set of tubes for your amp, do not pass it up. you may never have the chance again.

I have a pair of MI350s which I completely rebuilt and upgraded the design. For me they are the best sounding amps I have ever heard and after my mods they perform far better than original. Congratulations on the build, a fine effort.

Steve Mantz
Zed Audio CA USA