McIntosh MC-3500 Schematic Information

[Funker]

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
DF6ZC

[danFrank]

Hi, I have the operation and service manuals for both the MC3500 and the MI-350 and no AC voltage values. The only thing AC it has is 1.1 volts input signal for 350 watts output.

Do you have a pair of these?

Daniel

[Funker]

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 :


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

regards from Hamburg

73 Wolfgang
DF6ZC

[Bandersnatch]

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.
cheers,
Douglas

[danFrank]

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...
Daniel

[Funker]

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 .


@Douglas

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.

73
Wolfgang
DF6ZC

[Funker]

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:



Here a table of Split cores which are available here :

http://www.waasner.de/uk/start.htm

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
DF6ZC

[danFrank]

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

[danFrank]

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...
Daniel

[Kay Pirinha]

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