Newbie working on a Magna Varsity

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Newbie working on a Magna Varsity

Postby Cygnus X1 » Thu Apr 03, 2008 10:59 pm

As the title suggests...
New here, looks like a great community of folks!

A friend sent me a Magna Varsity 108 (Champ clone) to resurrect.
The large can cap had leaked all over the chassis, and there is some
moisture damage. The output tranny is shorted, so it goes, as well.
The PT looks ok, so far, I'm getting heater voltage and 400 plus volts from the 5Y3 rectifier.

This little amp is truly point to point, no eyelet or turret board!
The caps and resistors are simply strung across the pins of the 12Ax7
and 6V6. I went ahead and ordered some new axial electrolytics for the
filter caps (two), and the 6V6 cathode bias circuit cap. I'll mount these
where the canister cap was. Hopefully that will maintain the lead dress.

I'm in the process of a full rebuild. No questions yet, but I'm sure I'll run into some puzzlers somewhere.
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Postby Cygnus X1 » Sat Apr 05, 2008 1:02 am

Ok, first question.
This will be an awful indicator of my level of experience (I'm 45 and haven't been injured or killed yet)...

Wire: It has solid copper wires .022" diameter.
I have a lot of multistrand industrial wire that is .052" diameter,
likely aluminum. Is this a reasonable replacement?
I'm fairly confident the heat rating of the insulators of this wire I have
on hand is sufficient.

Is there any particular reason to use the old cloth covered wire?
I'm not touching the primaries going to the rectifier, they seem to
be in good shape. The rest are the ones going from pre- to power tube.
Those are the ones that I need to replace.

Since this amp is truly point to pont, I realize lead dress is critical.
Will the multi strand aluminum wire have an unexpected capacitance effect that solid copper wouldn't?
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Postby mesherm » Sat Apr 05, 2008 3:20 am

I'm sure the multistrand wire you have is probably 16 gauge tin plated copper and will work just fine although its a bit large. If it was truly aluminium you would not be able to solder it. The original wire was probably 22 gauge solid copper. Some people prefer stranded as it is less likey to break at nicks caused by stripping the insulation. The main advantage to solid wire is that it is slightly easier to wrap around terminals when doing point to point and it stays bent so your wiring looks neater.
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
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Postby cartoonweirdo » Sun Apr 06, 2008 10:26 am

The main disadvantage to solid wire is that it will break at the bends or the centers of long unsuported runs due to the mechanical shock of pitching the amp in and out of vans and up and down off the stages of wherever you are.

This is why PCB's have a bad name in the vintage style amp trade. The solid copper traces of less expensive boards bend every time you bang the amp around and after a year or two of touring they will crack on the axis of the bend. Some of the better ones (check out the milspec boards in Marshalls) tend to flex less and thereby keep this problem to a minimum. This is (I believe) the main reason that Leo Fender opted not to use PCB's (the ones he had available were even worse than modern ones).

When I first started opening up the amps to "improve" them (little did I know what I was starting out on at 16) I reasoned that larger wire was more conductive. Since there was some romex around the house that was apreciably larger than the 22 guage wires in all the amps I began to use it when I needed new wire. Most of those amps were back to me in under six months, and the only reason they came back at all is because I worked for free at the time. As you would expect the solid wire was breaking and creating intermitant problems.

Sorry to ramble on so long,
Carl
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Postby Cygnus X1 » Mon Apr 14, 2008 6:29 pm

Yellow_Light_Colorz_PDT_06
Yeah!
I went through, and replaced every resistor and electrolytics, (I kept the coupling caps in place), replaced the output tranny, formed it up, and...
No problems at all! It's quiet, and gives off a sweet sound. Clean up to about 4, then it breaks up nicely.
I spent a couple of days playing it, breaking it in, and it's shiiped to my friend.

Now, something else.
He's sending me a Califone Promenade record player that has 6L6's, mic and instrument inputs, to use for my own homebrew project. How cool is that?

http://img254.imageshack.us/img254/8867 ... in5kn0.jpg
http://img515.imageshack.us/img515/2332 ... in3bg0.jpg
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