Monkeymatic Mac Pro 2203

[xtian]

Q: What do you do with an old Mac Pro?

A: 2203.

[lord preset]

Everybody knows that.

[tsutt]

if you know what a 2203 is.

[Reeltarded]

First it won't connect with any standard cable and then it won't run the guitars you own.

[johnnyreece]

First it won't connect with any standard cable and then it won't run the guitars you own.

Will it also be obsolete next year?

Seriously, though, interesting idea!

[Reeltarded]

No more so than it was when it was released in 2005 with 1998 components.

[xtian]

Jokes about planned obsolescence, check!

Seriously, though, this is going to be very, very cool.

Think I've got enough airflow thru the chassis?

[xtian]

Mounted the trannies, pots, power switch/IEC/fuse assy, preamp sockets, input and output jacks.

[Bob S]

Mr Apple won't be happy.
Make it small, flat, thin & useless - then patent it.
Fans are your friend - think kt88s.
Get a good lawyer.
Looks good bud.

[xtian]

Mounted power tube board (reused from Bugera 1990, along with the power switch/IEC/fuse assy, and the trannies). Built filter cap board. Wired heaters.

Yes, the power tubes and filter cap board are mounted askew--using existing standoffs where possible to avoid putting any holes in the sides of the beautiful Apple chassis!

[xtian]

Finished up the power string, and everything checks out. I see 440Vdc, unloaded.

I have a question about the balancing resistors on the parallel/series caps. Each 47K, 2W MOX resistor sees a voltage drop of 220v, or 1.03 watts. While this is within tolerance, it's a lot of heat. Is this how you'd expect these balancing resistors to work?

I used 47K because it's what I had on hand. Marshall specs 56K.

I notice these resistors also serve as bleeders when you turn the amp off. Bonus.

[matt h]

(deleted)

[xtian]

Thanks, Matt. Next question:

This PT has a 20-0-20 secondary that was for the bias in the old amp. I want to maximize the DC voltage. I have two choices: full-wave, capacitor input; or full-wave bridge, capacitor input.

This Hammond document gives DC estimates:

http://www.hammondmfg.com/pdf/5c007.pdf

But I'm confused about the "peak" vs. "average" DC specs. Seems like I should ditch the center tap for this purpose, and go with the FWB for highest DC.

Other good resources for estimating DC output of rectifiers given unloaded AC voltages?

[Phil_S]

Man, that's a creepy lookin' build. <shudder>
I just discarded a couple of fill size Dell cases (not elegant like Apple, but similar in form factor) that must have been 15 years old. There were pretty stout. I stared at 'em for a while. Thought about it, but couldn't imagine it. Out they went. I'm rooting for you on this. I hope you get a great result.

[xtian]

What's creeping you?

I'm having a great time. The bias circuit is working now.

It's so spacious. And I just dove right in without a plan. The PT and OT first, positioned for least hum. Using parts of the old Mac as structure, support, anchor points.