Millett MOSFET-MAX Build

Very nice work!

 

About decoupling the bottom of the amp from the table - remember that there are cooling holes in the PCB at all heat sink positions.  Consequently, cooling will be greatly enhanced if you vent the bottom of the case.  This will allow heat-induced vertical-convection to flow at the heat sinks.  Likewise, you need sufficient standoff space between the tabletop and the bottom of the case to overcome a boundary layer effect.  I'm not sure exactly sure what height that should be, but probably 1/4" at least (more would be better).

Edited by tomb - 8/23/10 at 6:57am

Looking good so far. I like the clean look.

When you are drilling vent holes on the bottom, you might want to expose the test points as well. That way you can probe them with the top cover on when you bias your tubes. Of course you will still need top cover holes for the trim pots, so, maybe this works and maybe it doesn't. Just allows you to avoid test points on the back (tip jacks) or taking the top cover off when you roll tubes.

What is your goal with the faraday cage idea? Are you going to use it in a particularly noisy RF environment?

Just an FYI, even in the Hammond case, the amp picks up my blackberry quite easily.

You might need a lot of copper!

I think it looks pretty good, but if you're not happy with it - you could try some sort of stain and polyurethane and actually pick the look you want.

 

Glad you like it! About the voltage - 27VDC is ideal, but this depends greatly on the line voltage available. The power supply on the PCB is set up to work from a voltage difference (as are all linear-regulated suppplies in Head-Fi). The line voltage sets the upper difference with the 24VAC walwart. When rectified and smoothed with the large electrolytics in the power supply, that usually results in about 35VDC. However, the losses in the rectifiers and voltage drop required by the LM317 bring that down to about 28-29VDC as the maximum that can be set under ideal conditions. The Triad walwart is rated for 120VAC line voltage, however, and some people only get 110VAC at the wall. In those cases, you may lose a volt or two from that 28-29VDC. We chose 27VDC as a happy medium that we felt most people could achieve, but there are always exceptions.

 

I recently had a customer in the Phillipines who really struggled with this, because in converting from their 220V wall voltage, all of the available convertors went to 110V, not 120V. (It's easy to see why they did this, because the 2:1 transformer windings made things simpler.) Anyway, his voltage was down around 24VDC, but he really wanted to get to 27 at V+ and Gnd. After trying several convertors - each larger than the last - he ended up with a very large, floor-standing convertor whose amperage rating was so high that the voltage headroom finally resulted in the 27VDC. Short of all that though, you've done the correct thing - adjusted for as high as you can get and still be regulated, and adjusted the bias to one-half of that.

 

Just a guess, but I think you'll find that with that wood case, you'll be better off with feet that are screwed in, rather than glued/stuck on. Maybe you could let that determine your decision.

 

Those are interesting with those twists in the middle! I would only remind that if you have any hope of changing the tubes out with the cover on, you may need the two-per-tube approach. However, since you'd need to re-bias every time you changed tubes, you'd have to remove the lid anyway (in your case). So, either one would work fine.

 

No need to worry about filing down a flat on the Alps. The metal is actually so soft in that shaft that any set screw will dig in with very little effort - there's no movement. That's the proper approach - a set screw. If considering a wooden knob, I would drill a set screw hole and glue in a brass insert that can be threaded. You could even use a regular 4-40 socket head cap screw if you carefully measured and drilled a counter-bore.

Oops - it looked pretty light, so I guess I assumed it was unfinished - maybe a much darker color?  Still, it looks great to me, I was only responding to your apparent dissatisfaction.

 

From my experience, it seemed that the higher the bias, the better it sounds.  I went all the way up to 150ma (330mV) at one point, but backed off because it seemed to be too hot.  I wouldn't hesitate to get it up to 275mV, though.
 

So, if you want to "cheat", home depot has some all wood knobs that are directed at the ceiling fan crowd. Some are pretty funky, some, not so much. Might be an option. I used to have some extras before we remodeled, otherwise they would be yours. Now they are mulch. :-)

I've always like the "natural" wood look myself. My significant other, not so much. I used to have a pair of klipsch heresy II speakers I hand rubbed in a natural finish. They were birch as well . A great wood for speakers. The depth of finish really doesn't start to come out until you do a dozen or so coats. So, it you like the direction it is heading, just keep coating.

I still miss those speakers. *sigh*