Millett "Starving Student" hybrid amp - Page 47

So I finally got around to building one of these yesterday, and it is sounding really great. I am totally impressed with the quality of the sound considering the cost of the build. Thanks Pete for creating another brilliant design and giving it to the world to build. So far I have only used RCA's, but look forward to rolling others over the coming days. I find (at least with these RCA's) that I like listening with my AKG 701's better than my Beyer's. I'll see if that changes with other tubes.

This was my first time trying point-to-point, so it was good practice before delving into building my Bottlehead Seduction. Not really knowing much about it, I set out with the goal of trying to keep signal paths as short as possible, and for the most part I think I did okay on that. After doing this first one, I might do things a bit differently next time, but that's what this all about, right?

I decided to use a star ground layout, although in some ways it seemed redundant with an aluminum case. The only thing about this was it did create a spiderweb of ground wires, so what seemed like a pretty clean layout on paper ended up looking quite messy in the end, but oh well. I can say this, it is dead quiet. With the volume cranked to max, I can barely even hear a hiss on my 701's.

I also decided to build three little sub-assemblies on the bench first, and then wire them into the chassis as a unit. That worked out quite nicely, especially if I decide to play around with a different power supply configuration down the road. I can just swap out that assembly without much work at all.

I never found a "salvaged" case to use, so I went with the style that trains are bad used. I liked the look of it. I took some pictures, but realized I forgot to put on the volume knob when I took them, so use your imagination for that.

I'm waiting for my parts to arrive from Mouser. In the meantime, my power supply arrived from an eBay seller and my tubes from Radio Daze. The tubes were made by Hytron. I don't remember seeing that manufacturer's name in this thread. Does anybody know about these?

CBS-Hytron was a fairly common brand for many tubes. I haven't studied these tubes, yet, but it's quite possible that many of them are another case of wide subcontracting and re-branding. It might be more fruitful to note the construction details rather than the painted-on brand for distinguishing tube differences.

There are some distinguishing features that are consistent for almost any tube, though - acid-etched tube designators and white dots are certain to be of GE manufacture. An etched "squished" octagon with the tube designation inside is almost certainly an RCA.

These must be RCA. The 19J6 designation is inside a rectangle with diagonal corners (making it a quasi octagon).

Yep - that's a sure tell-tale for RCA manufacture.

So I determined that the FETS were cooked. I replaced them and now my left channel runs perfectly.

The right channel on the other hand...I get nothing but static/humming. I checked the voltage coming from the third pin on the tube and it reads around 40V, which is much higher than the desired 19V. This seems to correlate with the fact that the right channel tube burns significantly brighter than the left channel. I did read that MOSFETs tend to take out surrounding components when they go bad, so I checked all the of the right channel resistors and replaced the caps. The issue remained the same, so I'm thinking that it must be a wiring issue. Any other ideas? Thanks for all the help so far...this is my first DIY project, so I have a lot to learn!

^ Any chance that you could post some photos of your wiring?

I have a question about the grommets. I assume they're to keep the pins of the mosfet from grounding. Is this correct? The problem I have is when I put them in the holes, the pins for the heat sink just barely touch to top of the case. There's no way I can solder it. I can't find a good picture of the grommets and heat sink in this thread. Am I doing something wrong?

Edit: I see that some people have a notch in the bottom of their heat sinks. Mine doesn't have that even though the picture on the data sheet shows one. Could this be my problem?

I didn't use grommets, but I used heat shrink on the legs of the mosfets instead.

Scott

Nice looking amp, TimJo!

This is something in the air, yesterday i purchase tubes and today morning, before turning in PC I thinking about building amp in retro stile with sloping front panel and all inputs, outputs on it.
And than I turn on PC and see your design, very close shape to what I thinking about!

Thanks!

This case came from Mouser. MDC-642-PLAIN-ALUM It looks like they sold out, but they have more on order. Although, since you are in Latvia, you'll likely not be using Mouser.

scompton, that's what I did as well. As long as the leads are covered, and not touching the edge of the hole so the metal doesn't wear thruogh the heat shrink over time, you'll be fine.

Here are some pics of the wiring. The wires going into space are attached to the MOSFETs.

Entire board:


Lower Section (plugs, etc.)




Various angles of the right channel:




Left Channel:

Thanks for the wiring pics. That will hopefully help when I build mine in a week or so. Which leads me to my first silly question:

For the on/off switch, does it need to be rated to a certain voltage? I thought about using this:

Ring Illuminated - Red (12v), 16mm, SS, 2mm terminals (On/Off) [ASS162FARR12] - $9.85 : GAM Mods, PC Modding Products

Does it matter if it is only rated to 12v? I am guessing it could work for my amp 6 but not for the millett ss.

Anyone know of a similar switch to the vandals?

Only the LED is rated for 12v, the switch is rated for 240VAC, which is plenty.