Millett "Starving Student" hybrid amp - Page 237

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Edited by jon743 - 9/17/11 at 12:39pm

My guess is that the basic gain and voltage swing of the 19J6's in the Starving Student would be sufficient for the K240. Remember, there are some guys around here who've been using the SSMH to power some vintage orthos that probably exceed K1000 amp requirements.

About the pot - try not to think in terms of the impedance of the pot having an affect on the gain. Rather, the impedance of the pot combined with any input resistors you might use represent an overall increase in input impedance seen by your source (more noise, less detail). Dsavitsk may argue with me on this one, but I don't think you'd want the overall input impedance - pot + input resistors - to go much over 100K ohms. That's sort of the limit we put on our headphone pot impedances around here. Now, if it went to 150K or so - that's probably fine. However, if you're looking at controlling gain with low impedance headphones and start out with a 100K pot, then putting input resistors at 2-3 times the pot value would quickly give you a total impedance of 300-400K ohms as seen by your source. That's too much, IMHO.

So, low impedance/high efficiency phones - you might want a 10K pot and then 40K input resistors. The resulting total input impedance would be only 50K ohms, but you would have attenuated the gain by 4 times.

Of course, you'd never want any input resistors with a 600K headphone. So, I'd use a 50K or 100K pot, period - nothing else.

Finally, keep in mind that in my testing (I've lived with these two SSMH's for a couple of months now.), the 50K pot + 50K input resistors is the best balance for Sennheisers to Grado's. If you want something optimum for a single set of headphones, though - by all means try a different combination.


BTW - some of you have had trouble at Beezar with Paypal recently. I finally researched it today with the help of a customer. Turns out, Paypal changed the address of their payment server over the last few days. I confirmed this directly from Paypal and the same stuff was being reported on the OSCommerce support forums. Anyway - I've fixed the problem.

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Edited by jon743 - 9/17/11 at 12:39pm

I have the 600ohm k240s and I used just a 10k pot that I had lying around. It worked perfectly well if I remember though it was a bit much for other headphones.

As promised (but a little late), here's a pic of the custom Beezar/Hammond SSMH case:


(Click for a bigger pic)

It's getting about that time for the kits soon. I'll be out of the country (more or less) the next week, so I thought it was an appropriate time to post some stuff. This will give you all something to discuss while we're awaiting final shipment on the cases and following that, shipping your kits to you.

Besides the case pictured in the post above, here's what will constitute a kit:

(Click for a very big pic.)

The power supply comes complete with an AC cord, but was too big to photograph here along with the smaller parts. The only thing that won't be included is that the sockets won't be drilled as shown here. You'll have to do that yourself. There have been several questions about how to do that - and questions about building the SSMH, in general. So, I'm going to build another one using the kit above and the sample case pictured above in a step-by-step fashion. All of this will be posted on the SSMH website, too.


P.S. That's a machined aluminum knob, btw, and very hefty. Electroswitch makes some outstanding knobs for Mouser. It's just that they don't fit that well with the typical RK27. We don't have that problem, though, so this one works and looks great.

The first thing I do when building an amp with tube LED's - such as the SSMH, Millett Hybrid, MAX, MiniMAX, etc. is to drill the tube sockets. For one, you can't do that very well once soldered. Even if you could, chances are probably 1 in 4 that you'll break one. If it's already soldered to the board, that's bad.

Second, it takes awhile for the glue to dry. So drill them out, glue them back together, and let them dry/cure while you start working on the PCB.

Here's what the tube sockets look like before drilling:


Some people are able to pull out the center pin from the bottom (there are two halves to the pin - the bottom one has a flange). However, I usually only succeed in tearing up the flange until there's nothing left for my pliers to grip.

So I drill. Below shows a socket clamped between a couple pieces of pine in a drill vise on my trusty drill press. You don't need a drill press per se, but it's probably best to use some sort of vise to clamp the socket - gently - they are very easy to break.


Take it slow and don't push down forcefully! Let the drill bit do its work. I use a 1/8" bit. Smaller than that and you may just succeed in drilling a very small hole all the way through both pieces of the center pin, but perhaps making them even harder to get out. Larger bits are just too much and will break the socket.


Lift the bit out of the hole periodically to make sure you're not getting galling - the pins are very cheap pot metal and will easily melt to the bit if you let things get too hot.


The idea is to let the drill cut through the top half of the pin, allowing the bottom half to fall out/push out, or something similar. If you're lucky, you may end up with what's shown below - the bit will simply grab the top half of the pin and pull it out on one of your periodic bit removals. If so, then you're done drilling!

Here's what we have after drilling two sockets. (I confess, I went through 4 sockets today to get these. I was too distracted taking photos.) You can easily see the two portions of the removed pins:


OK - now that we've taken the pins out that held the two halves of the sockets together, we need to use something else that will bring the two halves together, but preserve the center hole. Epoxy works best for me. 15-minute is easily available at hobby shops and perhaps some craft stores. The bottles shown here are at least 5 years old. If stored in a cool place away from sunlight, they'll last a good long time. If you can't get the 15-minute kind, then use 5-minute epoxy - but be sure you're organized and work fast. Slower epoxies will dry more brittle and also ooze out of the socket until most of the glue is dripped out. It's important to use something that doesn't necessarily depend on soaking in, but cures with some flexibility. Don't use polyurethane glues - they will expand and get into the socket pins, ruining the sockets. Wood glues, on the other hand, will not soak into the grain-less ceramic, so they won't work either.

To mix up the epoxy, I use a simple sheet of aluminum foil folded over several times to give some thickness. Then I use a toothpick to mix the two parts of the epoxy together. The first step is to squeeze out two same-size blobs next to each other. Be very careful that you don't let the tips of the bottles touch the goo on the aluminum foil, or you won't get anything out of that tip again.


Next mix up the two parts until it's a single, contigous color and there are no streaks. Again, a toothpick works great for me:


The clock's ticking on the cure rate now, so start applying the epoxy to the tube sockets. After much trial and error, I've decided that the best way to do this is by applying epoxy only to the bottom half. Use the toothpick to smear a tiny blob between each of the pins - a little more on the socket key. Try not to get any any on the tips of the pins (ruins the tube connections), or near the center hole (you won't be able to fit the LED through). If you happen to get some inside the pins, use another toothpick dipped in rubbing alcohol and push it into the pins a few times. Work fast, because the epoxy is curing all the time.


Finally, very carefully fit the top half back onto the bottom half of each socket. Do this by lining up the keys that are molded into each half. Gently wiggle the top of the socket until the pins pop into place in each hole. Use rubber bands to clamp the socket halves together until the epoxy is cured. Let the epoxy cure with the sockets standing up straight! Otherwise, the epoxy may flow down into the pins, ruining them. Also, if you're in a hurry, this time of year things will speed up if you put the sockets out in the sun to cure.


Use the epoxy blob left on the aluminum foil to tell you when the epoxy is cured. The blob should feel smooth and slightly oily to the touch when it's fully cured.

Finally, when cured - do yourself a favor and test out the sockets before you solder them to the PCB:

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Edited by jon743 - 9/17/11 at 12:37pm

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Edited by jon743 - 9/17/11 at 12:37pm

tomb would`nt these bits be ideal for removing the pins?

yaka, though those extractor bits are designed for removing threaded screws with damaged/rusted heads, they might just work. The socket center pins appears to be press-fitted only so maybe once you get the bit's reversed thread in you can simply yank out the pins of the sockets? (sorry I don't have any sockets with center pins therefore I can't try it).

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Edited by jon743 - 9/17/11 at 12:36pm

tomb....having seen your pictures, I feel it might be worth ordering a couple of extra tube holders to go with my kit. Can I do this without incurring additional postal charges?

Do a couple of tube sockets weigh -0- and have no additional packaging costs?
No.

Would a couple of tube sockets added to an existing order cost less to ship than by themselves?
Absolutely.

P.S. Please understand - my purpose of posting the tutorial had nothing to do with trying to sell more tube sockets. On the contrary, it's only to detail everything that's involved. I've had many questions about how to drill tube sockets for LED's. At the same time, whenever someone's asked, I've always recommended buying extras. Some people never break one, some always do - YMMV.