[tomb]

Quote:

Originally Posted by cms5423 /img/forum/go_quote.gif Ok, but is there a pot option that mouser sells, that I can use without input resistors?

No.

Quote:

Like just get a 50k pot from alps? But which one? Can someone post a mouser part number for a good pot for this project?

... and no.

Mouser doesn't sell the Alps pot you're thinking about - nobody does but DIY suppliers like Tangent, Amb, or myself. The Alpha is the best compromise if you want to purchase from Mouser. DigiKey sells the Panasonic EVJ, but I think you'd be even less happier with that.

 

[cms5423]

Alright, sounds good to me, See I'd love to buy the pot that you sell on your beezar site, but then with shipping, it's not quite worth it, it pretty much doubles my cost.

 

[kuroguy]

maybe one of those home assemblled ebay stepped attenuators. I got one for $16 shipped. It'll certainly dictate the size of your enclosure, but it works really nice and adds 3 full evenings to the assembly (if you're low on projects and high on spare time).

 

[cms5423]

One more question. Why is this called a hybrid amplifier? I thought hybrids normally had a solidstate output stage, with tubes implemented earlier in the circuit? But from the looks of this project, there is no solid-state component

 

[smeggy]

well the solid state outputs are the MOSFETs attached to those heatsinks.

 

[cms5423]

ah ok. which also on that note (sorry for all the questions, I'm trying to actually learn the ins and outs rather than just put it together). The heatsinks, to work, how do you connect them to the mosfets?

 

[sachu]

Quote:

Originally Posted by cms5423 /img/forum/go_quote.gif ah ok. which also on that note (sorry for all the questions, I'm trying to actually learn the ins and outs rather than just put it together). The heatsinks, to work, how do you connect them to the mosfets?

You will need To-220 heatsink mounting kits to insulate the drain of the MOsfet from the heatsink else you can get a nasty shock if you accidentally touched the heatsink.
.
Make sure you pick the right type of heatsink, screw type or PCB mount type depending on how you plan to mount the heatsink.

If you want to go with the beezar PCB kit then go with the PCB mount type, else the screw type heatsink may be easier to deal with.

Tomb, how much is a PCB kit for the SS going to be sold for? And what doies it include? Can you just buy the PCB and a set of tubes?

 

[cms5423]

Ok, but what exactly will the mounting kit do? Like I guess my question is how does a heatsink work? I understand it gathers the heat and then disperses it through it's large surface area, but how does it gather the heat from the mosfets? Or am I completely wrong?

 

[sachu]

Quote:

Originally Posted by cms5423 /img/forum/go_quote.gif Ok, but what exactly will the mounting kit do? Like I guess my question is how does a heatsink work? I understand it gathers the heat and then disperses it through it's large surface area, but how does it gather the heat from the mosfets? Or am I completely wrong?

The Mosfet is basically screwed onto the heatsink so the back of the MOSFET is flush with the heatsink. So through the physics of conduction of heat from the MOSFET package to heatsink (the heatsink acts as an extension of the MOSFET package to dissipate heat), the MOSFET can be in the SOA (safe operating area)


All that the heatsink mounting kit does is insulate the drain of the MOSFET from the heatsink. most MOSFETs have the tabs shorted to their Drain. The mounitng kit comes with a mica insulator which goes inbetween the MOSFET tab and the heatsink. Then you take the small plastic washer which goes into the mounting hole of the MOSFET through which you wil put the screw. You need this plastic washer without which the screw would short the heatsink and the MOSFET tab.

 

[cms5423]

gotchya. Thanks

 

[tomb]

Sachu is correct. However - to make that happen - there are several things you need:

1. Bolt
2. Washer to protect the bolt head
3. Shoulder washer to insulate the bolt from the hole in the metal tab - preferrably, these are also heat-rated
4. Thermasil insulating pad or a mica insulator with thermal grease
5. Washer on the back side to protect the lock washer from cutting into the heat sink
6. Lock washer
7. Nut

The surface contact between the MOSFET back and the heat sink is all important. Actually, IMHO, the electrical insulating quality is secondary. The real task of the MOSFET-heat sink interface is to maximize the connection so that as Sachu says, the heat sink simply becomes an extension of the MOSFET.

Here's another diagram that may help:
MOSFET-MAX Heat Sink Mounting


EDIT: Looks like Sachu edited his post to include much of what I typed here.

EDIT2: I'm just repeating what Sachu already said, but note that the purpose of the heat sink is NOT to keep the MOSFET cool - far from it. The real purpose is to keep the MOSFET from exceeding safe operating temperatures. MOSFETs sort of like to run "hot." The heat sinks, when properly sized, provide a "heat inertia" that keeps things from running away and burning up. Improperly sized, a too-small heat sink can be like teetering on a cliff - one slight push and the whole thing crashes and burns. A properly sized heat sink slows this all down, and prevents any runaway temperatures.

 

[cms5423]

lastly, i tried ordering 19j6 tubes from this one site that were 4$, but I haven't heard from them, so does anyone know of any other place to buy them at a reasonable price?

 

[tomb]

Quote:

Originally Posted by sachu /img/forum/go_quote.gif <snip> If you want to go with the beezar PCB kit then go with the PCB mount type, else the screw type heatsink may be easier to deal with. Tomb, how much is a PCB kit for the SS going to be sold for? And what does it include? Can you just buy the PCB and a set of tubes?

It will be a fair price.

I'm not out to gouge anyone. I believe Dsavitsk and I agreed that the PCB itself will be $10. The 19J6 prices will be $5 each plus a $2 charge for matching. There's two triodes in each tube, so it takes awhile to test each tube. They will eventually run out, of course - before the PCB's do. I'm going to make one last call to my tube wholesalers this weekend for any that may remain out there, then we'll call it and start selling. I've also said this before, too, but 30 kits to start, then PCB's + tubes until the tubes run out, then PCB's.

My real concern is how to guarantee the tubes when the supply is finite. If I sell them all, then someone turns up with a microphonic tube, etc., I won't be able to replace it. You can't test microphonics in a tube tester. So, I guess I will also try out each individual tube in the actuall SSMH, but that's going to take even more time on my part. I think we'll pause for a week or two between selling the kits and the PCB's+tubes just to make sure that those who've invested in the kits have good tubes.

If any of you have a better idea, let me know.

P.S. I'm going to conduct a bit of additional testing before we do all of this. Frying another BantamDAC has got me a bit concerned. I'd like to know exactly what the phenomenon is that's causing it before we sell to the masses - and it's not power supply stuttering, because the current parts selection/caps have no stuttering at all. Besides, I blew the Bantam with the SSMH on and running and with the Bantam connected to USB and fully charged.

EDIT: If this offends a mod, I'm sorry. Because of the limited availability of the 19J6 tubes, this is a one time deal - almost the same as a group buy. Please delete this post if you feel the need.

 

[sachu]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif P.S. I'm going to conduct a bit of additional testing before we do all of this. Frying another BantamDAC has got me a bit concerned. I'd like to know exactly what the phenomenon is that's causing it before we sell to the masses - and it's not power supply stuttering, because the current parts selection/caps have no stuttering at all. Besides, I blew the Bantam with the SSMH on and running and with the Bantam connected to USB and fully charged.

Thisis exactly the reason I sold my Starving student amplifier.

I really liked the little amp. Punches well above its weight and would love to build another one. Is there a waiting list on the PCB+tubes? Is it possible for you to put me down for a PCB+ tubes set?

 

[smeggy]

me first