Looking for TO3's

[Sowhat]

I posted in another thread about FET's and Toroids and I have the Toroid thing all solved thanks to loudthud, but I have zig zagged a bit and instead of using Lateral MOSFET's, I'm going to go with a larger number of BJT's. The ones I want to use are MJL4281A/MJL4302A but just to be difficult, I want those in TO3 cases, not TO264's. Does anyone know of equivalent TO3 style transistors?

[Enzo]

Sure, use the VERY common MJ15024 and MJ15025. Or the equivalent MJ21194 and MJ21195. Those are 250v parts, and I can't imagine that you really actually needed 350v parts. And look on the data sheet for any part you chose for the SOA. If you want 16A from the part you can't do it at the 250 or 350v, and if you need the 250 or 350 aspect, you cannot draw the whole 16A through it.

[J M Fahey]

Just curious, are you designing a new power amp from the ground up, or following some previous schematic?

[R.G.]

+1.

If you're doing a new design, go with plastic. They're far cheaper and you can actually buy them. This is not necessarily true with TO-3 even if you can find parts that are theoretically the right part number, due to heavy counterfeiting.

I just looked, and On Semi (the artist formerly known as Motorola) and Toshiba don't seem to have any audio power devices in TO-3 any more. Sanken might. Or you could buy leftover stock. But if the objective is more reliability, using an extra pair or two of plastic devices is probably cheaper and more reliable, because of the effect of spreading the heat load out over the heat sink better, even for the same heat sink thermal resistance.

Or, if you insist on TO-3 because of looks, etc., ... well, best of luck.

[Steve Conner]

Last I looked, OnSemi were still selling the MJ15024 and suchlike. Well, technically the plain MJ-whatever are obsolete. The new part number has a G suffix designating a lead-free package. I recently bought some MJ15025Gs for a test fixture at work, a dummy load for a big battery charger.

Just don't buy any off EBay as they will almost certainly be shoddy fakes, made from dumpster-dived silicon dice packaged in a basement with silicone bathroom sealant gooped over the chip.

The plastic packages are certainly recommended for new designs, but the old TO3s are still alive and kicking.

FWIW, the mounting hole provided in larger plastic packages is simply there to confuse newbies, don't be tempted to use it! For optimum heat dissipation, you need some sort of clamp that exerts pressure squarely over the die.

[J M Fahey]

Hi sowhat
Just checked your "toroids" post and found

I have got the bug to design and build another amp, this time it's going to be a high power bass amp

so now I know what you're getting into.
Cool.
The practical problem will be, as usual, what I call the "mechanicals", which is the curse of the homemaker.
From the purely electronic side, you can design at will, whether it's MOS or Bipolars, simulate at will, and polish the design as much as you wish, but then you'll have to:
1) design a PCB, which is not a small task, specially when building "just one" (or a couple) amps.
2) have it made (or home make it).
3) solve the thermal problem: decide on heatsinks, how to mount them, use a fan? , etc.
Unless you already have a couple suitable ones, drilling for and mounting TO3 is way more complex than TO247.
So much so that I'd rather use 6 plastics instead of 4 metallics and still call it a good deal.
4) That said, and for a single build, I'd suggest you get for free a dumpster find such as:

which already provides you with a usable cabinet, transformer, drilled_for_TO3 heatsinks and fan.
You just clean it empty, junk the guts, keep only the "mechanicals" I mentioned and roll your own as far as circuit, sound and preferences.
Even in this example, which is clearly "too much", you needn't fill those 24 TO3 spaces (or 12), 4 or 6 devices are enough for 300W RMS.

There's one reason for everybody on the Net making Tube amps, beyond perceived sonic differences: circuits are simpler and classic, no need for PCBs, since easy turret or eyelet board or even PTP is fine, easy to mod and tweak after build, you can buy a blank chassis and all you need is a drill and 2 Greenlee socket punches (or a Unibit ), etc.
SS amps are more practical and cheaper to build but under a more "industrial" approach.

That's why I suggest you solve that aspect by recycling some "mechanicals" and concentrate on your personal contribution; the sound you create.
jm2c

[R.G.]

Last I looked, OnSemi were still selling the MJ15024 and suchlike. Well, technically the plain MJ-whatever are obsolete. The new part number has a G suffix designating a lead-free package. I recently bought some MJ15025Gs for a test fixture at work, a dummy load for a big battery charger.

Dang. I looked up On Semi audio transistors and was mildly surprised to not see either TO-3 or TO-204s listed. And they're there now.

OK, along with gray hair and needing bifocals, I can't google right either.

FWIW, the mounting hole provided in larger plastic packages is simply there to confuse newbies, don't be tempted to use it! For optimum heat dissipation, you need some sort of clamp that exerts pressure squarely over the die.

So right. I really like the MaxClips illustrated in Figures I and J here.

[imaradiostar]

4) That said, and for a single build, I'd suggest you get for free a dumpster find such as:

jm2c

Excellent idea! Of course, you can take care of some of the other stuff simply too, leaving a small protoboard or strip board (or even eyelets/turrets) for the preamp. I think an SVT style tube pre into a big solid state PA might be nice too.

I have a Hafler and a CS800 at home, both with good PT's, heat sinks and chassis. One of these days I'll build amps out of them. I'm seriously tempted to try some of the Chinese pre-built boards that utilize an off the shelf driver chip and a ton of plastic devices. Worst case would be I'm out the time and about a hundred bucks. Best case I've got a working amp again.

A few examples:

Assembled L20 VER7 Stero Audio power amplifier Boards | eBay

Assembled L20 Audio Power Amplifier Board x 2pcs 350W+350W With Heatsink | eBay

Stero L25 combined type power amplifier board mounted with Angle aluminum by LJM | eBay

700W Assembled mono Power Amplifier board | eBay

LME49810 300W Mono Toshiba High Power Amplifier Board | eBay

You can see how these could save you a ton of time. I don't know if they're worth a darn but it's worth trying for the money.

Jamie

[Steve Conner]

I wouldn't touch them, but that's only because I spent an excessive amount of time and money designing my own.

The LME49810 is an impressive chip. As far as I know it includes a Baker clamp for cleaner clipping, which would be good for musical instrument use. However, it doesn't include any protection for the power devices. That is left up to the board designer and you should check that they have implemented it. Audiophiles don't like protection, but IMO it's vital for any amp that has to earn a living in the real world. Especially around musicians

[Steve Conner]

I wouldn't touch them, but that's only because I spent an excessive amount of time and money designing my own.

The LME49810 is an impressive chip. As far as I know it includes a Baker clamp for cleaner clipping, which would be good for musical instrument use. However, it doesn't include any protection for the power devices. That is left up to the board designer and you should check that they have implemented it. Audiophiles don't like protection, but IMO it's vital for any amp that has to earn a living in the real world. Especially around musicians

[Sowhat]

Just curious, are you designing a new power amp from the ground up, or following some previous schematic?

Well, it's a borrowed design partly but my application simply wants TO3's even though the original design calls for TO264's. I use several wakefield 421 heatsinks in a modular fashion and I have found to make the amp very roadworthy these modules (for lack of a better term) must be quick swappable. It's not the use of a plastic package that concerns me very much but the physical mounting and connecting that presents some problems for me. I suppose I could develop some sort of quick disconnect system for a plastic package but I already have a tried and true method for TO3's so that's why I would rather find a TO3 than redesign a new quick disconnect system for let's say TO264's. This will be for a heavy duty 500VA 350 Watt minimum bass amp.

[loudthud]

I located some small eyelets. They are not quite small enough for DIP ICs, but small enough for just about everything else. Hole is .062. length .125 for .062 board material. Equivalent to Keystone #24, Link: Keystone Electronics Corp. - .250 [6,35] Quick-Fit Terminals

I tried setting them with a center punch but the results were not acceptable. I sketched out what I thought would be the tool needed to set these, sort of the underside of a mushroom, and took it to a machinist. He said no problem for to make one for $10. It worked so good I gave him $20! Now I have three sizes of eyelets to work with. Keystone 24, 34 and 44 (for .062 boards). For the bigger eyelets I use a "Darice" eyelet tool you can find on Ebay for about $7.

[Enzo]

So why must the individual transistors be what disconnects? If you plan to swap out entire heat sinks, why not wire them up and just use a connector? A Molex type or a similar one from AMP, their Universal Motor Mount series perhaps with the little locking tabs yo squeeze to release.

If you are really looking to repair a blown amp during a set break or something, then make power modules like in Hartke amps. One connector for power and output, and a small signal connector for input. Four screws and voila. I mean if you plan to swap out just the heat sink, then what if the failure also took out some resistor? You say it has to be swappable quick, so I must asssume you plan on doing it while in the middle of a performance.

Even if you want to only swap out output devices on a heat sink, look at what Peavey did on their very successful and reliable 400BH. The outputs are on one circuit board on the heat sink, a pair of three-pin connectors hooks it to the rest of the power amp. We have posted that schematic here before, look at post #5 here:

Problem with Peavey Mark3 400BH


ANother alternative would be instead of a molex connector, then have the transistors soldered to a board with 0.250 inch push on pins for the necessary power and output wires.
By swapping whole modules, then the shape of the part doesn't matter.

[loudthud]

I saw a guy on diyaudio was using these pcb terminal blocks as sockets for TO-247 or TO-3P transistors. The lead spacing isn't perfect, but pretty close. Just tighten the screws for a firm contact.

Phoenix Contact - 1715734 - Terminal Blocks and Strips - Terminal Block Connectors - Allied Electronics

[Enzo]

Sure, that ought to work.

[R.G.]

What Enzo said, again. If you simply must have one huge amplifier with different output stages, and it's un-roadworthy enough that you have to design plug-and-play output stages, there are more problems than an easy way to swap output devices.

Solid state amps which fail often have a chain of destruction leading all the way back into the front of the amp. Swapping just the output devices doesn't help, and may in fact kill new, good output stages if the voltage amp, pre-drivers and/or drivers were damaged. With today's parts, the whole front end of the amp before the output stage may be a trivial cost, so, as Enzo notes, why not replicate the whole power amp and mount that whole thing on a heat sink, plugging and unplugging the whole power amp? The connectors are much simpler and more reliable.

In fact, since it's almost certain that you use more than one huge speaker driven by this thing, why not use one smaller power amp per speaker, not many speakers per power amp? That way, a failure of either a speaker or a power amp doesn't shut you down. And bring more speaker/power amp modules than you need, or more power amps (and the associated power supplies, which is another high power/high stress item) as whole modules.

Finally, replacing a TO-3 -OR- a TO-247 or whatever in the field under time pressure and with poor tools, lighting, materials, etc. is a tough thing to do. If you get the fixing bolt tension incorrect, place the insulating wafer incorrectly, use too much or too little heat sink compound, or no heat sink goo, or let a conductive bit fall into the goo, or don't test the drivers, or don't fire it up and test it before frying... er, I mean trying it on an expensive speaker, that's going to make for a very unhappy amp owner.

By now, you've probably gleaned that (a) you asked a bunch of people with firm opinions and (b) many of us think you may be optimizing on the wrong point.

But we're always willing to talk about it.