a suitable transistor substitution for Delite amp

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Can someone please steer me towards a suitable transistor substitution for Nelson’s simplest delite amp. I’m not real concerned with the audio quality (perhaps wrongly), just that we get that magic moment when music comes pouring out of the speakers. Kind of a “hay look what you can do” project.
From Nelson’s Delite article, “This transistor is an unusual part, the Ixys IXTH20N50 depletion mode Mosfet. I got them from Digikey for about 6 bucks (and I had to buy 60).”
When I priced them they at mouser.com they were $21 and I had to get 30 and they were out of stock.
And so, …
I’m hoping to find something for a good bit less that I can use in my classroom to teach about electricity in an ‘introduction to engineering’ class that I’m putting together for next year. I pretty much have free rein to incorporate whatever projects I want to, but not such an open ended budget, so some of this may fall on me. If I/you can find something cheap enough I’ll get a bunch for the whole class and they can do little mods and see what happens etc.
http://www.firstwatt.com/pdf/art_delite.pdf

I don’t have a lot of experience (hence my rather basic question). I have built the F5 (I’ve meant to thank you for your help 6L6. You Rock!)

Thanks all in advance
George
 
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Why not make the zen with a light bulb? Then you've a mountain of parts that are cheap and available to choose from.
 

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PRR

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...“This transistor is an unusual part, the Ixys IXTH20N50 depletion mode Mosfet....”
http://www.firstwatt.com/pdf/art_delite.pdf
...use in my classroom to teach about electricity in an ‘introduction to engineering’ class

Note that this amp is suggested for 60V-100V. This is awful close to high voltage for a non-college classroom. (OK, we played with a 3KV supply in my high school....)

{sigh} Did you even look for the Fig 7 device, IXTH6N50D2? Mouser has 120 to sell one by one, six bucks.

{what I was gonna say} The description may be more helpful than the actual part #. Putting "depletion mode Mosfet" in Google turns up hits. One at Mouser, where you were. They come very small and very large. Selecting Pd 20W-100W gets all sub-Amp devices. Selecting Id >1A gives some oddballs, but IXTH6N50D2 and IXTH10N100D2 are promising. IXTH10N100D2 will sit at 1.5A with -2V bias, which is the right ballpark, except return gate to zero instead of to source. SADLY this is the 12-buck part, but they do have 120 and will sell you just one.
 

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PDR $6 is reasonable. Thanks for the pointers on finding replacements. I really want to keep this as simple as possible. Which leads me to thank aspringv. I had forgotten about the Zen amps. Read about them five or six years ago, but without the intention of building one. But now I think that’s what I want. Single transistor, a simple circuit without the need of a PCB (more $). Perhaps one or two of my budding electrical engineering students will get hooked and continue on. I teach in a poor district so seeing that light of excitement and understanding is just wonderful.
Of course I love it too. :)
 
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The heatsinks are the beefy bits, and I'd imagine you're going to have an interesting time with the eventual case and power supply too. Particularly if said students are going to take these home eventually?

Anyway. The most budget heatsink I've used was a piece of ally extrusion used for solar panel framing cut into pieces (the 'fins') and bolted to a ally plate. The extrusion pieces were free (offcuts) and the plate was a few bucks.
 
So far I've found parts for 15 Zen amps for about $100 - $130 each. I think that's pretty awesome!

Zen Variations 1 | Pass DIY

I do have a heat sink question.
I have a heat sink that was used for an old desk top computer (Intel® Celeron Dual-Core 3300/2.50GHz). It has a copper base and four heat pips that pass through 32 plates. The plates are 3.5in x 2.5in (9cm x 6.5cm).

It looks like the one towards the top of the page. Understanding CPU Heatsinks: Picking the Best CPU Cooler | GamersNexus - Gaming PC Builds & Hardware Benchmarks
The anatomy of a heatsink.


I have the fans if necessary (I was thinking they might be large enough with the fans and using a voltage source closer to 40V than 80V).

I was also wondering if a 20V laptop charger (with some smoothing of ripples) would work. The cost up above does not have a power source yet.
 
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I thought about the ACA but like the simplicity of the Zen. My students have (like almost all high school kids) no experience with resistors capacitors or circuits in general forget ever having soldered. The Zen has a single transistor which I think is an easier concept than the push pull of the ACA - and we can play with the circuit more easily and safely (I’ll have a few extra of parts).
Also the ACA kit is a kit. Just not as cool as a buch of parts and a diagram and point to point soldering. And it’s about twice as much money. Most of that is the case which is very nice, but I’m going for student creativity here. The only ground rule is to have (and understand) good grounding. :)
 
I do have a heat sink question.

My CPU heatsink copper plates had a poor surface finition, means poor heat transfers.
I treated them with my elbow grease (wet sand paper).
Extra measure has to be taken for active cooling system, an NC thermostat (a dollar or so) in series with the PSU in case of malfunctioning or forgot to turn fan on.
Cheers
 

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Very cool. I like the idea of repurposing the old computers. This week I’ll check out the voltage coming out of the PC power supplies (not the 19.5V laptop bricks - the internal power supply). It wouldn’t be the worst thing to have to rectify and smooth + and - 20 to 30 volts like I did with the F5. It would jacknthe price up to have to get the transformers and caps but good lessons.

As for goop I’m going to try and save their little hands and use thermal insulation pads, similar to the ones in the ACA kit. CD-02-05-190 Wakefield-Vette | Mouser

Thanks for the help and encouragement. Right now I’m thinking having my class creat accounts here and post progress on a class thread. If I get jammed up I know there is a wealth of knowledge here.
George
 
Very cool. I like the idea of repurposing the old computers. This week I’ll check out the voltage coming out of the PC power supplies (not the 19.5V laptop bricks - the internal power supply). It wouldn’t be the worst thing to have to rectify and smooth + and - 20 to 30 volts like I did with the F5.

An internal ATX-type computer power supply won't do it. 12V max, and the corresponding -12V is only good for an amp or so. The +12V output is pretty stout. You could run a handful car audio-type chip amps with it for a few watts of output each.
They do make for a quick and greasy benchtop supply with (IIRC) +12V, -12V, +5V,-5V, +3.3V available.
 
So I was looking at transformers (100 VA 20V) to get +/-28 V, so I’ve got 56V to power the amp. So I run close to 1A through the light bulb that acts as a 40-50ohm resistor to bring the voltage about 12V to feed the collector. (Do I have it right so far?)

So my question is why not ditch the light bulb and use a 12V supply? (Yeah, I know the light looks cool �� but it’s a more expensive transformer)
 
In this amplifier the voltage across that transistor is only 12v if no signal is being amplified. But when you are amplifying a signal the voltage across the transistor is changing perhaps from 2v to 30v . That light bulb just acting as a big resistor allows
for this voltage swing across the transistor. Without this swing there would be no ac
output that is NO music .
 
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