That's my experience with other minimal class A designs too.
The preferred MJ15003 or MJ15004 are the lowest Vce rated pair in a range of similar numbered On-Semi TO3 devices. The voltage ratings increase as the part numbers increase up to MJ15026. The highest ratings were intended for monster PA amps with kW rather than Watt ratings. Even a version of the humble 2N3055 and MJ2955 are included on some datasheets for the range: https://www.onsemi.com/pub/Collateral/2N3055A-D.PDF
The point is that any of these in the appropriate NPN or PNP group for your build could be substituted since low voltage performance doesn't noticeably change with higher voltage rating parts. A good and useful midrange type that has been available everywhere for many years is MJ15015 or complement MJ15016.
As I once did some PA and instrument amp repairs, I wound up with a collection of odd MJ150XX parts that were orphans from matched sets and ideal for this type of project. Perhaps others could check out repair shops as a source for these parts as this should be better than on-line stores with their cheapo poor copies and fakes 🙁
I think that may be guessing. If we look at the datasheets mentioned earlier, MJ15003 through to MJ15026 are all the same family of parts. Most have similar gains of 60-120 max. and Ft in the range of 1-4 MHz. The main differences lie in maximum voltage and power ratings. Generally with older generation semis, the bigger the chip and power rating, the higher the capacitance, the slower the device - and these include some of the biggest among TO3 audio BJTs.
We can build a JLH class A amp that uses moderately fast output transistors but is quite stable and sounds just as great. However, it just isn't necessary or advisable to do that without suitable test gear and experience. It will probably cost more and change the original appearance too, if it's important to you.
For reference, go to The Class A amplifier site archive: here and navigate to The Class A Amplifier Site > The JLH amplifier index > Additional information for the JLH > Transistor Substitutes.
Note; this doc. is all we have on the topic from Geoff Moss but it was last updated 16 years ago. Though the info is still valid in context, it's well out of date.
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i never try it but is very similar to 15003/15004 with more low collector current
(15A vs 20A for 15003/4)
(15A vs 20A for 15003/4)
I don't think I've ever seen MJ15002 or 15001 in local parts catalogues, even though still in production. It's down to the last shipments status at On-Semi but there's no reason you couldn't use what is available for a PNP version (reversed polarities, polarized parts and transistor types) and it will likely perform the same as MJ15004 in this application.
https://www.onsemi.com/pub/Collateral/MJ15001-D.PDF
https://www.onsemi.com/pub/Collateral/MJ15001-D.PDF
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I found some old MJ15002 produced by tesla in my stock
at least they aren't fakes
will try them
any advice on reversed transistor types for the imput?
at least they aren't fakes
will try them
any advice on reversed transistor types for the imput?
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The original input device is PNP 2N3906, updated to BC212 in 1996 and these are general purpose devices, not particularly low noise, so the usual NPN complements, 2N3904 or BC182 could be used. Other common low noise audio types could be a little better and types such as BC549, 550 have been suggested. However in Europe, different codes like BC413, 414 were used for similar parts and these could be included. If you have a choice of gain rating with B, C suffix parts, I would avoid the highest gain (C) types.
Compare datasheets for any small signal, NPN types you have in stock and just be sure the principal ratings like Vceo and Ic and Hfe are adequate and its a fairly low noise type. Certainly, look in the junk box too 😉
Compare datasheets for any small signal, NPN types you have in stock and just be sure the principal ratings like Vceo and Ic and Hfe are adequate and its a fairly low noise type. Certainly, look in the junk box too 😉
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Ah I see, slower device better for this project , I think I read it on some other post.. dunno remember
Thank you very much
Time to start collecting parts!
And perhaps some eBay pcb’s? Been trying to pick one of so many offered but can’t decide which one is recommended ..
The blue one: A lot of scratch-working available;-)
To get defined circuits, currents-regarding too.
The grey: too many breakthroughs - in my mind. A lot of circuit is/stays unknown ... To get defined circuits, currents-regarding too.
To get defined circuits, currents-regarding too.
The grey: too many breakthroughs - in my mind. A lot of circuit is/stays unknown ... To get defined circuits, currents-regarding too.