New MJ21193's and 94's are cased in steel? - diyAudio
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Old 24th January 2009, 01:55 AM   #1
SQLGuy is offline SQLGuy  United States
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Default New MJ21193's and 94's are cased in steel?

I just got the replacement parts for my PS Audio 200c. I'm switching to MJ21193's and MJ21194's for the outputs since the original NECs are pretty scarce and heavily counterfeited.

I was surprised that the cases for the new transistors are steel - both the cap and the base plate. Is this a new development due to the cost of copper, or have these always been steel cased?

I know a lot of purists that cringe at the thought of any steel anywhere in the current path. Is there any real concern here? Maybe bimetallic rectification effects between the steel cases and copper bus bars?

By the way, I checked the old transistors. The HSC's were all steel. The NEC 2SD555's were also steel, but the NEC 2SB600's were copper base plates with steel caps.

Thanks,
Paul
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Old 24th January 2009, 02:06 AM   #2
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New ?
I'm no expert on US TO3 devices, but i've never seen anything other than steel cases, for alloy you have to shopstick.
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Old 24th January 2009, 02:20 AM   #3
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Pretty much the standard practice for OnSemi and Motorola for years and years. I've used Texas Instruments power TO-3s that were copper, and I seem to recall running across some lower power TO-3s that felt light enough to be aluminum.

Cost probably has something to do with it, and possibly the tempco of steel and silicon are a better match. (Although the silicon is usually attached to a heat spreader, and I don't remember what material that is.)
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Old 24th January 2009, 03:10 AM   #4
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MJL21193 and MJL21194 are very good for Power Output.
Especially if you want to run Class A.

They are among the best TO-3 (steel) tranistors that are affordable.
More robust and can take more heat, higher temperature, than plastic packages ( like TO-247, TO-220, TO-3P ).

Also when attaching they are not as sensitive if you screw them TOO HARD.
They wont get as easily MECHANICALLY destroyed.

Robust and fairly fast and suitable for audio
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Old 24th January 2009, 04:17 AM   #5
SQLGuy is offline SQLGuy  United States
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Quote:
Originally posted by jacco vermeulen
New ?
I'm no expert on US TO3 devices, but i've never seen anything other than steel cases, for alloy you have to shopstick.
When this amp first came out, PS Audio made a big deal about their "solid copper" output transistors. Obviously this came off as a bit silly in the audio reviews, since there had to be some silicon, etc, in there with the copper, but they were using copper-cased transistors. At one point they were using NEC's, at another, Sanken's.

As I mentioned above, the NEC PNP's that were in the amp when I got it did have copper bases, though the NPN's did not. The PNP's also had red printing, which some have noted as a sign of a higher grade set of parts used by Mark Levinson et al.

This got me curious enough to go check some of my other stock of old MOSFETs and such, and sure enough, they're steel as well. I guess those copper TO-3's that were originally used really were somewhat special.
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