Threshold 400A transistors

[John]

Does anyone know what type of transistor should be used for Threshold
400A? Also looking for a recommended upgraded type of
transistor(250w). I have a semi-conductor test set to match but dont
want to buy a hundred TO-3's to get closely matched sets.
I think it may use a MJ15012 output @200w ea
What would be a good 250w transistor in lieu of the 200w for this amp?

[Pooh Bear]

Post by John
Does anyone know what type of transistor should be used for Threshold
400A? Also looking for a recommended upgraded type of
transistor(250w). I have a semi-conductor test set to match but dont
want to buy a hundred TO-3's to get closely matched sets.

What's printed on the can ?

Post by John
What would be a good 250w transistor in lieu of the 200w for this amp?

I'm curious why you think you need to replace them but the MJs are now
made by On Semi. They now have MJ21193/4 that are higher power
complementary pair with the latest diffusion technology. Might not be
stable in an old design though, due to higher ft.

http://www.onsemi.com/pub/Collateral/MJ21193-D.PDF

If your output stage is simply NPN only ( quasi-complementary ), I suggest
that a new amp may be in order if you value sound quality.


Graham

[François Yves Le Gal]

Post by John
Does anyone know what type of transistor should be used for Threshold
400A?

If I remember correctly, the 400A and the 4000A used multiple Motorola
2N5878/2N5876 complementary pairs. They are 80 V/10 A/150 W devices

http://www.centralsemi.com/leadedpdf/2n5875-5878.pdf

The schematics http://www.passlabs.com/np/400a-4000.tif only designate them
by A6 and A8

Post by John
Also looking for a recommended upgraded type of
transistor(250w).

Why? Pass used 12 NPN and 12 PNP transistors in the output stage, giving you
a more than adequate operation envelope.
The MJ series are much faster, with a higher Hfe and everything, which could
very well turn the amp into a fancy oscillator - or a very expensive fuse.

Post by John
What would be a good 250w transistor in lieu of the 200w for this amp?

Why should you want to "upgrade"?

Oh, and my favorite quote on properly setting bias on this amp, from Nelson
Pass, no less: "The bias should be set so that after the amp warms up the
heat sinks should be at a temperature where you can put your hand on them
for 5 seconds but not 10. (this, by the way, is the standard formula I use
for every amp)".

http://www.diyaudio.com/forums/showthread/t-40089.html

Is Nelson Pass a Fluke?

[John Walton]

Do the amplifier transistors have current limiting resistors -- usually 0.2
ohms -- these will help balance the load if you are using unmatched devices.

I have a few hundred of the 21193/94 devices in stock -- if you need matched
let me know.
http://www.tech-diy.com/hexfets.htm

Post by John
Does anyone know what type of transistor should be used for Threshold
400A? Also looking for a recommended upgraded type of
transistor(250w). I have a semi-conductor test set to match but dont
want to buy a hundred TO-3's to get closely matched sets.
What would be a good 250w transistor in lieu of the 200w for this amp?