Replacement for 70's Vintage Japanese TO-3 - diyAudio
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Old 6th February 2010, 11:36 PM   #1
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Default Replacement for 70's Vintage Japanese TO-3

I have a 70's vintage Kenwood KR-7600 receiver and an amplifier block from an old Pioneer SX-850 I'd like to bring up to speed. I'd especially like to upgrade the Kenwood, as I like its classic looks, and want to turn it into a stealth performer. The amp circuits are nothing really special, and the questions I have are regarding the output transistors. The Kenwood uses one each of 2SC1403 and 2SA745 per side, and the Pioneer uses a pir each of 2SB531A and 2SD371A per side. Is it worthwhile changing these out for some more modern TO-3 devices? The rest of the amplifier circuits are nothing really special, and I intend to have my way with them.
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Old 7th February 2010, 07:51 AM   #2
Mooly is offline Mooly  United Kingdom
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My feeling is there is nothing to be gained by swapping these.
In general, you would have to make other changes to get the benefits of faster higher gain devices. The circuitry as a whole is the limiting factor IMO.
To increase bandwidth, get faster slew rates etc requires careful modding and testing.
Anything is possible... but just swapping outputs won't do much on their own
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Old 7th February 2010, 10:02 AM   #3
djk is offline djk
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Just doing cap mods will sound a whole lot better on these amps. Fresh caps with film bypass.
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Old 7th February 2010, 10:37 AM   #4
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The power amp sections will be getting an extensive face lift, so if there are better TO-3 devices available, I'm game. However the original output devices are Toshiba or Sanken parts with 8MHz/15MHz ft, respectively. It's hard to improve that without moving to new TO-3P devices. I would be willing to trade better SOA or less beta droop for a somewhat lower ft.

The Pioneer amp block has lots of electrolytics in dubious places, as well as some ho-hum driver transistors. These will all go. There is a dodgy offset adjust at the amp input that will also go. I've been building amps for about 30 years - all I'm looking for are some alternate suggestions for output devices that don't involve ripping out the TO-3 sockets. All my amps lately have used mosfet output devices or tubes - my last bipolar amp used MJ802 and 4502.... It may well turn out that the output devices are the only things that stay, if I don't end up replacing them with lateral mosfets.
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Old 7th February 2010, 11:09 AM   #5
Mooly is offline Mooly  United Kingdom
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How about in that case, something like the MJ21193G and MJ21194G which are TO3 package and are "audio devices" as their intended use.
I believe these are more "complementary" than a lot and suffer less from beta droop...

MJ802 and MJ405... wasn't one of D Selfs designs was it ?
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Old 7th February 2010, 11:55 AM   #6
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Mooly View Post
How about in that case, something like the MJ21193G and MJ21194G which are TO3 package and are "audio devices" as their intended use.
?
those ones have a poor FT, they are in the same league as the
mj802/4502, but unfortunately, there are few if none, alternative,
as TO3 with good perfs are no more produced, at least, i don t
know better ones that the onsemi s mentionned..
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Old 7th February 2010, 03:12 PM   #7
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Quote:
Originally Posted by wahab View Post
those ones have a poor FT, they are in the same league as the
mj802/4502, but unfortunately, there are few if none, alternative,
as TO3 with good perfs are no more produced, at least, i don t
know better ones that the onsemi s mentionned..
This could be a good choice:
http://www.st.com/stonline/products/...44/2st5949.pdf
for more types go to my overview:
bipolar (bjt) transistor families for audio power output stages
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Old 7th February 2010, 04:56 PM   #8
wahab is offline wahab  Algeria
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yes, good device, it seems..
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Old 7th February 2010, 10:22 PM   #9
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For Mooly, the amp with the 802 and 4502 outputs was a Leach-like amp (somewhat) I designed and built back in 1979 with the materials I could grub up in the backwoods of Ithaca, NY. It looked like hell, but served me well for about 20 years, and still sits around disconsolately in a cupboard wondering why I abandoned it. No turn-on thump, and no offset drift after its initial adjustment.

I'll take a look at the ST devices. All the On Semi TO3 devices I've seen top out at 4 MHz. A plus for them is that I have scads of a couple of the NPN types courtesy of Papa Nelson at the first Burning Amp do.The complements aren't all that expensive form Digi-Key, especially in quantity.

Last edited by wrenchone; 7th February 2010 at 10:25 PM. Reason: spelling
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Old 7th February 2010, 11:10 PM   #10
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Nice call - the ST devices are in stock at Digi-Key, and the price ain't to bad. The SOA is about the same as the Motorola/On devices I have.
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