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.
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
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
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.
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.
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/literature/ds/14244/2st5949.pdf
for more types go to my overview:
http://www.diyaudio.com/forums/soli...istor-families-audio-power-output-stages.html
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.
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.
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There's such a thing as 'too fast', and if you've been building amps for thirty years you know this. I'd suggest that, although you may have criticisms concerning the components on the driver board (there's nothing wrong with the parts chosen by Kenwood...they are low Cob, wide bandwidth parts), the Japanese TO-3 output devices are very difficult to beat without reworking the heatsinks to use modern plastic devices. And the ST-Micro devices, nice though they are, are probably not a good idea in this receiver.
There's not a thing in the world wrong with the MJ21193 & 94. They are designed for audio, and very linear over a couple of decades of collector current. If the original TO-3's blew, these would be the first choice. I'd suggest that stability may well be a problem if the ST devices are substituted.
There's not a thing in the world wrong with the MJ21193 & 94. They are designed for audio, and very linear over a couple of decades of collector current. If the original TO-3's blew, these would be the first choice. I'd suggest that stability may well be a problem if the ST devices are substituted.
I will reiterate that the amp will be more or less completely redesigned (this is not a transistor-rolling exercise), so all bets are off. I will be hacking and adding bits of perf board as necessary.
I may keep the current devices if they are OK, as this is convenient. It is also easy to buy the ST devices and try them with my proposed circuit. I am very comfortable with high bandwidth circuits and have all the necessary equipment at work for detailed gain-phase analysis.
I will make over the Pioneer module first, as the risk factor is small. The Kenwood can wait a while, especially as there are other things in the innards that I'd like to change. I was lucky enough to score some of the modules for the Kenwood receiver from someone who parted one out, so I can work my devilry with them before I tackle the receiver itself. None of this stuff will happen in a big hurry.
After all this is said and done, I may just drop some lateral mosfets in place of the outputs anyway.... I have a few looking for a home. Then again, the heat sinks are probably not large enough to support the customary idle current for the mosfets.
I may keep the current devices if they are OK, as this is convenient. It is also easy to buy the ST devices and try them with my proposed circuit. I am very comfortable with high bandwidth circuits and have all the necessary equipment at work for detailed gain-phase analysis.
I will make over the Pioneer module first, as the risk factor is small. The Kenwood can wait a while, especially as there are other things in the innards that I'd like to change. I was lucky enough to score some of the modules for the Kenwood receiver from someone who parted one out, so I can work my devilry with them before I tackle the receiver itself. None of this stuff will happen in a big hurry.
After all this is said and done, I may just drop some lateral mosfets in place of the outputs anyway.... I have a few looking for a home. Then again, the heat sinks are probably not large enough to support the customary idle current for the mosfets.
Laterals are my outputs of choice... that's what my own amp uses.
I modded an old Rotel using HEXFETS,
http://www.diyaudio.com/forums/soli...l-ra820a-use-hexfetss-amongst-other-mods.html
If you use laterals remember to disable any temperature sensing of the vbe multiplier... negative tempco and all. Idle current of 100ma is ideal... no problem heatwise.
Hello all!
Just bringing back this thread because I have same problem with my vintage integrated amplifier (50Wpc RMS)
It has the TO-3 types of old 2SA745 and 2SC1403, so I'd like to know if I can replace them with the newer 2ST5949 and 2ST2121 without any problems.
Thank you in advance!
I believe the old TO-3s on your AU-517 are made by Sanken and high probability it's an LAPT type. I don't have the original datasheet so I can't back up my assumption, the floating PDF on the net are poor copies with no graphs . The ST parts you listed are nowhere close to the original, its got beta droop. LAPTs have gain/hfe that is consistent for 3 decades and are fast. A better choice if you can't find a reputable Sanken dealer try the Toshiba 2SA1962/2SC5242. They are not LAPTs but have better hfe linearity than the ST parts.
Sanken still makes LAPTs but they are either in MT-100/TO-3P and MT-200 epoxy packages -> SANKEN ELECTRIC : Power Transistors : For Power Amplifier
Sanken still makes LAPTs but they are either in MT-100/TO-3P and MT-200 epoxy packages -> SANKEN ELECTRIC : Power Transistors : For Power Amplifier
The ST Micro parts were not suggested as replacements, but to suit wrenchone's completely redesigned output stage. Having said that, later T03 power transistors will "work"to within their limits compared to the originals. These, I believe are loosely cloned, higher rated versions of Toshiba 2SC5949/A2121 in TO3 cases.
2SC5949 datasheet and application note, data sheet, circuit, pdf, cross reference Datasheet Archive
2st5949 datasheet and application note, data sheet, circuit, pdf, cross reference | Datasheet Archive
2SC1403 datasheet and application note, data sheet, circuit, pdf, cross reference | Datasheet Archive
In turn, those appear to be developments of 2SC5200/A1943.
Given the high Ft of 25 MHz compared to the original's 10 MHz, you might anticipate some stability issues by simply using them as drop-in replacements but that is likely simply resolved with a small bass stopper resistor, if necessary. As the alternatives are much lower FT On-semi parts as discussed, this seems a good, robust option. Frankly, I would use the MJ21193/4 option, which is a safer bet if you are inexperienced in substitution and resolving any problems.
I'm not sure about low beta droop or LAPT technology in 1980. I think it is a little later.
2SC5949 datasheet and application note, data sheet, circuit, pdf, cross reference Datasheet Archive
2st5949 datasheet and application note, data sheet, circuit, pdf, cross reference | Datasheet Archive
2SC1403 datasheet and application note, data sheet, circuit, pdf, cross reference | Datasheet Archive
In turn, those appear to be developments of 2SC5200/A1943.
Given the high Ft of 25 MHz compared to the original's 10 MHz, you might anticipate some stability issues by simply using them as drop-in replacements but that is likely simply resolved with a small bass stopper resistor, if necessary. As the alternatives are much lower FT On-semi parts as discussed, this seems a good, robust option. Frankly, I would use the MJ21193/4 option, which is a safer bet if you are inexperienced in substitution and resolving any problems.
I'm not sure about low beta droop or LAPT technology in 1980. I think it is a little later.
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I have quoted TO3 parts. They are all identically dimensioned. Note: MJ21193/4 are TO3, as are 2ST5949/A2121. Other types are only for reference.
Interestingly, TOP66 and TOP3 parts (which are similar to T0220 and T03P cases) were designed to adapt to the original TO66 and TO3 heatsinks. By trimming the central collector lead, the tab attaches directly to the nearest TO3 mounting bolt and the leads are bent and passed through the sink as previous. I can't recommend it for full metal case dissipation ratings, however.
Interestingly, TOP66 and TOP3 parts (which are similar to T0220 and T03P cases) were designed to adapt to the original TO66 and TO3 heatsinks. By trimming the central collector lead, the tab attaches directly to the nearest TO3 mounting bolt and the leads are bent and passed through the sink as previous. I can't recommend it for full metal case dissipation ratings, however.
Japanese designs historically focused on high Ft transistor developments. Some substitutions with On-semi types on other Sansui models such as AU517 have been claimed on various forums to sound no different. Some other designs do benefit from high speed, wide bandwidth topology but not all amplifiers are designed to make best use of such components, if I can put it that way. In fact, often the decision to fit them seemed unwarranted but that is how marketing and model variation costs are controlled too.
Then, there are guys that hear audio by the price and parts list too
The point is, that there is only a rule-of-thumb notion that high Ft is always necessary in every audio amplifier, yet this is plainly untrue. There is a small risk involved with fitting parts twice as fast as the originals and it is still your choice with unfortunately, few easy options.
Then, there are guys that hear audio by the price and parts list too
The point is, that there is only a rule-of-thumb notion that high Ft is always necessary in every audio amplifier, yet this is plainly untrue. There is a small risk involved with fitting parts twice as fast as the originals and it is still your choice with unfortunately, few easy options.
If I may add my $0.01...
Comparing Ft based on transistor substitution tables can often be misleading. Ft depends on current and remember that your amp actually mostly runs around idle, hence must be stable for these conditions.
There are plenty of devices to chose from as substitutes for older TO3 types electrically, but 99% of them are in a TO3P/MT100 or MT200 case. TO3P can be adapted to a TO3 intended heatsink in most cases but sometimes it can be an adventure. Many old japanese amps have transistor sockets which are designed to mate to the round TO3 pins, whereas TO3P have a square/rectangular cross-section and are made of softer material. Always check this, because problems can and do happen in some cases.
Actual TO3 devices to be used in older amps have unfortunately dwindled to perhaps 3 pairs, two of which are ONsemi and the third is the ST part. Some twilight tech factories like Semelab manufacture the old types but these are quite difficult to source, and can be extremely expensive because they are mostlyused in the medical, indistrial and military field where replacing a part with a different type means a whole lot of certificates have to be made again which costs huge money.
Fortunately, in MOST cases, replacing older high Ft epitaxial types (with Ft up to 15MHz or so) with MJ types from ONsemi will work just fine with no extra mods, and offer a far more robust output stage. Motorola/ONsemi have spent a lot of time optimizing these and if one looks closely in real world applications the difference in parameters is not that huge - besides, most japanese amps are quite tolerant of component tolerances, as would be expected if you want to have good production results and few rejects. Exceptions would be the 'special' amps, the TOTL ones which pushed the parts right to the edge, but in them you would mostly find things like gen1 LAPTs, MOSFETs etc. These tend to be very 'involved' designs in other ways and output transistor substitution is rarely the only problem. That being said, the ST parts can work in many of them. As a rule of thumb, one might take the advent of the LAPT as the actual serious turning point in designs, with regards to transistor substitutions - if you get an amp with a LAPT output, then you have to look a bit more deeply into finding suitable replacements. Fortunately, Sanken still make them and in fact continue to develop them so given a suitable source, even this should not be a problem.
Comparing Ft based on transistor substitution tables can often be misleading. Ft depends on current and remember that your amp actually mostly runs around idle, hence must be stable for these conditions.
There are plenty of devices to chose from as substitutes for older TO3 types electrically, but 99% of them are in a TO3P/MT100 or MT200 case. TO3P can be adapted to a TO3 intended heatsink in most cases but sometimes it can be an adventure. Many old japanese amps have transistor sockets which are designed to mate to the round TO3 pins, whereas TO3P have a square/rectangular cross-section and are made of softer material. Always check this, because problems can and do happen in some cases.
Actual TO3 devices to be used in older amps have unfortunately dwindled to perhaps 3 pairs, two of which are ONsemi and the third is the ST part. Some twilight tech factories like Semelab manufacture the old types but these are quite difficult to source, and can be extremely expensive because they are mostlyused in the medical, indistrial and military field where replacing a part with a different type means a whole lot of certificates have to be made again which costs huge money.
Fortunately, in MOST cases, replacing older high Ft epitaxial types (with Ft up to 15MHz or so) with MJ types from ONsemi will work just fine with no extra mods, and offer a far more robust output stage. Motorola/ONsemi have spent a lot of time optimizing these and if one looks closely in real world applications the difference in parameters is not that huge - besides, most japanese amps are quite tolerant of component tolerances, as would be expected if you want to have good production results and few rejects. Exceptions would be the 'special' amps, the TOTL ones which pushed the parts right to the edge, but in them you would mostly find things like gen1 LAPTs, MOSFETs etc. These tend to be very 'involved' designs in other ways and output transistor substitution is rarely the only problem. That being said, the ST parts can work in many of them. As a rule of thumb, one might take the advent of the LAPT as the actual serious turning point in designs, with regards to transistor substitutions - if you get an amp with a LAPT output, then you have to look a bit more deeply into finding suitable replacements. Fortunately, Sanken still make them and in fact continue to develop them so given a suitable source, even this should not be a problem.
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