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Old 8th February 2010, 06:48 AM   #11
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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.
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Old 8th February 2010, 07:11 AM   #12
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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.
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Old 8th February 2010, 07:40 AM   #13
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by wrenchone View Post
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,
Updating an old Rotel RA820A to use HEXFETS's amongst other mods :)

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.
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Old 4th January 2013, 05:13 AM   #14
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Quote:
Originally Posted by tiefbassuebertr View Post
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!
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Old 4th January 2013, 06:22 AM   #15
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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

Quote:
Originally Posted by Karl vd Berg View Post
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!
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Old 4th January 2013, 06:43 AM   #16
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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.
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Last edited by Ian Finch; 4th January 2013 at 06:54 AM.
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Old 4th January 2013, 07:17 AM   #17
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Yes, the TO-3s are Sanken inside my AU-317. They are rated 15Mhz of Ft

The options you guys advice are great, but they won't fit in the heat sink, unless under a strange and maybe difficult modify...

So, a higher Ft does not mean anything sometimes?

Thanks
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Old 4th January 2013, 07:58 AM   #18
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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.
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Old 4th January 2013, 08:40 AM   #19
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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.
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Old 4th January 2013, 09:41 AM   #20
ilimzn is offline ilimzn  Croatia
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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.
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