Replace transistors?

xyrium · 8th July 2006, 03:44 PM

Why is it that every time I read a thread about people modifying their equipment, it's always capacitors and Op Amps. It seems that it would be easier to replace a transistor in a circuit than most Op Amps, though I can understand doing the caps as well. Do transistors really have so little to do with the sound that most people don't care? In my mind, they are the valves/tubes of the solid state amplifier, and I would think they would have a similar impact of replacing tubes in a tube amp...

If they are as significant as I assume, can someone recommend a suitable replacement for a pair of 2SA1553 and 2SC4029 transistors?

Please feel free to simply point me to a link or thread or anything, I just couldn't find anything to answer my initial, and what may be, subjective question.

jaycee · 8th July 2006, 05:04 PM

You can't just switch any old transistors into a power amp design. The stability of the circuit is often critically defined by the speed and characteristics of the output stage. Simply dropping newer, faster transistors into a circuit will more than likely just make it oscillate and blow up.

On top of that, the output stage is only one part of the story. A power amplifiers "quality" is a function of many things, not just the transistors, but how they are used.

The only time it's generally acceptable to add superior transistors is when the superior part is something like max power dissipation eg. putting MJ15003/4 into an amp that used MJ2955/2N3055. This doesn't add anything to the circuit sonically, but improves reliability.

anatech · 8th July 2006, 05:53 PM

Hi xyrium,
Jaycee is right on the money! If you want to see how much a simple power transistor change can affect the circuit stability, check out the Symasym thread.

When servicing and old amp., I am doubly cautious powering it up if the outputs were changed. Oscillation may not occur until a certain load is attached and the amp excited (play music or tones). Even some STK direct replacements (same number, newer manufacture date) can turn into transmitters.

Never mind biasing and other issues that only a few technicians are really good at. Never mind the home experimenter. You are best not to mess with these things.

-Chris

jacco vermeulen · 8th July 2006, 06:40 PM

Quote:

Originally posted by xyrium
always capacitors and Op Amps.

That's a false assumption also.

The Mospec numbers you posted are reasonable fast devices, they probably can be replaced by some types with even higher Ft , but it depends on the circuit and it likely will require other changes. At some point it's easier to start designing them yourself.

xyrium · 8th July 2006, 08:33 PM

Thanks guys. I figured it would be sensitive to the overall circuit design, but I thought there might be superior replacements that have similar reactive characteristics yet better results due to design, materials, etc. However, I believe every word you've said that indicates it's simply not possible without knowing and undertstanding the circuit design.

I bought this amp used, and what concerns me is that one of the four transistors is green and not black like the other three. This has led me to believe that it's been in for repair at one point. I was hoping to install superior matched pairs to see if it made a difference. Myabe I'll simply purchase identical replacements for the current units if I can find them. It doesn't appear many people stock them anymore.

jacco vermeulen · 8th July 2006, 09:07 PM

Use Toshiba 2SA1943/2SC5200

Original Toshiba 2SA1553/2SC4029 will be hard to find and likely more expensive. Matching output devices is usually not done.
Only place i know that does have original 2SA1553/2SC4029 and not Mospec copies here in Europe charges $12 the pair.
Specs of Toshiba 2SA1553/2SC4029 are as good as identical to the 1943/5200 couple. Both 30MHz fast devices, same power, current and max voltage.

xyrium · 8th July 2006, 09:42 PM

Hey, thanks Jacco! I guess if a transistor was replaced at one point, something else may have been done as well, but what can I do, it works for now...

I'll get those just as a little replacement project. I assume I'll mount them to the heatsink first with grease and then solder the leads....

anatech · 9th July 2006, 12:49 PM

Hi Jacco,
Matching isn't done anywhere I don't think. I buy way more than I need and select them by hand. I always get a better repair this way. It is a requirement with some product also.

-Chris

LittleSickFish · 29th July 2008, 02:01 PM

Quote:

Originally posted by jacco vermeulen
Use Toshiba 2SA1943/2SC5200

Original Toshiba 2SA1553/2SC4029 will be hard to find and likely more expensive. Matching output devices is usually not done.
Only place i know that does have original 2SA1553/2SC4029 and not Mospec copies here in Europe charges $12 the pair.
Specs of Toshiba 2SA1553/2SC4029 are as good as identical to the 1943/5200 couple. Both 30MHz fast devices, same power, current and max voltage.

I found datasheets of Toshiba sa1553, there is said that it is 25Mhz with 360pF collector output capacitance. In datasheet of Toshiba sa1943, there is said that it is 30Mhz with 470pF c.o. capacitance.
I need to replace dead 2SA1553/2SC4029´s. As you mentioned, they are very hard to find, but I don´t know if the other type is suitable for this circuit with this different frequency and capacitance. Should I be worried about that?

LittleSickFish · 3rd August 2008, 04:15 PM

Well, I bought one 2SA1943 and one 2SC5200. I have spoken with some electricians and they said that the frequency difference is not problem, but here comes another thing.
In this mono amplifier there are two pairs of power transistors. One pair was dead and the othes pair seems to be OK, but when I unsoldered these good transistors and measured them with diode test on my multimeter, they had slightly lower parameters (Volts) than the new pair I bouhgt. So... Is this difference a problem? I have some experience with changing power tubes in my valve guitar amp, so I know that tubes must be paired or in quads. Is this the same case? Should a quad of transistors have the same values in diode test?
And another question of a beginner: There is a trim pot between transistors on a PCB. In a valve amp this trim pot is usually used for setting bias (which affects THD I think), does the solid state amp have bias setting too?
Thanks for reading and replying. I know, a lot of stupid questions, but I have to start somehow, right?

8th July 2006, 03:44 PM	#1
xyrium diyAudio Member Join Date: Apr 2005 Location: NJ	Replace transistors? Why is it that every time I read a thread about people modifying their equipment, it's always capacitors and Op Amps. It seems that it would be easier to replace a transistor in a circuit than most Op Amps, though I can understand doing the caps as well. Do transistors really have so little to do with the sound that most people don't care? In my mind, they are the valves/tubes of the solid state amplifier, and I would think they would have a similar impact of replacing tubes in a tube amp... If they are as significant as I assume, can someone recommend a suitable replacement for a pair of 2SA1553 and 2SC4029 transistors? Please feel free to simply point me to a link or thread or anything, I just couldn't find anything to answer my initial, and what may be, subjective question.

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8th July 2006, 05:04 PM	#2
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	You can't just switch any old transistors into a power amp design. The stability of the circuit is often critically defined by the speed and characteristics of the output stage. Simply dropping newer, faster transistors into a circuit will more than likely just make it oscillate and blow up. On top of that, the output stage is only one part of the story. A power amplifiers "quality" is a function of many things, not just the transistors, but how they are used. The only time it's generally acceptable to add superior transistors is when the superior part is something like max power dissipation eg. putting MJ15003/4 into an amp that used MJ2955/2N3055. This doesn't add anything to the circuit sonically, but improves reliability.

8th July 2006, 05:53 PM	#3
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi xyrium, Jaycee is right on the money! If you want to see how much a simple power transistor change can affect the circuit stability, check out the Symasym thread. When servicing and old amp., I am doubly cautious powering it up if the outputs were changed. Oscillation may not occur until a certain load is attached and the amp excited (play music or tones). Even some STK direct replacements (same number, newer manufacture date) can turn into transmitters. Never mind biasing and other issues that only a few technicians are really good at. Never mind the home experimenter. You are best not to mess with these things. -Chris

8th July 2006, 08:33 PM	#5
xyrium diyAudio Member Join Date: Apr 2005 Location: NJ	Thanks guys. I figured it would be sensitive to the overall circuit design, but I thought there might be superior replacements that have similar reactive characteristics yet better results due to design, materials, etc. However, I believe every word you've said that indicates it's simply not possible without knowing and undertstanding the circuit design. I bought this amp used, and what concerns me is that one of the four transistors is green and not black like the other three. This has led me to believe that it's been in for repair at one point. I was hoping to install superior matched pairs to see if it made a difference. Myabe I'll simply purchase identical replacements for the current units if I can find them. It doesn't appear many people stock them anymore.

8th July 2006, 09:07 PM	#6
jacco vermeulen diyAudio Member Join Date: Oct 2004 Location: At the sea front, just under Rotterdam	Use Toshiba 2SA1943/2SC5200 Original Toshiba 2SA1553/2SC4029 will be hard to find and likely more expensive. Matching output devices is usually not done. Only place i know that does have original 2SA1553/2SC4029 and not Mospec copies here in Europe charges $12 the pair. Specs of Toshiba 2SA1553/2SC4029 are as good as identical to the 1943/5200 couple. Both 30MHz fast devices, same power, current and max voltage.

8th July 2006, 09:42 PM	#7
xyrium diyAudio Member Join Date: Apr 2005 Location: NJ	Hey, thanks Jacco! I guess if a transistor was replaced at one point, something else may have been done as well, but what can I do, it works for now... I'll get those just as a little replacement project. I assume I'll mount them to the heatsink first with grease and then solder the leads....

9th July 2006, 12:49 PM	#8
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi Jacco, Matching isn't done anywhere I don't think. I buy way more than I need and select them by hand. I always get a better repair this way. It is a requirement with some product also. -Chris

3rd August 2008, 04:15 PM	#10
LittleSickFish diyAudio Member Join Date: Jul 2008	Well, I bought one 2SA1943 and one 2SC5200. I have spoken with some electricians and they said that the frequency difference is not problem, but here comes another thing. In this mono amplifier there are two pairs of power transistors. One pair was dead and the othes pair seems to be OK, but when I unsoldered these good transistors and measured them with diode test on my multimeter, they had slightly lower parameters (Volts) than the new pair I bouhgt. So... Is this difference a problem? I have some experience with changing power tubes in my valve guitar amp, so I know that tubes must be paired or in quads. Is this the same case? Should a quad of transistors have the same values in diode test? And another question of a beginner: There is a trim pot between transistors on a PCB. In a valve amp this trim pot is usually used for setting bias (which affects THD I think), does the solid state amp have bias setting too? Thanks for reading and replying. I know, a lot of stupid questions, but I have to start somehow, right?

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