Some questions about transistor replacement

Alienz

Super Member
Helllo,

I have some questions about transistor replacement. I did found some info on AK but just want to make sure I understand it correctly

-can a assume that trans. in an amp build around 1974 are not available anymore and its gonna be replacements no matter what? is it better to stick with the original or is it like with caps, the modern substitions are better quality?

-if i do find the original ones do i have change them on both channels? if i used substitions that would make sense to me but its not clear to me in case i do find some originals what i should do

-do stores look for substitions themselves if you ask them?

-how do you go about finding substitutions in general? i found some on the net for mine in a 'japanes transistor substitution handbook', i think it was from 1993? is this a good reference?

-i read something about 'pairing', how do you find out if the substitutions match up as a pair?

thanks,

Gerard
 
Can't comment on your other questions, but transistor "matching" is matching the HFE reading of a transistor. There's multimeters that measure this. A pretty cheap one can be had at harbor freight.
 
I'll take a couple of questions...

Some of the old transistor types are still available. Don't automatically assume that you must find substitutions.

Most solid-state audio gear is designed with high levels of negative feedback around every stage, so that transistor changes have near zero effect on absolute performance or channel balance. Keep in mind that huge production tolerances exist even within the original type number. There are exceptions, such as phono preamps and intermediate voltage gain stages operating very near the device limits, and power amps where faster transistors can cause violent oscillation. It's always wise to consult the schematic and learn a little about circuit design in order to predict cases where performance change is likely. It's even better to use test gear, so these problem cannot hide from you!
 
Dont know about Netherlands but in the US we have several lines of substitute transistors available from electronics dealers. I dont know if they even deal with European transistors equivalents though. Replacing the bad one would probably result in having to re adjust DC offset and maybe bias. And then listen to see if it sounds OK. Otherwise old or obsolete transistors can be found on ebay sometimes. Or salvaged from old equipment that might be identical to yours. Only problem I have seen with that is some notices of counterfeit or fake parts coming from China. I find China risky to deal with on ebay. THey are faking just about everything over there so you might get the right thing or not. Sometimes comparing the item to the photos supplied can tip you off.
 
Before you go hunting substitutes, a good understanding about what the device does and the measurable specifications would be the first step, IMO.
Intro to transistors
If it is a fairly common device, you shouldn't have a real hard time locating it. Yes, a good retailer should be able to advise for a substitution, if necesary. Matching pairs of transistors I leave to someone with more sophisticated equipment. It seems some of the better resellers will buy in quantity and do the matching, to sell those pairs at a small premium.

Do you have a canidate for a transistor transplant?
 
Thanks all :)
I just send an email to the local electronics shop to ask if the original ones are still available, I think it would be good to get an answer for this first. There arent as many retailers here as in US.

I did some research few weeks ago, when I checked the datasheet for one the suggested substitutions given by that japanese substitution handbook I found a lot of differences in specs with the original one which confused me, is this normal?

I'm looking for the
-NEC 2SD218 , book suggests the Toshiba 2SD425
-NEC 2SA649 ---> Sanyo 2SB775
 
Already got the answer in from the store, they dont have them and advised me to check forums on the net about it :)

They are not really specialized in audio parts, there is a store in Amsterdam which is, might give that a try.

But I also tried a spanish site, that one does have the 2sd218 but not the 2sa649, this last one might be really hard to find i suspect. The 2sd218 was also priced at around 16 british pounds, bit more than i expected.

http://www.reparacionlcd.com/transtandar.php

I guess I do have to find some substitutions
 
Have you tried Here

Your suggested equivalent for 2SD218 looks fine (TO3 mounting) - but I don't like the other equivalent which is TO3P (P=Plastic mounting) although the spec's seem OK. I am guessing you will want 2 TO3 types.

If you don't like the cost of these - then you will have to go for modern equivalents which will be MUCH more robust - simply due to the fact that the somewhat older spec originals are just not made any more - they are extinct.
 
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Have you tried Here

Your suggested equivalent for 2SD218 looks fine (TO3 mounting) - but I don't like the other equivalent which is TO3P (P=Plastic mounting) although the spec's seem OK. I am guessing you will want 2 TO3 types.

If you don't like the cost of these - then you will have to go for modern equivalents which will be MUCH more robust - simply due to the fact that the somewhat older spec originals are just not made any more - they are extinct.


Thank you John! I was actually just looking over the printed datasheets of those subs, that second one is indeed plastic, seems not a good idea, some other candidates are also plastic :(

In principle I'd like to keep things that I restore as original as possible but since that doesnt seem possible I have no problem using modern ones. If they are also more robust than thats even better. Now where and how to find those? :) I found those mentioned subs in that handbook but thats from 1983 , pretty outdated by now too I gues.

I ended up on the Littlediode site via that spanish site, they do have the 2sd218 but i end up at their help page when i search for the 2sa649

Thanks again,
Gerard

Edit: by modern ones you mean those plastic ones? That would mean I would have make changes to the circuitboard, heatsink etc? I'm not sure if I would like to do that, that would change a bit too much of the original 'setup' for me. Are those plastic ones getting also less hotter than the TO-3's btw?

And I did find that sub for the 2sd218 for half the price on littlediode afterall. Maybe I have another candidate for the 2sa649, the BD318, around 4 euros, gonna look at those specs tomorrow
 
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Nothing wrong with plastic cased transistors - which can dissipate huge amounts of heat if required when properly mounted. But if they are replacing metal cased TO3's you really only want to be forced to do this.

If you are replacing NPN/PNP complimentary pairs - and can only get one substitute (2SD218) then you need to disregard that one - and start looking for pairs of transistors that are made to work together - this is very often quoted on the data sheets for either transistor. The BD318 will very likely have a compliment - this is the one you should use with it.

I replaced some NLA TO3 transistors (NPN/NPN - a bit different to your circumstance) in my Sansui 1000X with much more modern On-Semi TO3 replacements - they work fine. But it is a bit odd protecting 15A transistors with 2.5A fuses :D - as that is all they need to be :music:
 
Matching high power transistors should be done with the aid of a curve tracer. The schemes you see posted on the Internet for transistor matching are for small signal transistors. The current needed to test power transistors creates heat and that heat will shift parametric measurements.
Today, devices within the same lot are very close in tolerance with standard deviations for critical parameters typically around 1%. Matching transistors in a quasi-complementary output stage is probably unnecessary. It is probably prudent to try to secure matched pairs for a full complementary output.
 
If you are replacing NPN/PNP complimentary pairs - and can only get one substitute (2SD218) then you need to disregard that one - and start looking for pairs of transistors that are made to work together - this is very often quoted on the data sheets for either transistor. The BD318 will very likely have a compliment - this is the one you should use with it.

Thanks John, thats exactly the kind of simple info i need, I hadnt even noticed those datasheets mentioned a complementary trans.! :)

I used this site for the datasheets: http://www.datasheetarchive.com

As I now see it its either gonna be:

2SD218 --> 2SD425, suggested partner: 2SB555 or 2SB556 (last one not in stock atm)

or

2SA649 --> BD318, suggested partner: BD315 or BD316

Pricewise the last option is just a bit cheaper unless i go for the BD315, which is twice the price of the BD316.

I think you already said the 2SD425 would be ok to replace it so I'm kinda leaning towards that option, not sure about the BD318 yet, I still don't know exactly what to look for when comparing specs.

Thanks again, already learned so much again last few days :yes:
 
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If you do chose to use a 2SD425 you should use its compliment as the other one of the pair - using a 2SDxxx and a BDxxx wont work too well.

The giveaway for complimentary transistors is that the spec for the NPN mirrors that of the PNP - they are just about identical - and you have to check the top of the datasheet to see which one you are looking at !

Matching transistors can be a good idea but as kconnor says the devices 'out-of-the-box' are so close you very seldom need to do that. (unless you are a perfectionist - which I try to be :smoke:)
 
With the possible exception of the very latest generation of bipolar power transistors, it is essentially futile to match NPN/PNP pairs for the output stage, even with a curve tracer. If you examine the datasheets closely, you will see that the curves of current gain vs collector current have different shapes. This makes it unlikely that you will find pairs that are truly matched over the entire map. If I was looking to upgrade performance, I would spend my time looking at other aspects of amplifier design.
 
If you do chose to use a 2SD425 you should use its compliment as the other one of the pair - using a 2SDxxx and a BDxxx wont work too well.

The giveaway for complimentary transistors is that the spec for the NPN mirrors that of the PNP - they are just about identical - and you have to check the top of the datasheet to see which one you are looking at !

Matching transistors can be a good idea but as kconnor says the devices 'out-of-the-box' are so close you very seldom need to do that. (unless you are a perfectionist - which I try to be :smoke:)

I found that complementary 2SD555 on the specsheet of the 2SD425. I also printed out the sheet for that 2SD555, it says 'complementary to SD425 en 426'
I also compared all the specs and low and behold, just like you said, they are all exactly the same but negative! :)
 
Just a little recap:
-I needed to replace a 2SD218 and 2SA649

-only the 2SD218 is still available, no To-3's found for the 2SA649. I havent been able to find any other complementary to the 2SD218. I have to look for an alternative pair that is complementary to each other

-a substitute suggested by the japanese transistor substition handbook for the SD218 is the 2SD425

-checked the specsheets of the 2SD425, it says the complementary is the 2SB555 and vice versa

-all specs of these two are identical but one is all negative

-both are available at littlediode, i think im gonna check if i can get them from a dutch shop too

I also suspect i misunderstood the word 'matching', looks like you mean the proces of comparing all the stats and graphs till you find a matching one? I think I thought 'complementary' and 'matching' just meant the same Being dutch doesnt make understanding the technical 'lingo' easier :)
 
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2SD425 and 2SB555 make excellent substitutions for the output devices in a very wide range of Japanese amplifier designs. If I'm not sure of what to use, I will generally try these first (assuming adequate voltage ratings).
 
2SD425 and 2SB555 make excellent substitutions for the output devices in a very wide range of Japanese amplifier designs. If I'm not sure of what to use, I will generally try these first (assuming adequate voltage ratings).

Thanks, thats good to hear! :)
They even seem to be available at a specialized store here in holland, i still have to replace some resistors first.
 
I smell smoke & burnt resistors already.

I'd would assume right off the bat that anyone selling 2SB555's and 2SD425's is selling Chinese counterfeits, as they have been out of production for decades.

Use modern On-Semi MJ21193 & 94 devices purchased from a reliable source, and you will avoid a lot of potential disasters.
 
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