Hello,
I am trying to buy lateral mosfets for my next amp projects but since they are obsolote devices i was not succesful. I could probably buy a set of them if i visit enough shops around also with very inflated prices.
Other option is Exicon devices which i would normaly prefer but purchasing them from profusion is not appeling due to high shipping prices.
So the thing is, i came across a used PA amp in local ebay equivalent. It is a beefy amp with 12 pair of Hitachi 2sk135/j50 laterals, a toroid transformer probably 2KVA range and other stuff found in PA amps. Seller is claiming amp is working and in good condition other than cosmetic issues. With a price of approx. 150 usd it attract my attention. But the question is;
Do you trust 30 year old transistors which probably worked in harsh conditions?
I am trying to buy lateral mosfets for my next amp projects but since they are obsolote devices i was not succesful. I could probably buy a set of them if i visit enough shops around also with very inflated prices.
Other option is Exicon devices which i would normaly prefer but purchasing them from profusion is not appeling due to high shipping prices.
So the thing is, i came across a used PA amp in local ebay equivalent. It is a beefy amp with 12 pair of Hitachi 2sk135/j50 laterals, a toroid transformer probably 2KVA range and other stuff found in PA amps. Seller is claiming amp is working and in good condition other than cosmetic issues. With a price of approx. 150 usd it attract my attention. But the question is;
Do you trust 30 year old transistors which probably worked in harsh conditions?
Hitachi 2sk135/j50 laterals are the worst sounding transistors I know, old or new. No matter what others say. I would prefer simple IRFs in a TO-247 package, they sound much better.
There are a lot of ugly sounding Hitachi 2sk135/j50 amps around. There is a simple cause: They were promoted to be voltage driven and many designed the driver stages like that. Which lead to a typical "mosfet" sound.
Anyway, with a good design, this transistors can sound just fine.
Usualy they last forever. If you want to use them in a class A design, this may not be true. Anyway, used or new, they are the same breed. For that price I would consider buying the whole thing. Just don't tell the seller there is gold hidden in his roadside find and you pay that much money, just for the few parts inside.
Anyway, with a good design, this transistors can sound just fine.
Usualy they last forever. If you want to use them in a class A design, this may not be true. Anyway, used or new, they are the same breed. For that price I would consider buying the whole thing. Just don't tell the seller there is gold hidden in his roadside find and you pay that much money, just for the few parts inside.
It is OK to have used transistors but their reliability (or equivalently their remaining useful lifetime) depends on:
One advantage of salvagied components is that they're highly likely to already have crossed the 'infant mortality' stage of the bathtub curve.
https://en.wikipedia.org/wiki/Bathtub_curve
- Their consumed lifetime.
- The stress endured (hitherto) during usage.
- The stress they'd endure in future, while being part of the new equipment.
One advantage of salvagied components is that they're highly likely to already have crossed the 'infant mortality' stage of the bathtub curve.
https://en.wikipedia.org/wiki/Bathtub_curve
Tubes maybe, not transistors. If in working state and not abused in the new setup, transistors will last longer than the average user.
All electronics, no exceptions. Everything is designed to last for sometime not forever. Refer to the MIL Handbook 217 for categories.
https://www.quanterion.com/wp-content/uploads/2014/09/MIL-HDBK-217F.pdf
However, this has become old and a modern reliability standard would be FIDES, rumoured to have been developed by Airbus.
https://www.fides-reliability.org/en/node/9
The power transformer plus the chassis/case/housing alone would probably be worth more than USD 150...
Best regards!
Best regards!
Salvaged from working equipment or salvaging from damaged equipment is what makes the difference. If a piece has a pair of fried output transistors, the ones in the other channel are likely fine. Ones in parallel with or driving the burnt ones are NOT. At least not for anything serious. They may have just barely escaped what caused the failure. And there is no way to know. You could analyze the failure mode and see how far back into the circuit the parts would have been operating within safe regions. Like the VAS could be ok, especially if just boot strapped or if it’s current source is still good.
Agreed, however the above possibilities still fundamentally influence the normal (or accelerated) ageing and thereby the reliability of the transistors.
Overload/overheat burns off a lot of lifetime - quickly. That’s why one fails in the first place - it’s all time dependent. The MTTF curve is just that - median time to failure. There is a distribution. The ones that would have failed first did already. The others are lurking somewhere close if conditions were similar.
Ageing of transistors should be taken with a grain of salt. I have been scavenging electronic equipment for several decades and failed so far to observe a working transistor die of old age before my eyes.
I scavenge too. But too many times I’ve seen someone replace only the one or two outputs that failed out of a group, only to have to fix it again not long after. When I fix something I don’t want to see it back again in a couple of months. “Working” but potentially stressed parts will not get used in a customer‘s amp, nor will they get used anything I’ll be using in the field. On the test bench, prototyping, tinkering, sure. Pulls that came from working equipment might get used in a customer repair, when the choice is between the used part, having to N-rig it with something that was never intended to fit or otherwise inadequate, or scrapping the unit entirely. I may use those parts in one of my PA amps, too. Especially if it was in something I built myself and decommissioned due to building an improved design.
I just cleaned the switches and pots in a 1980-1982 Technics SA-202. I saw evidence that one device - a transistor used for temperature sensing - had been replaced. The rest of the receiver worked just fine. This in spite of containing a couple of custom ICs, circuit modules, and such. I bet that thing will last another 40 years.
Parts aren't designed to fail after a certain amount of time. They're designed to last a minimum amount of time (usually 10 years) when operated at the worst case operating conditions. No reasonable designer would ever operate the part at that extreme, so the parts will usually last much, much longer.
Tom
Parts aren't designed to fail after a certain amount of time. They're designed to last a minimum amount of time (usually 10 years) when operated at the worst case operating conditions. No reasonable designer would ever operate the part at that extreme, so the parts will usually last much, much longer.
Tom
What about transistors that are old, but never used?
E.g. I purchased a batch of germanium transistors made in the 60s, never used.
I expect them to last many many years to come, is that reasonable expectation?
E.g. I purchased a batch of germanium transistors made in the 60s, never used.
I expect them to last many many years to come, is that reasonable expectation?
Thank you for all of the replies. I always thought solid state semiconductors have virtually faultless given they are working within predetermined safe boundaries. But my doubt was based on;
-This amp is a PA amp working close to limits,
-An article or a forum post that i read about corrosion or material degradation in older semiconductors.
But i guess these are not that much of an ancient devices being manufactured in the 80's. So i think its time to bid and buy the amp and check it myself. 🙂
-This amp is a PA amp working close to limits,
-An article or a forum post that i read about corrosion or material degradation in older semiconductors.
But i guess these are not that much of an ancient devices being manufactured in the 80's. So i think its time to bid and buy the amp and check it myself. 🙂
Heat cycling is a cause of wear out.
Non-hermetic plastic cases are prone to humidity creeping up the connection terminals.
TO-3 are hermetically tight for my knowledge.
I do not know whether electron-migration is an issue.
btw, I have a NOS collection of 2SK135/2SJ50 from the 80's and some yrs ago I built my reference mono block with one pair of these.
There have been no issues so far.
Non-hermetic plastic cases are prone to humidity creeping up the connection terminals.
TO-3 are hermetically tight for my knowledge.
I do not know whether electron-migration is an issue.
btw, I have a NOS collection of 2SK135/2SJ50 from the 80's and some yrs ago I built my reference mono block with one pair of these.
There have been no issues so far.