2SA798 opinions

[quaddriver]

I have recently seen a thread where the input pair represented by the 2SA798 dual transistor is a 'shoot on sight' item.

Is that true? I know they CAN go noisy and KSA992s in matched pairs are good replacements, but I have an amp benched right now with perfectly fine 798s as per listening to it, and the thread from late last week has made me wonder if the conventional wisdom has changed?

swap or not? Id rather not if I dont have to....this amp was never blown up and its only crime is being 40 years old (heck I myself was guilty of that, 13 years ago....)

Yes it is being capped and flowed, but nothing was ever blorched in it.

Ps the non adjustable DCO is appx 10mv, well within echos 'dont mess with it' state in his thread...

 

[Eastham]

I've replaced the 2SA798's in two receivers now and heard no difference in noise floor. MY SX-5560 and recently my SA-400 in which I replaced every transistor in the audio path. But there is the chance they may go bad in the near future and you'd have to open 'em back up and replace them. There's a pair in my Mitsubishi and they seem to be fine so far, but I've already made the replacements so I may aswel replace them. Collecting all the "good" ones and keeping them aside, why? I dunno. Suppose they may come in handy for something.

 

[slimecity]

I guess opinions on this may differ. I did the replacement on the input stage of one channel of my M22 pioneer amp - that channel was crackling now and then with DC offset going thru the roof. If figured the other channel would soon follow so I did both channels. I was pretty there was also a sonic improvement but may have been imagining that.....

However if the amp is a certain age with these devices in there and you are looking to future proof in general then I would say odds are that at least one of these devices will give you trouble in the near future unless you shotgun them.

 

[sregor]

IMHO the 2SA798 get a bad rap. I think that when something goes in an amplifier, which causes large DC, the input pair takes a hit in performance, whether it be 2sa798, 2sa720, or whatever. So if an amp isn't running well or had a blown channel, the input pair still works but not up to stuff. They should really be tested with a curve tracer, but it is much simpler to replace them if there is any reason to suspect them. YMMV

 

[zebulon1]

The 992's are exceptional as a replacement for the diff pair. If you have the capability to gain match for amps without a Balance adjustment they are very stable.
I have the same thought when fixing a set for a client or even a flip. I'll think about swapping them out just because I don't want a call back.

 

[markthefixer]

I have recently seen a thread where the input pair represented by the 2SA798 dual transistor is a 'shoot on sight' item.

Is that true? I know they CAN go noisy and KSA992s in matched pairs are good replacements, but I have an amp benched right now with perfectly fine 798s as per listening to it, and the thread from late last week has made me wonder if the conventional wisdom has changed?

swap or not? Id rather not if I dont have to....this amp was never blown up and its only crime is being 40 years old (heck I myself was guilty of that, 13 years ago....)

Yes it is being capped and flowed, but nothing was ever blorched in it.

Ps the non adjustable DCO is appx 10mv, well within echos 'dont mess with it' state in his thread...

Please provide a link to that thread.

 

[quaddriver]

Upon further review, Its a 2016 thread I had missed that got bumped last week, If I did this right:

http://www.audiokarma.org/forums/in...ilure-prone-whatever-and-replacements.731653/

#4 in the list....and it refers to other threads...

 

[c.coyle]

I've found them in Nikko gear and replaced them with KSA992s heatshrinked together. I gotta admit they weren't causing noise or high offset. I did it just because that seems to be the soft consensus, here and on DIY, although I haven't heard any actual evidence of widespread failures. It's cheap and easy to do, so why worry about having to open up the unit again somewhere down the road?

I think we do it because they look different.

 

[tranguru]

I have recently seen a thread where the input pair represented by the 2SA798 dual transistor is a 'shoot on sight' item.

Is that true? I know they CAN go noisy and KSA992s in matched pairs are good replacements, but I have an amp benched right now with perfectly fine 798s as per listening to it, and the thread from late last week has made me wonder if the conventional wisdom has changed?

swap or not? Id rather not if I dont have to....this amp was never blown up and its only crime is being 40 years old (heck I myself was guilty of that, 13 years ago....)

Yes it is being capped and flowed, but nothing was ever blorched in it.

Ps the non adjustable DCO is appx 10mv, well within echos 'dont mess with it' state in his thread...

I have asked that question a few times...

the answer I get very often is replace them.
on the other have though: an amp designer told many years ago that these are "little jewels" because they make the perfect diff amp and not adjustments/matching/... are needed
so the answer might it depends on your particular case - no right or wrong answer.

I bought a dozen of them - for about $4 each.

 

[NesanM]

This is my first input in any forum as my way of thanking this community.
This is an old thread but just in case it is useful to somebody, I had a problem with my Pioneer SA-6700 amplifier which suddenly started to make large crackling noise on the speakers and located the issue to couple of 2SA798 that activated or cut the relay. I replaced them with 4 nos 2N5401 (need to be careful with them as there are apparently 2 versions of the same transistor with different order of pin outs). The Amp is now working fine.

 

[markthefixer]

This is my first input in any forum as my way of thanking this community.
This is an old thread but just in case it is useful to somebody, I had a problem with my Pioneer SA-6700 amplifier which suddenly started to make large crackling noise on the speakers and located the issue to couple of 2SA798 that activated or cut the relay. I replaced them with 4 nos 2N5401 (need to be careful with them as there are apparently 2 versions of the same transistor with different order of pin outs). The Amp is now working fine.

Just curious as to what steered you to the 2n5401 transistors?

 

[Skywatcher]

Just curious as to what steered you to the 2n5401 transistors?

Yeah, they are a pretty "mundane" transistor with a max hFE of around 200. But they would "function" fine, just not optimal. Still better than snap, crackle, and pop followed by silence.

 

[blhagstrom]

I ran into enough noisy ones that they always get the evil eye when I find them.

If I’m doing a rebuild/restore/recap, I seriously think about replacing them.

 

[Skywatcher]

I ran into enough noisy ones that they always get the evil eye when I find them.

If I’m doing a rebuild/restore/recap, I seriously think about replacing them.

I've had too many bite me soon after, so I don't even give a 2nd thought anymore to changing them.

 

[rhodus]

Any opinions on the current 798s for sale now?

 

[Skywatcher]

Any opinions on the current 798s for sale now?

My opinion is there is a 99.99% chance they are counterfeit. But that doesn't necessarily mean they won't "work". There is simply a 0.01% chance you know what they actually are.

 

[rhodus]

My opinion is there is a 99.99% chance they are counterfeit. But that doesn't necessarily mean they won't "work". There is simply a 0.01% chance you know what they actually are.

I wonder if anyone has done any "bench testing" of them if that's possible.

 

[blhagstrom]

Any opinions on the current 798s for sale now?

I wouldn't trust them.

We know how to gain match a pair of KSA992 so why not do what we know works?
Unless you are not set up to gain match KSA992s. I suppose there is a market for someone to sell matched ones for that purpose.

 

[rhodus]

I wouldn't trust them.

We know how to gain match a pair of KSA992 so why not do what we know works?
Unless you are not set up to gain match KSA992s. I suppose there is a market for someone to sell matched ones for that purpose.

What do you think about this homemade tester?

Hi Allan, hFE measurement. Take two meters. One to measure the Base current, another one to measure the Collector current. Use the potentiometer to adjust the Base:Collector current you wish to make the measure. Take the two reading, divide the Ic/Ib = hFE. Take a look at the schematic, Make sure you connect the battery for proper polarity. Here it is design for an NPN transistor. For PNP reverse the 9V battery. If your meters are digital it will show a negative reading. If meters are analog you will need to reverse both meters polarity in order to have a readable value,

 

[Skywatcher]

View attachment 2139829
What do you think about this homemade tester?

Hi Allan, hFE measurement. Take two meters. One to measure the Base current, another one to measure the Collector current. Use the potentiometer to adjust the Base:Collector current you wish to make the measure. Take the two reading, divide the Ic/Ib = hFE. Take a look at the schematic, Make sure you connect the battery for proper polarity. Here it is design for an NPN transistor. For PNP reverse the 9V battery. If your meters are digital it will show a negative reading. If meters are analog you will need to reverse both meters polarity in order to have a readable value,

Then increase the current until they pop. Then you'll know their limit.