If you don't insist on having a TO-202 package (truly obsolete), you could make do with TO-106 video driver transistors with a bit of metal for heat sinking. The leads would need to be formed differently, but that's not difficult. Search out the threads on VAS transistors and you'll find a slew of suggestions, a lot of which are currently available.
OK. In fact, the leads on the 2N6556's were crossed to fit the PCB layout of the amp to start with. But why would those video drivers be better than the 2SA1220/KSA1220 devices I'm looking at now?
I was hoping to find originals so I didn't have to replace the other five 2N6556's that aren't blown. Which I will do if I have to sub in parts, so that they at least behave the same between channels.
Thanks,
Paul
I was hoping to find originals so I didn't have to replace the other five 2N6556's that aren't blown. Which I will do if I have to sub in parts, so that they at least behave the same between channels.
Thanks,
Paul
You'll have to look at some of the suggestions and judge for yourself. The substitutes are cheap enough so that it is no big thing to replace all the TO202s with TO-126 (correction) video drivers. I've standardized on the KSA1381 and KSC3503, which are 300V, 150 MHz devices available from Mouser for beer change. There are also faster, lower voltage devices available. Check the threads if you are interested.
I looked at the KSA1381. Sounds like a pretty nice device as well, but I don't need the 300Vceo, and I do need the higher Ic (I think).
KSA1220 is 1A, 2N6556 is 1A, KSA1381 is 0.1A.
But in circuit, the resistance on the collector should limit Ic to about .5A, and the Pd of the collector resistor says that it really shouldn't be more than 35mA.
The two things I prefer about the KSA1381, looking at its data sheet, are the specific mention of low THD and the lower Cob.
I must say, though, that I don't really understand the role of Cob. My expecation is that it's tied to Ft to some extent, and would also affect impedance of the transistor at higher frequencies. The drive of these transistors uses 1.4K pull-ups at the base. From 1/2piFc I get a corner frequency of about 600Mhz for 26pF. If I did that correctly, then I don't think the 18pF versus 26pF Cob would be an issue in subbing in the KSA1220.
The phrase used in the KSA1381 data sheet is, "Excellent gain linearity for low THD." DC hFE is similarly linear for both devices (effectively flat from 0 to 20mA for the 1381, and from 0 to 200mA for the 1220). Gain products are harder to compare, because the scales are different owing to the different Ic ratings, and their run at different Vce as well, so I'm not sure how useful a comparison is... FWIW, the curves are the same shape.
So, that's where I am now. Price on the KSA1220A's is about 25% higher than the KSA1381's (a whopping 41 cents a piece). And I can't say whether the closer match of specs makes the KSA1220 a better choice, or the specific mention of THD should push me towards the KSA1381, since I don't seem to actually need the 1A Ic in my amp's circuit. I still wish I could just find another 2N6556 somewhere.
Cheers,
Paul
KSA1220 is 1A, 2N6556 is 1A, KSA1381 is 0.1A.
But in circuit, the resistance on the collector should limit Ic to about .5A, and the Pd of the collector resistor says that it really shouldn't be more than 35mA.
The two things I prefer about the KSA1381, looking at its data sheet, are the specific mention of low THD and the lower Cob.
I must say, though, that I don't really understand the role of Cob. My expecation is that it's tied to Ft to some extent, and would also affect impedance of the transistor at higher frequencies. The drive of these transistors uses 1.4K pull-ups at the base. From 1/2piFc I get a corner frequency of about 600Mhz for 26pF. If I did that correctly, then I don't think the 18pF versus 26pF Cob would be an issue in subbing in the KSA1220.
The phrase used in the KSA1381 data sheet is, "Excellent gain linearity for low THD." DC hFE is similarly linear for both devices (effectively flat from 0 to 20mA for the 1381, and from 0 to 200mA for the 1220). Gain products are harder to compare, because the scales are different owing to the different Ic ratings, and their run at different Vce as well, so I'm not sure how useful a comparison is... FWIW, the curves are the same shape.
So, that's where I am now. Price on the KSA1220A's is about 25% higher than the KSA1381's (a whopping 41 cents a piece). And I can't say whether the closer match of specs makes the KSA1220 a better choice, or the specific mention of THD should push me towards the KSA1381, since I don't seem to actually need the 1A Ic in my amp's circuit. I still wish I could just find another 2N6556 somewhere.
Cheers,
Paul
Hi
Google says they have'm at Cricklewood Electronics
London England
For 9 pounds sterling each "ouch"
Here's the link in case you are independently wealthy
http://www.cricklewoodelectronics.c...php?printable=Y&productid=3070&cat=175&page=3
Good luck
B D
Google says they have'm at Cricklewood Electronics
London England
For 9 pounds sterling each "ouch"
Here's the link in case you are independently wealthy
http://www.cricklewoodelectronics.c...php?printable=Y&productid=3070&cat=175&page=3
Good luck
B D
No, I'm certainly not worried about the cost of the transistors. I am more worried about the cost of other parts (and speakers) that might be damaged during an experimentation run. Also, these PCB's are pretty low quality... they're not through-hole plated, and their traces are thin and tend to come loose from the substrate with mild soldering heat.
Lastly, while I have some decent spectan software and an M-Audio outboard sound card, I don't think I can do any really accurate THD measurements with that. So, experimentation would involve probably switching one channel to KSA1220's, leaving the other channel with the 2N6556's, checking for any obvious differences in behavor (DC offset, noise, obvious distortion of test waveforms, etc), then making some measurements to guestimate Pd and Ic in circuit to make sure the devices were operating within deisgn limits, then a lot of listening to make subjective evalautions (switch left and right and repeat), then switch out the KSA1220's for KSA1381's and repeat the whole suite, then finally, switch in whatever transistors had been selected as the final choice into both channels...
Mostly I just want the amp to work reliably and sound good, and use it. It sounded pretty good with the 2N6556's and they had lasted for 20+ years. I expect what I'll do is to do about half the process listed above: swap KSA1220's into the damaged channel, and, assuming they seem to work as well as the devices in the undamaged channel, then swap out the 2N6556's in the undamaged channel. If the KSA1220's don't seem to work for some reason, then I'll start experimenting with other devices, or perhaps bite the bullet on a $150 minimum order from one of those places that stockpiles obsolete parts.
Paul
Lastly, while I have some decent spectan software and an M-Audio outboard sound card, I don't think I can do any really accurate THD measurements with that. So, experimentation would involve probably switching one channel to KSA1220's, leaving the other channel with the 2N6556's, checking for any obvious differences in behavor (DC offset, noise, obvious distortion of test waveforms, etc), then making some measurements to guestimate Pd and Ic in circuit to make sure the devices were operating within deisgn limits, then a lot of listening to make subjective evalautions (switch left and right and repeat), then switch out the KSA1220's for KSA1381's and repeat the whole suite, then finally, switch in whatever transistors had been selected as the final choice into both channels...
Mostly I just want the amp to work reliably and sound good, and use it. It sounded pretty good with the 2N6556's and they had lasted for 20+ years. I expect what I'll do is to do about half the process listed above: swap KSA1220's into the damaged channel, and, assuming they seem to work as well as the devices in the undamaged channel, then swap out the 2N6556's in the undamaged channel. If the KSA1220's don't seem to work for some reason, then I'll start experimenting with other devices, or perhaps bite the bullet on a $150 minimum order from one of those places that stockpiles obsolete parts.
Paul
beardawg said:
Thanks. I might do that. I only need one, and the pound is pretty low right now versus the dollar. Probably want another 10 pounds for shipping, though!
EDIT: Oops, no, they don't even ship to the US. I have friends in the UK that could handle this by proxy... but what a pain!
Searching further down Google, I found another place in the UK that has them: http://www.dalbani.co.uk. Seeing the Dalbani name, I checked the US company Dalbani, but they don't list 2N6556 at all.
Anyway, the UK Dalbani has them for £3.85, and they do ship worldwide. £5.99 for shipping.
So, I ordered a couple and I should now have all my bases covered.
Cheers,
Paul
Anyway, the UK Dalbani has them for £3.85, and they do ship worldwide. £5.99 for shipping.
So, I ordered a couple and I should now have all my bases covered.
Cheers,
Paul
Hi Paul -
The PNP 2N6556 in TO-202 case indeed seems to be truly difficult to find. I've got the NPN 2N6553, but none of the PNP part.
One near equivalent is the 2SB649 which is ECB but rated at 120V/1.5A/20W in a To-126 case, somehow I don't believe that rating of 20 Watts in a To-126 case, think a misplaced decimal point... I've got some of those.
But the closest equivalent and best choice would seem to be the MPS-U57, which is also obsolete and also unobtainium. However, Central Semiconductor also made this part under using the designation: CEN-U57 which is an EBC in a TO-202 case, and also has a comparable NPN complement, the CEN-U07.
The specs, the CEN-U57 data sheet, says it is: 100V/2A/50Mhz. minimum, 1.75W/30 pF./hFE 80 minimum @ 50 mA. so it is really, really close to the 2N6556, or maybe it really is the MPS-U57, or at least the same die, but in order to use their own number, Central had to change the specs a few percent.
Mouser has them, but as a non-stocked part with a minimum order of 340 pieces at $1.68 !~! each I'm guessing you don't want to spend $580. or so with the shipping for them.
I've got a eleven (11) NOS CEN-U57 if you or anybody wants them for $20.usd which includes shipping in the lower 48 States.
-Steven
The PNP 2N6556 in TO-202 case indeed seems to be truly difficult to find. I've got the NPN 2N6553, but none of the PNP part.
One near equivalent is the 2SB649 which is ECB but rated at 120V/1.5A/20W in a To-126 case, somehow I don't believe that rating of 20 Watts in a To-126 case, think a misplaced decimal point... I've got some of those.
But the closest equivalent and best choice would seem to be the MPS-U57, which is also obsolete and also unobtainium. However, Central Semiconductor also made this part under using the designation: CEN-U57 which is an EBC in a TO-202 case, and also has a comparable NPN complement, the CEN-U07.
The specs, the CEN-U57 data sheet, says it is: 100V/2A/50Mhz. minimum, 1.75W/30 pF./hFE 80 minimum @ 50 mA. so it is really, really close to the 2N6556, or maybe it really is the MPS-U57, or at least the same die, but in order to use their own number, Central had to change the specs a few percent.
Mouser has them, but as a non-stocked part with a minimum order of 340 pieces at $1.68 !~! each I'm guessing you don't want to spend $580. or so with the shipping for them.
I've got a eleven (11) NOS CEN-U57 if you or anybody wants them for $20.usd which includes shipping in the lower 48 States.
-Steven
SQLGuy said:
Well, the good news is that, in initial testing, the KSA1220A's seem to work just fine.
The bad news is that the ones Mouser sent me have really short legs and don't fit too well in my application. Would that be because I ordered tube stock instead of bulk stock? I definitely need the ones with the longer legs to be more comfortable with this installation.
Also, the noise is still there, but that doesn't particularly surprise me. Next thing will be to try swapping out some output devices, one by one, I think, to see whether that's where the noise is coming from.
Cheers,
Paul
The bad news is that the ones Mouser sent me have really short legs and don't fit too well in my application. Would that be because I ordered tube stock instead of bulk stock? I definitely need the ones with the longer legs to be more comfortable with this installation.
Also, the noise is still there, but that doesn't particularly surprise me. Next thing will be to try swapping out some output devices, one by one, I think, to see whether that's where the noise is coming from.
Cheers,
Paul
Further update: Mouser's product manager came back to the customer service person I was working with and told her that, for KSA1220A's, the tube stock is supposed to have really short legs. They were updating their description information to indicate that.
Their suggestion to me was to return the ones I'd ordered for a refund, but the shipping would have cost me about the same as the parts (10 pieces @ 39 cents), so I'm just keeping them and have ordered another set of the bulk stock parts, which have the full-length legs.
Their suggestion to me was to return the ones I'd ordered for a refund, but the shipping would have cost me about the same as the parts (10 pieces @ 39 cents), so I'm just keeping them and have ordered another set of the bulk stock parts, which have the full-length legs.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.