Any ideas where I can get a 2N6556? - diyAudio
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Old 6th February 2009, 11:09 PM   #1
SQLGuy is offline SQLGuy  United States
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Default Any ideas where I can get a 2N6556?

These are used in the pre-driver section of my amp. They seem to be well and truly obsolete, and most of the suggested equivalents are not very close in specs.

PNP 100V 1A 250MHz 2W 18pF hFE 80 - 300

I am planning to try KSA1220A if I can't find the actual part.

Thanks,
Paul
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Old 7th February 2009, 01:16 AM   #2
llwhtt is offline llwhtt  United States
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Motorola/ON suggests their 2N4919 as a replacement. Mouser has them for $0.76.

Craig
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Old 7th February 2009, 02:06 AM   #3
SQLGuy is offline SQLGuy  United States
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Odd recommendation from them... 60V and only 3MHz.

Thanks, though, but I think this means that On doesn't have a real sub to offer.
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Old 7th February 2009, 05:26 AM   #4
es44 is offline es44  Denmark
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VRT-disk says: BD 420, BD 530 for comparision types.

Best regards
Ebbe
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Old 7th February 2009, 05:38 AM   #5
SQLGuy is offline SQLGuy  United States
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Yes, those are much closer subs, but, they're also obsolete. In fact, one listing I found suggested replacing the BD420 with a 2N6556!

Do you know any good sources for these numbers?

Thanks,
Paul
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Old 7th February 2009, 05:49 AM   #6
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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.
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Old 7th February 2009, 07:25 AM   #7
SQLGuy is offline SQLGuy  United States
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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
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Old 7th February 2009, 07:48 AM   #8
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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.
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Old 7th February 2009, 07:41 PM   #9
SQLGuy is offline SQLGuy  United States
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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
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Old 7th February 2009, 08:22 PM   #10
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Why don't you buy some of both and test - it's not as if the cost is gonna break you. You may end up kissing the 2N6556 happily good-bye, for all I know. Certainly you will at least have devices in place that you can replace in case of mishap, providing you buy up a small hoard.
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