Transistor equivalents

[jbmdisco]

I am rebuilding a Marshall VS100 power amp board after the customer decided to put a rather large fuse in after the original went pop . . . . . . sorry, BANG !!!! and done 'some further' damages to the amp. Have now gotten it down to the amp board and basically replacing all semis on the board but having problems locating 1 transistor type - 2SA872 (I have cross referenced these with the following - 2SA941...942, 2SA970, 2SA1016, 2SA1038 BCX30 2N3930 KT667A9 - none of which my normal suppliers can produce ie Farnell + RS comps)

Any ideas ???

Happy new year to all or 'Bliadhna mhath ur' as some of us kilt wearing savages say

[diydidi]

Order them from UTsource.net

[J M Fahey]

1) please post amp schematic.

2) meanwhile I can guess, given parameters, that they may be input differential pair transistors (they are rated 50mA , 300mW , 90V)

IF so, and considering I'm in Argentina and too cheap to spend U$10 minimum order + U$4 minimum post + estimated 15 days delay, I'd grab a pair (you'll need to replace bothn even if just one is bad, to keep input pair balanced) of my trusty old workhorses : 2N5401 .

Mind you, pinout will not match , so a little leg bending is in order, no big deal.

Since they cost some 50 cents each and I have them at arm's length in 100 piece bags, what's not to like about that?

All this idle morning wanderings, before my coffee heats up; please confirm by posting schematic.

By the way, if they are input pair transistors and since you have less than 45V rails, you may use even easier to get (in Europe) BC556 or BC557 .

Pinout won't match either; no big deal.

[km6xz]

RS Components has the Fairchild KSA992 so try that before trying such a hit and miss outfit like UTSource.

[km6xz]

Yes, post the schematic for those playing along at home. The requirements of the circuit are not likely to be demanding the high collector-emitter rating of the original. The 120v rating plus high beta is not a common T0-92 300mw PNP. In fact TO-92 cases are being phased out since they are only used in maintenance of old equipment and not new designs. The TO-3 is just about gone in most lines except ON Semi. In 10 years common audio transistors will not be available in older styles at all. Hold onto old stocks of transistors since the cork sniffers will be seeking Vintage NOS transistors for that classic organic sound instead of the sterile integrated sound.

[Jazz P Bass]

Schematic attached.

[g1]

They are the diffy pair (T1 & T2 of power amp), and amp supply is somewhere less than +/-50V.

Edit: simulpost, I'll leave the xtra schem here for backup .

[R.G.]

JM is correct - the specific type number is not terribly critical there. Pretty much any (1) modern (2) highish hfe transistor with a BVceo over about 50-60V will work. Matching is only important for worrying about offset. Some usable devices are 2SA992, KSA992, BC556.

If -LT is not more than -50V, you can probably use 2N4250, and 2N5400, 2N5401 and also MPSA05 would probably work as well.

[Dave H]

Hold onto old stocks of transistors since the cork sniffers will be seeking Vintage NOS transistors for that classic organic sound instead of the sterile integrated sound.

Lord give me strength! Please say that is not true.

[J M Fahey]

Lord give me strength! Please say that is not true.

It has already been going on for many years on Germanium transistors, as soon as they started becoming hard to find, then with Hitachi MosFets, so it's the logical next step when TO92 starts disappearing.

FWIW in cranky DIY Forum, where the crop of the crop meet, they agonize on old Hitachi, Sanyo and Telefunken silicon transistors, go figure.

The idea is, for example, to copy the Line input stage of some old Akai or Revox tape recorder, to which some magical properties are assigned.

So the Future is already here, folks.

[Mick Bailey]

FWIW in cranky DIY Forum, where the crop of the crop meet, they agonize on old Hitachi, Sanyo and Telefunken silicon transistors, go figure.

And JFETS now, too. J201 especially.

It isn't all empty though; certainly the old 2N3055s were constructed differently than new ones. RCA used a homotaxial construction with high collector-base capacitance and in some cases a modern version isn't a good swap in a power amp without other changes.

[J M Fahey]

True, but in that case there was a real Technological difference, and transistors were labelled different to reflect that.

When newer, much cheaper, faster but weaker Epitaxial construction ones came to dominate the market, for some time they used the letter H or E to differentiate them ... now there are no more Hometaxials that I'm aware of.

Around 2003 or 2004 I bought a carton (some 300/350 units) of USHA India made 2N3055H (labelled UR 2N3055H) , they lasted a couple years, very robust and still working today under harsh conditions.

When they run off, I bought ST 2N3055 , originals, in bulk cartons straight from the Factory ... they started to die like flies.

When I complained, Distributor told me: "in the old days, competition was based on quality and customers would pay extra for extra quality , transistors would surpass nominal specs by a wide margin ... but in nowadays cutthroat market they match just published specs ... and no more"

Of course, I had bought the old ones between 1$ and 2$ a piece, new ones were 67 cents each, in bulk.

Eventually I had to switch to TIP142/147 which in practice were stronger (imagine a plastic 10A plastic transistor being stronger than a 15A metallic TO3 one ) and now use IRFP240/250 Mosfets.

Tomorrow? .... who knows?