EARTH super bass producer series output transistor ?

[Tilman]

Hi,

I try to fix this old bass amplifier and I found several diodes and transistors blown.
It has 8!! NPN output transistors labeled SENT 172. Google does not give me anything with this number.
The rail voltage is something like +-40V and it says it can handle 200W @ 2 ohms.
The transistor tester says something like hfe 46 for the good ones.

Does anyone know the specs of that transistor or even a suitable replacement for it?
A schematic would be the crown....but I guess it would be to asking for a miracle...

Thank you!
Tilman

[Steve Conner]

Try the MJ15003 or even the 2N3055 if you can still get them.

[Tilman]

Thanks Steve!

Would you think it is safe to mix new/replacement transistors with the old ones that measure still good ?
Or should I replace all of them ? (only two out of eight are dead).

Tilman

[drewl]

Do yourself a favor, replace them all with newer robust ones.
The main reason being matched ones will have even current draw, preventing premature failure.

Make sure you check the emitter resistors where the two bad transistors were....check them all.

[Steve Conner]

If only two are dead, I'd first get the amp working on the remaining ones. It should work fine with a few missing, but stick to a 4 ohm load.

Once it seems to be working OK, the rule is that you shouldn't mix different devices in parallel on the same side of the output stage.

[Tilman]

Yes, one 0,33ohms 10W emitter resister is dead too.
I will follow your advice and get 8 new transistors although I wont be able to actually match them.

Thanks!
Tilman

[Steve Conner]

In my experience, if you order the transistors all at once from a big distributor, you're likely to get them from the same batch and they will be well enough matched.

Anyway, 0.33 ohms is a generous emitter resistor that will take care of any slight mismatch.

[teemuk]

The amp is probably a Xerox copy of a similar Peavey amp (Musician Bass Mk III?) so I'd look what Peavey used and go with that. As said earlier, probably a pair of some very generic power transistors was used. If possible replace them with an even more robust type.

[Enzo]

No one expects you to find 8 matched of some particular transistor in te4rms of gain and such. What we want is that they all match in terms of type. In other words 8 of MJ15003 or 8 of 2N3055.

Agreed, the power amp is probably a copy of a Peavey design. And I bet with a pencil and paper you could probably draw out the schematic from the board.

[R.G.]

Yes to the advice so far, except that the 2N3055 does not in general have enough voltage rating to live long and prosper in a +/- 40V supply environment. Do yourself a favor and get the more modern high-voltage, high-SOA parts. They're really not all that expensive, about $3.00 - $5.00 each.

[Tilman]

Hi,

I bought 8 MJ15003s installed them and replaced some other defective parts and burned away PCB copper traces. The amp does not have fuses in the +/-35V supply so the rectifier diodes together with the PCB copper function as a "fuse" there .
This was before I read R.G.s reply.
To my great surprise the amp is alive and kicking ! It even sounds really good !

There is one strange thing about it : There is NO idle current running through the NPNs. I do not measure a single millivolt across the emitter Rs.
The scope does not show any crossover distortion and as I said - it sounds good and it provides the power it is rated for.

I will leave it as it is now and hope it will run another 40 years.

Thank you all for your help!
Tilman

[J M Fahey]

Don't worry, it's Peavey Magic.
In fact , not that mysterious.
Its output could be labelled "layered Darlington" or something.
The drivers drive a TO3 power transistor pair which on its own already can fully drive a 4 ohm load, or a 2 ohm load at low power, and *they* are properly biased.
When load current demands become higher, the other 6 TO3 power transistors which are connected to the first two emitter resistors kick in, supplying extra current and power dissipation.
At said current levels, you are already way beyond crossover problems.

[RedLemonSonny]

Just bought an not working Earth 440 producer head. checked all the transisors and everything else on the board but everything seemed to be okay.. it only makes a humming noise and doesn't react to plugging in a bass guitar, in fact it doesn't react to anything, it's just humming... hope you guys can help me out.