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Old 15th January 2002, 02:15 PM   #1
ih is offline ih
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Default TO-3 transistors and high voltage DC

Finally bought all the stuff for my amp today (Rod Elliot P3A).

Got home, and of course, there's something wrong - I've got the wrong PNP transistors (Jaycar's database and catalogues say different things). I was going to use 2N3055/MJ2955, but I've got a pair of 2N3055's, and a pair of MJ15004's.

Now, I expect this is a stupid question, and I expect the answer to be a resounding NO, but can I still use these together? The figures for them aren't *that* dissimilar (mostly power handling), but things like gain would be quite critical, wouldn't they?

Also, how do you connect the rest of the circuit to the transistor case? Do I use a solder lug on one of the mounting holes?

Final question (for now): I'm using +/-35VDC rails. How dangerous is this to come into contact with? I've taken extreme caution when buying parts for the mains side of things to make sure there's absolutely no way anything can go wrong, but 70 volts across the chest still sounds pretty nasty. Should I be worried about brushing the circuit board while the amp is running?

Thanks

Ian
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Old 15th January 2002, 02:55 PM   #2
Geoff is offline Geoff  United Kingdom
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Ian

Your 2N3055/MJ15004 combination will work, though output stage distortion might be a little higher than normal. However, having said that, the 2N3055 and MJ2955 are not a very good complementary match by modern day standards so there may be very little difference. Try the devices you've got. If you don't like the result, change the 2N3055s for MJ15003s or better still buy some MJ21193/MJ21194 pairs (or MJL3281A/MJL1302A if you don't mind changing to flatpack devices).

Yes, use a solder lug under one of the TO3 fixing screws for the collector connection.

+/-35V is not particulary hazardous, just make sure you don't inadvertently put a short (like a screwdriver) across the power supply smoothing capacitors. Otherwise, reasonable care is all that is required.

Geoff
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Old 15th January 2002, 04:27 PM   #3
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under the right set of conditions almost any voltage can prove fatal given enough current but it is generally considered that you need @ least 12Vac or 30Vdc to get a bad shock.... the 70Vdc your using, while not huge is enough to potentially do you some damage so be careful. Having said that you shouldnt have to go to extremes but my rule of thumb is that if it is capable of giving me a bite i should avoid getting bitten and do what is required accordingly... 24V directly across the chest stings.... 70V would not be @ all pleasent.
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Old 15th January 2002, 08:13 PM   #4
tiroth is offline tiroth  United States
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A word of warning...if you do short out one of the rails, your protection fuse will blow, and it will swing to the voltage of the opposing rail, destroying the output device and probably the load. The same thing will happen if you are using test clips to supply the rail voltages and bump one off. My advice is to use extreme care in testing, as I destroyed the outputs of a perfectly good P3A amp this way during testing.

Good luck with the amp!
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Old 16th January 2002, 03:14 AM   #5
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Another question: when soldering to the pins on the TO-3's, are there any guidelines for the cable I should use from there to the amp board? Like, I know they should be short, but would I be better off with thick DC cable, or shielded cable (for the base I guess this would be good), or what?
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Old 16th January 2002, 04:03 AM   #6
bawang is offline bawang  Malaysia
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Ian, why not use the MJ15003/4 combination? Just exchange the wrong set for a pair of MJ15003s. You can use smaller cables for the base connections, but heavier cable are needed for the emitter and collector connections. Keep them under 6 inches in length. Don't have to use shielded cables, ordinary cables will do as long as the insulation is good enough. At + - 35V, this means (almost) any cable. The currents flowing thru the emitter/collector cables will be less than 5 amps.
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Old 16th January 2002, 04:24 AM   #7
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Most places wont let you simply exchange the items because you can damage semi's with no external evidence of the fact....
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Old 16th January 2002, 12:36 PM   #8
swede is offline swede  Switzerland
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Hi there,

Where is the best place to buy different transistors.

The MJ21193 and MJ21194 are about $8 a piece where I easily can get hold of them.

Are there any good discussions on the specs and internals of different power transistors? (i.g why does Nelson Pass use this kind of transistors and why does Mr. Leach use that?)

Is there any disadvantages in using "flatpack devices" vs. TO3?

Well, thats my thoughts for now,
//magnus
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