substitute for RCA36892?

[Sowhat]

I have a Kustom 150 with a blown output transistor, an RCA36892 and I was wondering if I could sub this with a 2N3716. Rummaging through my parts I found a bag of these and although the NTE sub for the RCA36892 is an NTE130, and I may have a few of those in my parts bin somewhere, I don't trust them. I have the 2N3716's right here in my hand and they look like overkill for this but they still should do the job, shouldn't they?

[R.G.]

I would be very cautious about that. The Kustom 150 schemo I looked up has about +/-40V power supplies, the 2N3716 is rated for 60V and 80V BVceo. It wasn't known at the time of the Kustom 150s, but high BVceo lets you get larger safe operating area in bipolar output transistors. The old practice was to choose outputs with Vceo of equal to the whole power supply. Today, you can do much better at a good price. You'd be much better off with some of the newer devices, which run about $3.00-$4.00 each for a 160V to 200V device rated at 15-16A and 150W, with much extended safe operating area.

You might get lucky. But I never trust my luck in musical amps.

[Enzo]

The Kustom 36892, also part number 007-0000-000, is pretty much just a 2N3055. That is a 15 amp 115 watt transistor.

The 2N3716 is a 10A 150W part. An amp like this is all about the current, and i am not sure how close we would be pushing its SOA.

You have A bad one? If you are changing types, you really ought to change its mate too. Have them the same type.

An MJ15015 might be a better swap - more robust too. Even some MJ15003 should work well.


Asteroids arcade video games from the early 1980s used the 2N3716 (and 2N3792) for deflection in the video monitors. I used to carry them in my tool kit for field repairs to same.

[Sowhat]

Thanks Enzo, Those 2N3716's were probably in my parts bin because I have done my share of videogame and monitor fixin as well. Seems that business has dropped off sharply in recent years although I still have a bunch of vector and raster monitors laying around here plus lots of parts. I believe I have a bag of 3055's somewhere as well but I just ran out of MJ15003/4's... guess I need to do some more digging. To have all my parts in nice orderly bins is impossible, just too much junk! It's all in bags and boxes all over the place, what a mess.

[Gtr_tech]

Asteroids arcade video games from the early 1980s used the 2N3716 (and 2N3792) for deflection in the video monitors. I used to carry them in my tool kit for field repairs to same.

The color vector monitor in Tempest used to blow the shit outta those pairs. I bulletproofed my Tempest by adding larger finned heatsinks and a fan for each one. Along with the later software revision that limited the offscreen deflection from the board, it ran flawlessly until the high voltage transformer started arcing.

[Enzo]

Ah, the glory days...

[Sowhat]

I have a habit of communicating with my customers maybe too much and in too much detail as I repair their equipment but I feel this puts them at ease with me thrashing through their precious collectors item. Here's a snippet of an email chain I have going with the owner of this Kustom 150 who sent it to me in California from his place in Chicago...

Hi Paul,

The output transistors are the last part before the speakers and driver transistors are the current booster before the output transistors. These things usually blow along with the output transistors because they are in the high current loop of the amplifier. Emitter resistors usually burn out as well but I have not checked those yet so well see. Those are the big rectangular tan colored resistors just below the driver transistors and have a very low resistance like 0.5 ohms.

This is a typical transistor cascade failure which I saw hundreds of back when I repaired automotive 8 tracks for Stereo City. This failure was so common with those because the output transistors were on a heatsink running inside the case just opposite the 8 tracks mounting screw holes. People would use the wrong screws to mount their tape players in their cars and run longer than required mounting screw right through the case of the output transistors. Granted it was a big design flaw but It kept the repair centers humming for a while.

A short circuit on the output is the only real cause for this type of failure. The amp is designed to run fine at full volume for an eternity and back in the 60's the physics of this type of circuit was not as well understood as it is today and the manufacturers used parts that were much stronger than really needed to assure the amp would not blow even in the most extreme situations... of course theres no protection from a short circuit with any grade of electronic device short of a good super fast acting fuse which would probably false trip with these old circuits. My Parsons Mark II has two fast acting fuses (one for the +V rail and one for the -V rail) in it for this very purpose but then again it's design is much more modern and high tech. The Mark II is short circuit protected and is one of the things I like about it's design among lots of other features.

[R.G.]

The Kustom 36892, also part number 007-0000-000, is pretty much just a 2N3055. That is a 15 amp 115 watt transistor. The 2N3716 is a 10A 150W part. An amp like this is all about the current, and i am not sure how close we would be pushing its SOA.

The 2N3055 was - deservedly - the workhorse of the era. But SOA was not well understood. Second breakdown SOA limitations happen when a bipolar device has to support currents at high voltages. The power and current can be surprisingly small if the voltage is near the BVceo of the device. It bears not much relationship to the nominal maximum power and current of the device. It's related more to the fine geometry of the collector/base/emitter junctions, and arises when one spot has microscopically more current/power than its neighbors and goes into small-spot thermal runaway; at least that's what I remember from long-ago semiconductor physics.

That's why today's better high power audio outputs have Bvceo in the range of 160-250V. It runs the SOA up back in the range where the device is really being used. As a rule of thumb, you want your output devices to have twice the BVceo as the combined power supply it withstands. What is associated with that high Vceo (but not necessarily a direct result of it) is a much higher current capability at about half the BVceo without going into second breakdown runaway.

You have A bad one? If you are changing types, you really ought to change its mate too. Have them the same type.

+ many times. Don't replace only one.

An MJ15015 might be a better swap - more robust too. Even some MJ15003 should work well.

+ many times!

[Sowhat]

Wow! thanks for all the info guys... I feel like I'm back in college with all this talk about safe operating areas and as such. I have fixed it using a pair of MJ15003's which I though that I had run out of but when digging around I found a few more... as I said, my parts bins are a mess. I also replaced the drivers because one of them was toast as well. I used the original heatsinks for the drivers by pressing out the old transistors and then carefully reaming out the heatsink hole for a press fit for the new transistors... a technique I picked up somewhere along the way which works amazingly well and keeps things looking as original as possible.