2N3055 alternative in a TO-247 or TO-220 package...?

Thread Starter

bobshbob1

Joined May 2, 2022
5
Greetings,

Hopefully this is the right place for this, and apologies in advance for the noob-ish question... I'm repairing a tube guitar amp (schematic attached) which uses a regulated 12VDC supply to power the preamp tube heaters, some relays, a fan, and some logic/switching stuff that is out of my depth of understanding (although I'm willing to learn).

Anyway, the regulator for this 12V supply is supposedly a 2N3055 in what appears to be a TO-220 case, bolted to the side of the chassis. The collector connects to unregulated ~20VDC, the base is connected to a 13V zener to ground, and the regulated 12V (I guess more like 12.4) is taken from the emitter. This part of the circuit is near the bottom left of the schematic.

So the 2N3055 in this amp is dead, shorted base to emitter.

What I'm having trouble with is finding a replacement in the same form factor. Seems like 2N3055 is only available in a TO-3 case. I found an alternative, TIP3055, which is in a TO-247 case, but I'm not sure if that would be an appropriate substitute...

I'm also wondering why they used such a high-power part (~100 watts, 15A) in what seems like a low-power application (600mA for the heaters, plus a little 12V fan and a couple relays and logic stuff). And if it was indeed overrated for this application, why did they choose it and why might it have failed?

I'm going to go ahead and order a few TIP3055's, but if anyone could give me some reassurance that that's an okay substitute, or suggest a better alternative, it would be very much appreciated. :)

Thanks in advance!
 

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Papabravo

Joined Feb 24, 2006
18,422
At one time they, 2n3055's, were cheap and available. The circuit you are describing is a very poor regulator and using a "beast" of a part was supposed to offer some form of "protection". Obviously, the original designers were misinformed. Chances are that when it went bye-bye, it took other parts with it. Are you sure you want to go down this rabbit hole?
 

Thread Starter

bobshbob1

Joined May 2, 2022
5
At one time they, 2n3055's, were cheap and available. The circuit you are describing is a very poor regulator and using a "beast" of a part was supposed to offer some form of "protection". Obviously, the original designers were misinformed. Chances are that when it went bye-bye, it took other parts with it. Are you sure you want to go down this rabbit hole?
Hmmm...I suspected it might not have been the best way to get a +12V supply going... Leave it to modern designers to over-complicate a freaking tube guitar amp that doesn't even have any onboard effects besides reverb. Perhaps the part was overrated to handle the cold-start current of the tube filaments...?

I guess it depends on which rabbit hole you're referring to - finding a replacement part? Or figuring out what else got taken out when the transistor failed?

Re: the second rabbit hole, the transformer winding has 2x 5-amp fuses, which are both intact, and all the loads coming off of that supply are on separate boards, with a separate connector for the +12 on each one, so if there is a short downstream at least it won't be hard to isolate...
 

LowQCab

Joined Nov 6, 2012
2,050
The part is having to dissipate ~8V X 600ma = 4.8-Watts.
The Transistor package by it's self, can dissipate maybe 1-Watt, and survive,
the other 3.8-Watts has to be removed by way of a proper Heat-Sink.

A Stamped-Steel-Chassis makes a poor Heat-Sink,
especially with other hot parts near by, and no Air-movement.

Cheap Heat-Sink-Goop gets hard and shrinks with age,
and deteriorates faster with increased temps,
rendering it useless at transferring Heat.
Doohh !!! there it is ........ the infamous "Blue-Smoke" escaping !!!
.
.
.
 

dl324

Joined Mar 30, 2015
14,462
I found an alternative, TIP3055, which is in a TO-247 case, but I'm not sure if that would be an appropriate substitute...
It has a lower maximum power dissipation but should be okay in your case. The benefit is that TO-247 was designed to mount in places designed for TO-3
 

Papabravo

Joined Feb 24, 2006
18,422
Hmmm...I suspected it might not have been the best way to get a +12V supply going... Leave it to modern designers to over-complicate a freaking tube guitar amp that doesn't even have any onboard effects besides reverb. Perhaps the part was overrated to handle the cold-start current of the tube filaments...?

I guess it depends on which rabbit hole you're referring to - finding a replacement part? Or figuring out what else got taken out when the transistor failed?

Re: the second rabbit hole, the transformer winding has 2x 5-amp fuses, which are both intact, and all the loads coming off of that supply are on separate boards, with a separate connector for the +12 on each one, so if there is a short downstream at least it won't be hard to isolate...
Actually both. One rabbit hole is as good as the next.
 

Thread Starter

bobshbob1

Joined May 2, 2022
5
The part is having to dissipate ~8V X 600ma = 4.8-Watts.
The Transistor package by it's self, can dissipate maybe 1-Watt, and survive,
the other 3.8-Watts has to be removed by way of a proper Heat-Sink.

A Stamped-Steel-Chassis makes a poor Heat-Sink,
especially with other hot parts near by, and no Air-movement.

Cheap Heat-Sink-Goop gets hard and shrinks with age,
and deteriorates faster with increased temps,
rendering it useless at transferring Heat.
Doohh !!! there it is ........ the infamous "Blue-Smoke" escaping !!!
.
.
.
Heat sink goop? What's that? :p

Here is a photo of where it was attached - looks like just a piece of clear tape behind the part... perhaps a proper heat sink is in order with the replacement, if it will fit.PXL_20220503_000413283.jpg
 

crutschow

Joined Mar 14, 2008
29,792
You might consider converting to the common LM7812 12V regulator.
It's rated for 1A and has both short circuit and over-temperature protection.
The 1A limit would reduce the surge current when power is first applied to the cold heaters, possibly lengthening the tubes' life.
 
Last edited:

MrSalts

Joined Apr 2, 2020
1,762
Heat sink goop? What's that? :p

Here is a photo of where it was attached - looks like just a piece of clear tape behind the part... perhaps a proper heat sink is in order with the replacement, if it will fit.
Heat sink goop is a thermally conductive paste - usually a talc-filled silicone that is pretty thermally conductive. It is intended to be spread in a very thin layer and be just enough to fill the air gaps when the flat transistor body is set against a flat chassis or heat sink. It is not thermally conductive enough to be a heat sink, it is just way more conductive than air gaps between the metal flats.

as for the "tape" - it is likely a mica sheet. Mica is a mineral that exists in a crystal form that can delaminate in sheets - very thin and even (flat) sheets. The mica is there because it is non-conductive and keeps the tab of the TO-220 package (usually electrically connected to the collector) from touching the heat sink (commonly connected to ground).
 

Thread Starter

bobshbob1

Joined May 2, 2022
5
Thanks for the replies all! I was joking about the heatsink goop, I know what it is, but I did not know about the mica sheet being an alternative...I do have some goop on hand; would that be preferable to the mica sheet? I mean it's probably what I'm going to use because the current mica is no longer intact.

I had considered a 12V regulator too, but that would actually require more changes to the existing board than just dropping in a TIP3055. I'm gonna order a couple TIP3055's and a couple 78S12 regulators (2A version because I'm honestly not sure what the current requirements of this circuit are, other than the heaters) as an alternative.
 

Irving

Joined Jan 30, 2016
2,668
A screw-less TO-247 package with a pressure clip and Kapton tape isolation is significantly better thermally than a TO-220 with mica insulation and thermal paste. There are much better BJT in a TO-247Plus case than the 3055 part.

If the heatsink is grounded then a 3-terminal regulator won't need insulation (tab = centre ground pin) just paste and is probably the best long term option, IMHO.
 

DickCappels

Joined Aug 21, 2008
8,709
Thanks for the replies all! I was joking about the heatsink goop, I know what it is, but I did not know about the mica sheet being an alternative...I do have some goop on hand; would that be preferable to the mica sheet? I mean it's probably what I'm going to use because the current mica is no longer intact.
(Some text removed for clarity)
They are used together. The heatsink compound improves thermal coupling. The sheet of mica is to provide electrical insulation between the transistor and the metal chassis or heatsink.

Instead of mica and heatsink compound sometimes silicon insulation pads are used, they are glass fiber filled so they are tough. What looks like filaments in the insulator in the photo may be glass fibers.
 

crutschow

Joined Mar 14, 2008
29,792
I had considered a 12V regulator too, but that would actually require more changes to the existing board than just dropping in a TIP3055
You just connect the regulator input to the unregulated DC source, the regulator ground to circuit common, and the regulator output to where the 3055 output connected.
 

Irving

Joined Jan 30, 2016
2,668
You just connect the regulator input to the unregulated DC source, the regulator ground to circuit common, and the regulator output to where the 3055 output connected.
Like so... really not that difficult. Some regulators like a 0.1uF - 10uF capacitor across eithe input or output pins if connected via long leads - check data sheet - you can solder one if need be direct to the pins.

1651595407637.png
 
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