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Vintage Silvertone Power Transistors

Started by VintageSilvertone, March 27, 2015, 12:03:01 AM

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VintageSilvertone

I have a vintage 1967 Silvertone 1465 head and cabinet in nearly mint condition with all 6 original speakers.  It is stated to be a 150 watt amp with two channels of 75 watts each.  I've had it since about 1982 and I'm doing some work on the head.  I have the schematic and owners manual, which indicates that there are two main output power transistors mounted on heatsinks and one lower power transistor mounted on the main board.  The transistors are identified using Sears part numbers, and there is no reliable cross reference.  I'm wondering if anyone has replaced the power transistors in a Silvertone 1465 before.  The two higher power transistors on the heat sinks are labeled 9158.  From reading other online discussions, I've deducted that these can be replaced by NPN MJ15015 transistors.  However, I'm not sure what the single lower power transistor (labeled 3155) should be replaced with.  I've seen smaller Sears amps that use 3155s as their main output power transistors.  I wonder if the 3155 is equivalent to a 2N3055 NPN transistor.  I'd welcome any suggestions or commentary.

VintageSilvertone

Here are images of the relevant part of the schematic as well as the two 9158 power transistors and the 3155 lower power transistor.

Roly

#2
Hi VintageSilvertone, welcome.


Output power:

+/-35V rails, 5.33 ohms load

35/1.414 = 24.75VRMS

P = E2/R

(24.75^2) / 5.33 = 114.93watts RMS.

So yeah, "150" is peak power, it's a 100 watt amp.


All these devices have to do is meet or exceed the specs of the transistor they are replacing; maximum voltage (35*2=70), maximum current (35/5.33=6.57amps) and maximum power (about the same as the output power, split between two devices, 115/2=57.5watts).

As for the driver, as long as it will physically fit I'd try another MJ15015 (which is generally slightly superior to the 2N3055).

MJ15015
IC(A) = 15, VCBO(V) = 100, VCEO(V) = 60, PD(W) = 115, Package = TO-3, HFE(Min/Max) = 20/70



HTH


ed to correct
If you say theory and practice don't agree you haven't applied enough theory.

phatt

But what is wrong with the amplifier? Headscratch??
Phil.

DrGonz78

#4
The 3155 is a PNP type germanium transistor. I had worked on a 1464 Silvertone and replaced the 3155's with MJ2955 transistors. The difference here is that you are replacing a germanium with a silicon type transistor and there is bound to be biasing issues. It seems that 3155 was a Sears part number but I am not 100% on that either. Attached is a data sheet that I found while working on the 1464 amp and it shows 2N3155 as a Germanium PNP transistor. To me it seems that there really was a 2N3155 and it was not just a Sears part house number. Just my take on it...

Also don't replace the 3155 with a MJ15015 NPN, but rather the PNP compliment MJ15016. Or use the MJ2955 PNP transistor which is the one I used. 

Edit: Also note that I have attached the 1423 Silvertone schematic that uses 3155's on the outputs. Enzo was the one that pointed out this schematic when I was working on my amp. Note that they are PNP transistors and are not 2N3055's.
"A person who never made a mistake never tried anything new." -Albert Einstein

J M Fahey

Fully agree, noticing that Q7 emitter goes to positive, and Q6 collector goes to negative, confirming PNP  :)

Biasing must be increased for silicon, but kept slightly underbiased, because there is no thermal compensation nor a sensible way to add it.

VintageSilvertone

Thanks all for the detailed and thoughful replies.  I am doing a few improvements to the head because I've had it in storage for several years and my son wants to start playing through it.  The head seems fine, but I want to put a grounded plug on it and I'd like to swap out the speaker wires exiting the head with a 1/4 pin plug.  As long as I had it apart, I thought I might replace pluggable parts such as the transistors and the large capacitors.  However, I may leave the germanium 3155 transistor alone and just replace the 9158s with a matching pair of MJ15015s

In any event, I really appreciate the comments about the 3155.  I would never have guessed that it was a germanium PNP.  I assumed germanium would have been too expensive for a Silvertone.  As you can probably tell, I have basic electronics skills and don't know solid state amps well.  I didn't realize that the replacement transistor simply needs to exceed the specs of the original.  Very important to know.  I always found the old Silvertone to deliver way below its stated output.  I have a 50 watt Laney tube amp from the early 80s that drives the 6 Jensens in the Silvertone cab way harder than the solid state 1465 head.     


Roly

{oops.   :-[ }

Quote from: phatt on March 27, 2015, 04:25:17 AM
But what is wrong with the amplifier? Headscratch??
Phil.

Quote from: VintageSilvertone on March 27, 2015, 11:31:36 AM
As long as I had it apart, I thought I might replace pluggable parts such as the transistors and the large capacitors.

Nooooo.  If it ain't busted, don't fix it!  It is easily possible to do more harm than good (e.g. fitting Si NPN transistors to a working Ge PNP amp   :-[ , making it "not working", and maybe terminally damaging it in the process).

If any amp has been in storage for a long time then there is very good reasons to be suspicious of the power supply electrolytics, and bring it up initially on a Limiting Lamp, then replace them if they shows signs of being faulty - and that goes for all components.  If they look okay and test okay then leave them alone.  (one exception to this is waxed paper caps in very old valve gear, but that's another story)

There are an awful lot of guitar amps that don't make the watts claimed, and frankly screwing 100WRMS out of a pair of old Germaniums seems like a bit of an ask to me.  You certainly wouldn't get it from a pair of 2N3055's in a similar circuit; in practical terms they run out about the 50 watt/8 ohm level (and don't like 4 ohms much).


If you say theory and practice don't agree you haven't applied enough theory.

VintageSilvertone

I put her back together tonight and powered her up without any issues, so I may just leave well enough alone.  The only thing I noticed was a fairly prominent hum when the volumes were all the way off.  Not sure what that's from, but I could probably live with it.  The amp has really nice tone.  She still has all the original transistors and caps, which all seem in fair shape.  I still think it would be safe to replace the 9158s with a pair of MJ15015s, as they are not germanium.  I plan to leave the 3155 alone.  Still, I've not heard that transistor issues cause hum.  I've heard that it's usually the capacitors getting old. 

Enzo

Agree.  When the amp works, stop fixing it.  As was said above, 40-50 year old amps probably could do with some new electrolytic filter caps, but there is absolutely no reason to remove and replace perfectly good transistors.

New output transistors will not increase the power or anything, power comes from the power supply.

The two transistors are the output transistors.  The lone third one is the driver.


yes, hum is likely from weak power supply filter caps.

Roly

Quote from: VintageSilvertoneI still think it would be safe to replace the 9158s with a pair of MJ15015s, as they are not germanium.

Perhaps, but only if they have failed.  Otherwise leave well enough alone.


Quote from: VintageSilvertoneI noticed was a fairly prominent hum when the volumes were all the way off

Ah.  Now you have a symptom that does point to the power supply electros and is not surprising in a vintage amp.

On the bench I'd pull out a couple of chunky electros and tack them across the existing ones just to prove the point - there should be a notable improvement.  Or you could just replace them on spec, they aren't terribly expensive, and it's certainly not a bad idea given the age of the originals.
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

Or we could MEASURE the ripple on the power supplies to verify the problem is there.

Nothing wrong with paralleling fresh caps, of course, just in my shop the scope and meter are closer than my caps collection.