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 Post subject: output transistor replacements
PostPosted: Jul Wed 08, 2020 12:47 am 
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I'm trying to find out what can replace these blown output transistors ,No other physical marking other than 630 032 and 630 033.

It's out of a 1977 Barcus-Berry 1720 / 2X10 Combo Guitar Amp
Schematic here https://www.dropbox.com/s/itmw0mv95mx3tzh/Barcus-Berry%201720.jpg?dl=0

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 Post subject: Re: output transistor replacements
PostPosted: Jul Wed 08, 2020 1:15 am 
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If they're TO-3 devices I'd use a 2N3055 and Mot MJ2955 devices. That's a pretty old design.

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 Post subject: Re: output transistor replacements
PostPosted: Jul Wed 08, 2020 11:46 am 
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What he said, or search for a NTE replacement such as this one:
https://www.nteinc.com/specs/200to299/pdf/nte284.pdf

I think this complimentary pair may be a bit of overkill for this application, but just try and match up specifications. You can do that by searching on the NET website.

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 Post subject: Re: output transistor replacements
PostPosted: Jul Wed 08, 2020 7:41 pm 
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I don't want to scare you off, but this deceptively simple circuit is full of pitfalls. It's DC coupled so the DC gain (Beta) of each transistor sets the bias for the next one. Consequently if you install transistors having different gains than the originals, the bias will be off. Another issue would be the fT or gain-bandwidth product of the original transistors. It was probably pretty low when the amp was made, but over the years manufacturers have pushed the numbers up higher and higher. Problem is, if you replace old output transistors that had an fT of around 100-kHz with modern ones which might have an fT of 1.5 MHz, you could end up with a nice AM broadcast transmitter when it breaks into parasitic oscillation. Which it may do for a while until the new transistors overheat and fail.

As a first step, did you test the driver transistor (Q4) and pre-drivers (Q2 and Q3)? It often happens that when the outputs in this kind of amplifier blow, the damage can back up and kill the transistors in the prior stages. Working from speaker terminal back, when you get to the first good transistor or the input jack of the amplifier, whichever comes first, is when you can stop testing. To replace the outputs there is no easy way around it; you will need to find a pair of TO-3 transistors, one PNP and one NPN, with relatively low to medium beta and fT, and which match on Beta reasonably well.

Keep in mind that on most spec sheets, beta is given as a minimum value; the transistors you actually get could have considerably more and that is not necessarily a good thing. Do you have access to a curve analyzer or at least a transistor tester that measures DC current gain to match the transistors with? Is there anything in the service literature about how the bias control and the pot from base to ground of Q2 should be adjusted? My guess is that with no signal input to the amp, the bias control would be set for equal voltage drops across each emitter resistor and the other control (if it needs to be set) would be adjusted for a specified output level with a known input voltage.

(Note added later: Per Ted's post below, I agree TR-1 is probably to be adjusted for 0 volts DC offset on the speaker line, but that is in lieu of any better instructions from the manufacturer.)

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Last edited by Chris108 on Jul Thu 09, 2020 12:46 am, edited 2 times in total.

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 Post subject: Re: output transistor replacements
PostPosted: Jul Wed 08, 2020 11:54 pm 
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Attachment:
Barcus-Berry 1720.jpg
Barcus-Berry 1720.jpg [ 535.89 KiB | Viewed 363 times ]


Q5 and Q6 must be Darlingtons. Simple transistors such as the 2N3055 & MJE2955 will not work.

The circuit will provide too much standing base emitter bias voltage and far too little base current unless you use Darlington transistors.

Note the string of four forward biased diodes in the base bias network. And, with a 70V total supply voltage and a 4 ohm load, you are talking 7 - 8 amps peak collector current.

At a bare minimum, Q5 and Q6 should be rated for 80V and 10A.

Yes, do check the rest of the transistors. Q4 is most at risk. The rest are not so stressed when the outputs fry.

The selection of TO-3 parts is getting pretty thin, and expensive. It may be worthwhile considering plastic packaged parts instead.

The adjustment controls should be clean and provide a smooth adjustment. An analog ohmmeter can be used to judge the smoothness. The value should not jump around as the control is turned.
TR-1 sets the DC output voltage. This should be about zero volts with no signal.
TR-2 sets the standing current in the output stage. It would be nice to have a spec for this current. Lacking an official number, 50mA is probably safe enough. Too high and things melt. Too low and it sounds bad. But that might be good in this application.

You do want to get the thing all working before connecting the speaker. If the amp misbehaves, the speaker is easily toasted.

Ted


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 Post subject: Re: output transistor replacements
PostPosted: Jul Thu 09, 2020 12:11 am 
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One possible choice would be these ON parts: MJ11015, MJ11016.
The are likely to work and have pretty good ratings.
They run about $15 a pair. You might wanna buy two or three sets.

https://www.onsemi.com/pub/Collateral/MJ11012-D.PDF
Attachment:
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 Post subject: Re: output transistor replacements
PostPosted: Jul Thu 09, 2020 12:15 am 
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I’m not so sure those outputs should be darlingtons. Let me get home and use my glasses lol

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 Post subject: Re: output transistor replacements
PostPosted: Jul Thu 09, 2020 12:41 am 
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Here's a picture of one https://reverb.com/item/20915461-1970-s-barcus-berry-1715-guitar-amp Nameplate power input is 160 watts at 120 volts.

If I were working on it, I'd probably try it out with a pair of ordinary transistors first. If the guess is wrong, it will perform very weakly or do nothing at all. Then Darlington transistors might be the answer. But if you put Darlingtons in and the guess is wrong, there could be a blown fuse, some burned wires and/or other collateral damage.

The advice not to test it on its speaker is sound (pun unavoidable). There is no DC protection circuit here so if anything goes wrong, it'll blow the voice coil of the speaker through the front grille. A load resistor (value is not critical, could be anything from four ohms to 32 ohms) across the output terminals will allow you to measure the output and ensure there is no DC offset or voltage there. Bringing the unit up on a variac is highly recommended, until you know you can trust it on full line voltage. An AC ammeter in series with the amplifier power would also be a really good idea. With no audio applied, the power draw should be very low, perhaps a quarter of an ampere or maybe a little more. But if you see that it's taking one amp or more with no audio, it's a bad sign and you should back the voltage off and find out why. Doing so could save you from blowing up a new set of output transistors.

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 Post subject: Re: output transistor replacements
PostPosted: Jul Thu 09, 2020 2:34 am 
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Have a look at these:
https://www.nteinc.com/specs/200to299/pdf/nte280.pdf

I don't see, after studying this a bit more, that a darlington is indicated. I suppose we are assuming that these are silicon, not germanium, output devices.

Are you SURE the originals were blown? If so, both? or just one? If you have a good one, some information can be found by testing it.

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 Post subject: Re: output transistor replacements
PostPosted: Jul Thu 09, 2020 2:37 am 
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I posted the question for someone from another site an have invited him to follow and participate in the discussion.
You guys are a wealth of knowledge.

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 Post subject: Re: output transistor replacements
PostPosted: Jul Thu 09, 2020 5:55 am 
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Quote:
...I suppose we are assuming that these are silicon, not germanium, output devices...

This is a 1977 design, and "Geraniums" had long since fallen out of favor. Look at the input devices, they're in the 2N5000+ category, most definitely silicon.
============
Having also looked at the schematic for a while, I think the "Darlington camp" may well be correct.

When looking at the PNP output device's available base drive, something on the order of 8 mils, just do the math, its not going to happen with a with a single bipolar device.

Image

I certainly don't think AC gain @ 1kc is going to be an order of magnitude better.

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 Post subject: Re: output transistor replacements
PostPosted: Jul Thu 09, 2020 11:34 am 
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You're making the assumption that this is some kind of earth-moving equipment. It's not. I've never worked on this exact model but I have handled similar. Maybe good for about 20-30 watts RMS continuous output. Roughly the equivalent of a pair of 6L6GCs.

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 Post subject: Re: output transistor replacements
PostPosted: Jul Thu 09, 2020 12:27 pm 
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If they ARE darlingtons, that would explain the 4 diodes from base to base (where we more typically see 2 diodes)

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 Post subject: Re: output transistor replacements
PostPosted: Jul Thu 09, 2020 12:33 pm 
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I've seen the 4 diode setup on lots of that era equipment, without Darlingtons as the outputs. Going one step further, I do not recall seeing the diode across the ouptuts on any Darlington design either. That was my thinking when I mentioned my feeling that they were just POT's. (Plain Old Transistors).

And a big DUH to me regarding the germanium comment. DUH!!.. long gone from equipment by that timeline. Sometimes I say the silliest things....

I'm wondering if we've scared away the OP yet..... :-D

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 Post subject: Re: output transistor replacements
PostPosted: Jul Thu 09, 2020 5:56 pm 
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Depending on whether or not the transistors are completely vaporized or shorted you may be able to measure the base emitter junctions to see if they are darlingtons.

RRM


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 Post subject: Re: output transistor replacements
PostPosted: Jul Fri 10, 2020 8:31 am 
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Greetings to the Forum:

Given the cost of replacement devices, especially in amps with stepped power output and four or more power devices per channel, I can say from experience that such amplifiers are not cost-effective to repair. Especially when you take into account Chris108's well taken points about cascade failure and gain and DC bias considerations. Tell the owner to buy a new amp. He will be money ahead.

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 Post subject: Re: output transistor replacements
PostPosted: Jul Fri 10, 2020 5:25 pm 
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It's like an old radio. You may never be able to sell it and get back enough to cover the parts you put into it, not to speak of the labor.

Probably the electrolytics are about due, and the case is scuffed. Are you sure the speaker didn't get it when the transistors failed?

However, this forum is not really about helping commercial repair shops make business decisions.

Someone may be working on the amp for fun. And if what you want is an old amp, this one can be repaired without too much trouble and expense. At least when compared to a classic car or boat.

You will need the big darlingtons, and perhaps some more parts, as well, to get it going. When working correctly, this amplifier will put out over 100W. (Even more, when stated as W RMS.) I suspect that the power supplies and heat sinks may not allow continuous operation, so best not to leave it running into the dummy load while you go to lunch!

Ted


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 Post subject: Re: output transistor replacements
PostPosted: Jul Fri 10, 2020 8:03 pm 
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According to the USPTO, BBE Sound in California licenses the name Barcus Berry - perhaps they may have the info you need:

BBE Sound Headquarters:
2548 Fender Ave.
Fullerton, CA 92831
Phone: 1 (800) 233-8346, (714) 897-6766
Fax: (714) 895-6728

http://www.bbesound.com/

EDIT: Service dept: service@bbesound.com

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