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Matched output transistors: does gain change with age?
Matched output transistors: does gain change with age?
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Old 14th March 2019, 02:33 PM   #1
Mayank is offline Mayank  United States
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Default Matched output transistors: does gain change with age?

Hi:

I am restoring a 30+ year old Son of Ampzilla. The amplifier was still working/passed DBT check/reasonable DC offset prior to my decision to do a complete recap and replace meter lamps with LEDs. I know that the matched input differential pairs often drift apart with passage of time, but was surprised to find a significant variation in gain in the output pairs:

Channel A

-NPN- -NPN- -PNP- -PNP-
30 73 95 72

Channel B

-NPN- -NPN- -PNP- -PNP-
51 128 167 79

Yes, the transistors have original GAS 118/119 numbers; there was no sign of previous repair/replacement.

So question: is it normal to see such large variation in gain (the factory obviously had hand matched them 30+ years ago)?

Rgds
Mayank

Last edited by Mayank; 14th March 2019 at 03:13 PM.
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Old 14th March 2019, 03:10 PM   #2
anatech is offline anatech  Canada
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Matched output transistors: does gain change with age?
Hi Mayank,
No, those were not hand matched, or not matched properly if they were matched. You will tend to see changes if that output stage failed and only the shorted or open transistors were changed.

New transistors have better performance, including being able to hold their beta up at higher currents. When I restore an old amplifier of mine, I do install a set of fresh output transistors that are hand matched. That's even if I ever get my dream amplifier - a Marantz 500 and all it's output transistors!

New transistors generally have less spread on their beta values, so it is easier to get matched sets while buying fewer over all to achieve that goal. All good things for me and you. I would install a matched set of outputs and try to match the complimentary driver transistors as well. The input diff pair is critical to match, and a jig is needed in order to get the really tight matches that really improve performance. Another pain in the rear end, but well worth the effort.

-Chris
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Old 14th March 2019, 03:11 PM   #3
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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There is no element in the Earth that support the ageing without consequences. Why transistors will?
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Old 14th March 2019, 04:22 PM   #4
mchambin is offline mchambin  France
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I am curious to know what is the actual ( measured ) benefit of pairing output transistors.
Are these amplifiers low feedback loop gain ?
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Old 15th March 2019, 05:56 PM   #5
Mayank is offline Mayank  United States
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Thank you for your comments Chris!

Quote:
Originally Posted by anatech View Post
No, those were not hand matched, or not matched properly if they were matched.
The transistors did have the tell-tale red number markings which had led me to presume they might have been, but perhaps they weren't matched properly or have just drifted apart.

Quote:
Originally Posted by anatech View Post
You will tend to see changes if that output stage failed and only the shorted or open transistors were changed.
Agree. I have found this to be in several "serviced & working" amplifiers with one/two outputs replaced.

I have already replaced the VAS and drivers with newer OnSemi devices. I will be replacing the outputs as well once I receive my package from Mouser.

Good luck on your hunt for the Marantz 500 !!

Rgds
Mayank
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Old 15th March 2019, 06:01 PM   #6
Mayank is offline Mayank  United States
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Quote:
Originally Posted by mchambin View Post
I am curious to know what is the actual ( measured ) benefit of pairing output transistors.
Are these amplifiers low feedback loop gain ?
I will defer technical clarification on this to Chris and other "Yoda's" here. I've always replaced all the output transistors with similar gain whenever I find 1 or 2 bad in the bank.

Attached is the schematic of the outputs of this GAS Ampzilla Son amplifier.

Rgds
Mayank
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File Type: jpg Output Schematic.jpg (417.3 KB, 293 views)
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Old 15th March 2019, 06:35 PM   #7
grimberg is offline grimberg  United States
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Quote:
Originally Posted by anatech View Post
... That's even if I ever get my dream amplifier - a Marantz 500 ...
Chris,

In the 70s and 80s I owned various Marantz amplifiers, including the model 250. I have never seen or listened to the model 500. What is so especial about it? Is its schematic available?
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Old 15th March 2019, 08:11 PM   #8
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by Mayank View Post
was surprised to find a significant variation in gain in the output pairs:

Channel A

-NPN- -NPN- -PNP- -PNP-
30 73 95 72

Channel B

-NPN- -NPN- -PNP- -PNP-
51 128 167 79

Yes, the transistors have original GAS 118/119 numbers; there was no sign of previous repair/replacement.
Your very high Hfe numbers make me think you measured at very low current, a few mA , probably a handheld meter or even the one included in a multimeter.
Those current levels are not realistic at all.

To be relevant you should measure power transistors at a current level compatible with what they will deliver to speakers, say 4 to 10A or thereabouts.
Quote:
So question: is it normal to see such large variation in gain (the factory obviously had hand matched them 30+ years ago)?
Variation in gain?
Quite possible, but to be certain make a jig to measure them at high current.

Hand matched? ... maybe, but leave "obvious" out, you are just assuming it

I bet there was a reasonably wide window of tolerance, but in any case the great equalizer is the individual emitter resistor.

In any case no significant variation or degradation with time, what you have there is basically what was fit on Day 1.
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Old 15th March 2019, 09:42 PM   #9
stocktrader200 is offline stocktrader200  Canada
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if Vbe and hfe are linked then matching becomes more critical. The higher beta transistor will drive somewhat more power due to the emitter resistors which mitigate current hogging in one device.
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Old 15th March 2019, 10:26 PM   #10
mchambin is offline mchambin  France
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Quote:
Originally Posted by stocktrader200 View Post
if Vbe and hfe are linked then matching becomes more critical. The higher beta transistor will drive somewhat more power due to the emitter resistors which mitigate current hogging in one device.
I wonder
_what is dominant for load sharing Vbe or Beta ?
_is there a dependance. I mean do we have usualy hight beta with low Vbe or usualy high beta with high Vbe or nothing usual ?
_is 0.2 ohm emitter resistor, high enough to force load sharing safely ?
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