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Ugh - my ampzilla died again
Ugh - my ampzilla died again
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Old 7th December 2017, 01:31 AM   #21
danschy is offline danschy  United States
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Originally Posted by llwhtt View Post
Do you remember what the past failures were? All the same or different? Fuses should have opened if there were bad outputs.

Craig
I've replaced output devices, schottky diodes, driver devices and input devices; I guess most all types of semiconductors have failed on one channel or the other, except for small signal diodes. In my younger years I built and played around with numerous amps and was not always as careful as I should have been. It is possible some of the failed devices were due to my fumble fingering something or other. At this point I don't recall - I can just see what was replaced. Given my intended use, which would be driving "normal" speakers at sane levels, I'm guessing a properly rebuilt Ampzilla would outlast me. I'm still on the fence, but leaning towards some level of fixing/rebuilding, so really appreciate everyone's comments on device selection.
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Old 7th December 2017, 01:55 AM   #22
Bare is offline Bare  Canada
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Time... for a walk to the Trash Bin?
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Old 7th December 2017, 02:00 AM   #23
danschy is offline danschy  United States
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Originally Posted by wg_ski View Post
I would use MJ15022/15023 as my primary output choice. The hFE at 15 amps is higher than the 2119x series...
Per the ON spec sheets, hfe looks better on the MJ21195/96, although spec'd at slightly different Vce:
MJ21195/96: 5V/16A - hfe>8
MJ15022/23: 4V/16A - hfe>5
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Old 7th December 2017, 02:01 AM   #24
danschy is offline danschy  United States
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Time... for a walk to the Trash Bin?
Too heavy! It would break my trash bin and my refuse collector may fine me for excess weight!
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Old 7th December 2017, 02:21 AM   #25
indianajo is offline indianajo  United States
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Anybody running 15 amps through a TO3 transistor in class AB hasn't read the soa tables. At 75 v about 2.5 amps, details may vary. Use the DC soa curve for best service life, Andrew has said I believe. 15 amps is in heavy saturation, which AB amp output transistors do not do.
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Old 7th December 2017, 03:12 AM   #26
danschy is offline danschy  United States
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Originally Posted by indianajo View Post
Anybody running 15 amps through a TO3 transistor in class AB hasn't read the soa tables. At 75 v about 2.5 amps, details may vary. Use the DC soa curve for best service life, Andrew has said I believe. 15 amps is in heavy saturation, which AB amp output transistors do not do.
Interesting. I guess I have not spent enough time looking at SOA curves. Assuming an 8-ohm load, 70V rails (mine are actually 69 and would probably droop a little but I'll ignore that), the voltage is equally spread across the stacked output devices (and I can still do simple math), then ... at an output power of 130W I hit the 1sec SOA curve (Vce=58, I=4). I guess 200W is a little optimistic, like everyone has been saying.
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Old 7th December 2017, 11:42 AM   #27
indianajo is offline indianajo  United States
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Output transistors have more gain now than 1974, so you don't need the high rail voltages to achieve max wattage limited by soa. Then OT's could get output about a third of the rail voltage, now devices can go about half rail voltage. (35v^2)/8=153W, divided by two output pairs, is 76W per pair. sqrt(153/8)=4.3 amps, kind of pushing dc soa at 70 v.
BTW, a 2n5631/6031 datasheet from motorola shows 0.95 A soa at 70v.
A PV-4C is 120 W/ch @ 8 ohms, and has 4 pairs of MJ15024/25. And a 5" fan.
So assuming your temp sense IC's are good (and I don't know how to test them) you could improve reliability of ampzilla by cutting rail voltage. Either regulate, which you don't have heat sinks for, or increase output pairs which you don't have heat sinks for, or a different transformer, which is $75 with freight. 55 v rails gets 6.9 A on two pairs or 3.5 each, down in the DC soa limit. I use the rail voltage when I'm reading the soa table incidently. Inductive speakers can increase the current over resistive loads. A +-55 v rail comes from a 41 vac nominal transformer with two windings, since they are sold by full power average voltage.
If changing to a toroid transformer, put a surge limiter NTC resistor in series with primary windings like a GE CL50, because they surge pretty badly at turn on. Toroids also pass lightning strike surges worse than E-frame too, IMHO, so I put a 275 vac MOS supressor primary in parallel after the fuse. Cuts the pop volume of a nearby strike, anyway. I solder these accessories on a cinch 4 terminal strip near the AC fuse.
And stuff some speaker DC protection in there too, see the $5 protection board thread. My speakers cost $300 each used, the amp was $100 + repair work.
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Last edited by indianajo; 7th December 2017 at 12:01 PM.
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Old 7th December 2017, 12:55 PM   #28
danschy is offline danschy  United States
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The significant SOA violations, especially with the original output devices (2N5631/6031), certainly lends some understanding as to why Ampzilla failures are common, especially in commercial applications. With my very low power usage the last decade or so I'm sure I never violated SOA, but after the earlier rebuilds I did a lot of high power testing so may have weakened the devices right from the start. Thanks for your help and insight. I worked for 40 years as a EE (RF comm equipment), retired a few years ago, and miss the daily technical challenges.
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Old 8th December 2017, 12:07 AM   #29
wg_ski is offline wg_ski  United States
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The original thinking was that each 5630 would see only 35 volts or less at any significant current. At 35V they are good for about 5A. But at 5 or 10 amps the gain falls enough that the base current upsets the bias thru those 2 watt resistors, and it doesnt divide evenly anymore. Put a heavy and reactive enough load on it and poof. The trouble with second breakdown is that you don't even have to run it hot to run into danger. With a standard parallel stage, it will divide the current evenly if you have a reasonable vbe match and a high enough emitter resistor value - even if the stage is running out of beta.

Leach solved tha problem in the Suoeramp by using a triple. But as many of them oscillated as didn't - it is real sensitive to construction technique.

Last edited by wg_ski; 8th December 2017 at 12:08 AM. Reason: Typo
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Old 8th December 2017, 01:28 AM   #30
indianajo is offline indianajo  United States
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I couldn't believe the output transistor stacking was real. I thought perhaps though some off schematic wiring the output transistors were really parallel. 2n5631/6031 was only Vceo of 140 v, but with MJ15024/25 ON semi solved that problem, with 250 v Vceo. In 1974 there were various theories why transistors blew up - that was put to rest with Motorola's famous App note of about 1980 where 2nd breakdown & soa limits were introduced (no I don't have it).
Danschy, I'm a physicist, who haven't even read Doug Self or Bob Cordell's amp design books. I'm sure you could pass me up if you did. But people tend to obsess over 3rd decimal place harmonic distortion and various ideals, whereas I find some commercial products in the PA market quite acceptable sounding. Considering that speakers generally distort so badly. I'm not golden eared, but I am silver with 14000 hz response still age 67. I can quite easily hear the 1% hd of my tube amp, and have moved on to transistors and op amps in the 0.3% HD class & below. And if I want to hear some real sound, my piano is sitting right there, totally authentic 0.000% HD.
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Last edited by indianajo; 8th December 2017 at 01:41 AM.
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