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Ugh - my ampzilla died again
Ugh - my ampzilla died again
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Old 2nd December 2017, 08:55 PM   #1
danschy is offline danschy  United States
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Default Ugh - my ampzilla died again

Bought my Ampzilla (kit) back in late 1974. Over the years I've had it it's failed 3 or 4 times requiring extensive troubleshooting and repair. One time it additionally took out one of my speakers which was luckily covered under warranty. The last 10 or so years it has been driving my subwoofer, but alas it has died again. I'm trying to figure out if it's worth repairing. In previous repairs I replaced only the failed components, but I'm thinking if I talk myself into fixing it again, it should be a more substantial rebuild including all caps, etc. I'm open for suggestions, or if somebody knows of a good home for a failed Ampzilla I could probably be talked into parting with it. I think a rebuild would be a fun project, but there are other fun things I could do with my time.
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Old 2nd December 2017, 09:39 PM   #2
indianajo is offline indianajo  United States
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Version no suffix from eservice info seems to have no thermal sense on the heat sink. If you have version 2,3,or 4, you may be better off. The biphase drive to the output transistors is odd, also. If it is version 1, salvage the transformer case, power switch, heat sink, build some other driver board.
No speaker protection other than fuse shows either, I won't connect an amp to anything but resale shop trash speakers without it. I built an AX6, which has a speaker cap. My ST120 boards were peeling copper from all the repairs. My output transistors are located remote from the driver, too. I used a salvage inductor on the double diode heat sense coming back from the remote heat sink. My driver/sense leads are about 5", not run close parallel.
List of amp projects you can buy boards for is here:
Class AB amp recommendations
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Last edited by indianajo; 2nd December 2017 at 10:05 PM.
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Old 2nd December 2017, 10:29 PM   #3
danschy is offline danschy  United States
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Pretty sure I have version 1 since I bought it in 1974. I don't see any version number; serial number is 100370; manual is titled "Ampzilla Model GAS 101". There is a thermal cutoff on the heatsink, but no speaker protection other than a fuse.
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Old 2nd December 2017, 11:53 PM   #4
indianajo is offline indianajo  United States
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Yeah, that design is primitive. Forget what I said about biphase drivers. Setting the O.T. Idle current at first, then collapsing the drive voltage split when the parts heat up, is a key to longevity. I like 20 ma idle current, and have my double diodes on a cinch terminal strip right over the output transistors. Image your heat sink and see what people think of it. My ST120 heat sink was **** and has been suppliemented with PCAT fans.
Measure your board space and see what new design you can fit in there, then peruse the available boards on the link. I drill holes in the board corners, make tiny steel angle brackets, screw the driver in the case with 6-32 screws & elestic lock nuts, so they wont come unscrewed & short.
If you're going to stay with split supply, (+v, -v, speaker ground) look at the thread titled $5 protection board. I don't like hard contact relays, they oxide up and drop sound after years, but it is better than a fuse. Or you can disconnect the center tap from the power supply caps and build a speaker cap design - full speaker protection for $4/channel.
Update the O.T. mica insulators and all electrolytic caps as you go, they deteriorate with time. Use heat sink grease, wash up afterwards, it's poisonous.
Remember to use safety glasses desoldering, it splashes. Use one hand at a time when measuring voltage, and a clip lead for meter return. Current across the heart can kill. Use a light bulb box in series with 120 VAC when starting up, gives you time to measure errors before the parts smoke. most amps can deliver 1 watt on the light bulb, if the idle current is right, the output DC voltage (before cap) is ~zero, and the music is good you can ramp up.
Lots of fun options for redesign on this forum.
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Last edited by indianajo; 3rd December 2017 at 12:07 AM.
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Old 3rd December 2017, 05:53 AM   #5
PRR is online now PRR  United States
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I think if your Ampzilla ran 40+ years, you more than got your money out of it.

I won't say it was a "cheap" amp. It was a very good price for its power. I wondered at the time: how could he do that and be reliable? In a few years it seemed to be only marginally reliable. Some ran fine. Others died too easy, and took out speakers. They had a real bad rap in "professional" work where amps are worked hard.

In retrospect, we have learned a lot since those days. 350 Watts from just two-pair of transistors seems incredibly even stupidly "bold". I have no doubt it would cook all day on test-bench, and early reviews noted problems on reactive loads that James quickly corrected. I still think he was skating too close to the edge, and that we have learned a lot about repeated insults to Silicon eventually causing a blow-up. Today a minimal design leans to 75W/pair, and the declining cost of transistors vs heatsinks and warranty suggests even less per pair (even as audio power transistors have got a bit better).

It IS a mighty pretty box, and probably the PT has many miles left in it. It is a shame there is no pre-made solution for +/-75V rails that can just drop-in.
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Old 3rd December 2017, 03:30 PM   #6
indianajo is offline indianajo  United States
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+- 75 v wasn't on the eserviceinfo schematic. PA quality amps with those rails tend to have 4 pairs MJ15024/25, which you probably don't have room for. Output transistor gains are higher these days; in 1974 gain 5 was a premium part.
So probably $75 on a new transformer with lower rail voltages would be a good place to start. AN-6458 - 600VA 58V Transformer - AnTek Products Corp
That would allow you to stay with two pairs OT.
I've got a 1970 amp with 70v rail, too high for reliability except I listen to classical music, with very low crest factor. So the actual average power delivered is much lower than the peaks, like the cannon shot in 1812 Overture.
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Last edited by indianajo; 3rd December 2017 at 03:33 PM.
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Old 3rd December 2017, 05:07 PM   #7
prairiemystic is offline prairiemystic  Canada
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I have one in for repair, it has tantalum capacitors in the feedback network

PM me for the original 1974 Popular Electronics construction article, it's 4.8MB
I had a real hard time finding the schematic.

It doesn't look that terrible a design, just a weak output stage I think.
But ahead of it's time in some ways.
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Old 4th December 2017, 03:39 AM   #8
danschy is offline danschy  United States
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Thanks prairiemystic, but I already have the article, which is where I first learned of the Ampzilla and prompted me to purchase the kit. I've repaired it several times and also modified it years ago with the Jung improvements. It performed very well - I seem to recall distortion in the 0.002% region, the limit of my measurement instruments. I have never used it with any kind of exotic speaker load or at very high power levels, so am disappointed at the multiple failures. It's always run very cool due to the fan. Failures could just be a result of my soldering skills or inevitable device failure after many years of service.

Thanks for the suggestion indianajo - hadn't thought of that route to higher reliability, but I think the highest value item of my Ampzilla is that huge transformer.
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Old 5th December 2017, 12:38 AM   #9
djk is offline djk
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The GAS 100 or 101 bias IC must be in contact with the heatsink or the amp will blow up.

Use MJ21195/96 for outputs.

Replace all the small electrolytics and carbon film resistors.

(the copper striker coat on the copperweld leads fail)
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Old 5th December 2017, 03:23 AM   #10
danschy is offline danschy  United States
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Thanks, djk. MJ21195/96 look good for the outputs. Any suggestions for T0-66 driver replacements - 2N6316/18? I plan to use MJE15032/3 for the lower power TO-66 devices as recommended on another thread.
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