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Ugh - my ampzilla died again
Ugh - my ampzilla died again
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Old 5th December 2017, 04:29 AM   #11
bwaslo is offline bwaslo  United States
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Ugh - my ampzilla died again
Back when I was a repair tech and had to handle these, dying was what those amps did best. It got to be where we just shipped them to the factory for repair (wholesale replacement of silicon inside, basically), they didn't pay enough to cover all the time and the high likelihood of it appearing on the service shelf again soon after. People laughed at the Phase Linear 400, as it was cheaper than the zilla, but the Phase 400's seemed to hold up a lot better for the customer. (Of course, some of that might be because the name "Ampzilla" was just an invitation for frat party abuse!)
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Old 5th December 2017, 12:42 PM   #12
danschy is offline danschy  United States
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Yeah, bwasio, that seems to be a common opinion. I'm pretty sure abuse was not the cause of the latest failure, although who knows if something was "weakened" earlier in its life. My last rebuild was in 2001 (my son had it at college, so maybe there was some partying going on). The last ~15 years it has been sitting in my family room driving my dual-voice-coil subwoofer - 8 ohms per side. It is on probably 12 hours a day, but mainly (and sadly) very little music, mostly my wife is watching CNN, so the average output power has to be tiny. I am probably more intrigued to do the rebuild just because I like rebuilding things, rather than it being a good use of my time. But hey, I'm retired, and working on the Ampzilla brings back some great memories from earlier in my life. (Still on the fence about the rebuild.)
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Old 5th December 2017, 02:49 PM   #13
indianajo is offline indianajo  United States
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Age doesn't hurt properly rated silicon, see PRR in post #5. Amps that keep the output transistors below the SOA current even with reactive loads tend to not blow them up. SOA wasn't invented in 1974, started with about the MJ802 as far as I know. I'm working on a 1980 Allen organ amp this week that the silicon is all okay, 100 w all day from one pair OT.
MJE15032 is about as tough a driver as you can get, but don't fit TO66 holes very well. And the exact match transistors you get from central semi and NTE are missing a lot of specs on the datasheets, like soa. I like the specs on the ON semi parts I'm getting.
If it were me, I't put the ampzilla boards away in the attic and buy new boards that fit TO220 drivers . And a transformer with lower voltage, actually suitable to 2 pairs/ channel output transistors. That temp sense "IC" is a bit mysterious, at least renew the heat sink compound on it (every where else).
Follow djk instructions, he is very knowledgeable. Ie all your resistors are suspect, apparently. I haven't had trouble with suitably rated resistors, but I work old dynaco & hammond stuff with premium mil-spec clone carbon comp resistors. BTW, metal film resistors now are the best, have lower hiss than carbon comp. Quite audible above 100kohms. Vishay Welwyn and multicomp have been shipping from factories in countries where I trust the QA dept.
BTW with a new antek transformer, you can use the +-18 winding included to produce +-15 for op amps. Which can sound cool if you spend as much as $.28 each for 5532 4560 or 33078. Or drop $2 on LM4562. Stay away from surface mount if you're my age.
As far as new boards in old iron, look at the price of new enclosures - $$$. Heat sinks and fans aren't cheap, either.
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Last edited by indianajo; 5th December 2017 at 02:52 PM.
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Old 5th December 2017, 04:50 PM   #14
llwhtt is offline llwhtt  United States
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When I get an SAE/GAS amplifier in I usually just strip it down and start over, it's just easier that way. On the Ampzilla the lower four drivers on each drive board can be MJE15028/29s, the upper two need to be high voltage devices, MJE15032/33s, two are the VAS and the other two are in the VI limiter circuit. There are several options for the Uniwatt devices. I usually replace all of the resistors with metal film, pf size capacitors with silvered mica and .01-.1 ceramic bypasses with film like WIMA or Panasonic. And of course all of the small electrolytics with whatever floats your boat, Panasonic, Nichicon, or Elna. I have both versions of the bias transistor arrays, CA3046/86 or MPQ6001, if you need any.

In the driver circuit there six large resistors in each channel, 300, 750, and 1K. I've been using 3W Vishay metal films for those.

I'm rebuilding an SAE 2400 at the moment, that was SAE's closest relative to the original Ampzilla. Many things apply to both so that is another source of info you can look for.

Let's keep this legend alive!

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Old 6th December 2017, 01:19 AM   #15
indianajo is offline indianajo  United States
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Found a successful amp schematic that has +-75 v rails. It has four pairs of output transistor per channel, 2sd555/2sb600. The mark levinson ML-9. You'd have to buy a blown up one to get the boards, though.
I doubt if that, or a honeybadger sold by diyaudio supply with 4 pairs would stuff in your case, though.
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Old 6th December 2017, 05:09 PM   #16
llwhtt is offline llwhtt  United States
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Ampzilla only has room for four output transistors per channel. So that means leave it with series output transistors as designed or a complete redesign with two parallel pairs which is not enough for a 200W amplifier. Best to just rebuild it stock with modern parts. The Dynaco ST400, SAE2400, and Ampzilla I all have the same output stage and there plenty out there that are trouble-free.

I'm waiting to hear the results of the autopsy. I wonder if it's the same failure every time.

Craig

Last edited by llwhtt; 6th December 2017 at 05:12 PM.
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Old 6th December 2017, 06:12 PM   #17
danschy is offline danschy  United States
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I agree, I need to get going on the autopsy. Unfortunately today I'm at hospital with sister for a shoulder replacement. I took some quick measurements on one of the channels and DC seemed approximately correct so I don't think any power transistors have died. The other channel has a couple smoked components - have not made any measurements.
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Old 6th December 2017, 06:21 PM   #18
llwhtt is offline llwhtt  United States
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Do you remember what the past failures were? All the same or different? Fuses should have opened if there were bad outputs.

Craig
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Old 6th December 2017, 11:59 PM   #19
wg_ski is offline wg_ski  United States
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I would use MJ15022/15023 as my primary output choice. The hFE at 15 amps is higher than the 2119x series, and generally the lower voltage grade has better gain. The problem with the originals (output and driver) was the low gain at high current. At high current it stresses the bejeezus out of the drivers, which can cause thermal runaway. If the driver TJ is higher than that of the outputs, the bias becomes under compensated. And if the composite gain is too low you don't get proper voltage division in the series stack. Then there's not enough SOA. With modern outputs AND MJE15030 drivers it's less likely to give any trouble.

The 2N5630/6030 outputs were often used because the hFE is about as high at 15A as you can get with "enough" SOA. Still needs better drivers than stock. And regardless, you're stuck putting in TO220s where 66's are supposed to go.
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Old 7th December 2017, 12:37 AM   #20
djk is offline djk
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Why did Bongiorno recommend the MJ21195/96 over the MJ15022/23?

"James Bongiorno, the designer of this amp, suggests using 2SB595/2SD525 as drivers and MJ21193/MJ21194 for outputs with MJ15024/MJ15025 as alternate outputs."
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