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.
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
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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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.
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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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.
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.
+- 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.
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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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.
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.
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.
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.
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!)
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.)
(Still on the fence about the rebuild.)
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 - $$$. Heat sinks and fans aren't cheap, either.
MJE15032 is about as tough a driver as you can get, but don't fit TO66 holes very well. And the exact match transistors you get from central semi and NTE are missing a lot of specs on the datasheets, like soa. I like the specs on the ON semi parts I'm getting.
If it were me, I't put the ampzilla boards away in the attic and buy new boards that fit TO220 drivers . And a transformer with lower voltage, actually suitable to 2 pairs/ channel output transistors. That temp sense "IC" is a bit mysterious, at least renew the heat sink compound on it (every where else).
Follow djk instructions, he is very knowledgeable. Ie all your resistors are suspect, apparently. I haven't had trouble with suitably rated resistors, but I work old dynaco & hammond stuff with premium mil-spec clone carbon comp resistors. BTW, metal film resistors now are the best, have lower hiss than carbon comp. Quite audible above 100kohms. Vishay Welwyn and multicomp have been shipping from factories in countries where I trust the QA dept.
BTW with a new antek transformer, you can use the +-18 winding included to produce +-15 for op amps. Which can sound cool if you spend as much as $.28 each for 5532 4560 or 33078. Or drop $2 on LM4562. Stay away from surface mount if you're my age.
As far as new boards in old iron, look at the price of new enclosures - $$$. Heat sinks and fans aren't cheap, either.
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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!
Craig
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!
Craig
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.
I doubt if that, or a honeybadger sold by diyaudio supply with 4 pairs would stuff in your case, though.
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
I'm waiting to hear the results of the autopsy. I wonder if it's the same failure every time.
Craig
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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.
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.
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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