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Ugh - my ampzilla died again
Ugh - my ampzilla died again
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Old 8th December 2017, 02:59 AM   #31
wg_ski is offline wg_ski  United States
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In 1974 140 vceo was high for a PNP. So was a 30 volt 2nd breakdown point. They had NPNs that were more rugged, but they were only NPNs and had fT around 200 kHz. There were other ways of blowing them up - playing it loud and running multiple speakers wouldn't do it, but running a 20 kHz square wave with no load sure would. The Mj15024 came along and solved that problem. but the pnp still took a while - it wasn't available immediately.

I once tried building a stacked output stage out of all NPNs. Don't try it. You can't make it stable. There's a reason why no one does or did it.
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Old 8th December 2017, 07:07 PM   #32
Michael Chua is offline Michael Chua  United States
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Quote:
Originally Posted by indianajo View Post
I couldn't believe the output transistor stacking was real.
That was a long time ago where transistors were scarce. Today, we have the luxury of transistors that can operate at +-70V rails.

I personally would change the output to a more conventional design with one pair of drivers and 4 pairs of outputs. I would change the bias as well.

This would of course mean that the amp needs to be re-stablized.
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Old 8th December 2017, 07:19 PM   #33
llwhtt is offline llwhtt  United States
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All versions of vintage Ampzilla heatsinks only have room for four output transistors per channel. All of the related amplifiers, Dynaco, SAE, and Ampzilla have the same output arrangement, they are what they are. There's not much that can be done without drastic mechanical changes that will ruin any value the amplifiers may have. Like I said previously there are plenty of all those amplifiers still out there working fine and people enjoying them. Obviously nobody would design an amplifier that way today but there was not much choice 40-50 years ago. Let's help the OP repair/rebuild his the way JB made it without redesigning this classic.

Craig
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Old 8th December 2017, 08:13 PM   #34
danschy is offline danschy  United States
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Thanks Craig. I have greatly enjoyed the discussion, and appreciate everyone's comments. The soa violations were new to me, so feel I now have a much better appreciation for the amp's limitations, and also suggestions for better devices. I agree with your opinion, and will likely do a minimal rebuild, recognizing that anything much above 100w-8ohm will potentially shorten it's life, but I don't see myself ever going there outside the test bench.
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Old 12th December 2017, 01:06 AM   #35
danschy is offline danschy  United States
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Have some other stuff going on so haven't had a chance to troubleshoot my Ampzilla, but I have a good idea for replacment power devices based on above comments. Any suggested for replacement for schottky diode 1N5823? Just use 2N5551/5401 for the input devices (UPSU06/56)?. A few years ago I replaced the fuse bulbs for the meter panel with some LED bulbs, but they have also died. Any recommendations? Thanks!
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Old 12th December 2017, 03:17 AM   #36
PB2 is offline PB2  United States
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If you look at the 2N5630 and consider that the output stage is stacked it will handle
about 4A at 40V (roughly half of the supply) or 160W for just a minor SOA derating from
the 200W spec.
MJ15024 handles about 6A at 40 volts, 240W and has not hit the SOA limited area of the curve at that voltage.
Similarly the MJ21195 shows about 5.5A at 40V, 220W on the SOA curve and is also not limited.
Both of the newer parts should do their rated 250W and they would be like having 250/160 = 1.6 times as many of the older output devices.
Also consider that straight thermal derating is required, fan cooling is a big advantage over
other power amps without fans.

Bongiorno probably recommended the MJ21195/96 over the MJ15022/23 because of their higher speed and therefore a reduction in cross conduction.
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Old 12th December 2017, 05:38 AM   #37
nigelwright7557 is offline nigelwright7557  United Kingdom
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I would be looking into why certain components are failing or keep on failing.
Is they amp on the edge of oscillation ?
Is it struggling with the speaker load ?
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Old 12th December 2017, 07:41 AM   #38
QSerraTico_Tico is offline QSerraTico_Tico  Europe
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Quote:
Originally Posted by danschy View Post
Have some other stuff going on so haven't had a chance to troubleshoot my Ampzilla, but I have a good idea for replacment power devices based on above comments. Any suggested for replacement for schottky diode 1N5823? Just use 2N5551/5401 for the input devices (UPSU06/56)?. A few years ago I replaced the fuse bulbs for the meter panel with some LED bulbs, but they have also died. Any recommendations? Thanks!
Omit the Schottky diodes. They make no sense.
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Old 12th December 2017, 05:53 PM   #39
llwhtt is offline llwhtt  United States
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1N5821 for the Schottky diodes. I think JB recommended and was using MJ21193/4 in his updates.

Craig
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