Phase Linear 700B burns speakers

Collector value on a PL700 is too high to justify gutting it, IMO. You could probably get $300 for it on eBay in this condition, which is enough to buy a better chassis and transformer.

Besides, the heatsinking on the PL700 isn't really what you want.
 
Hey, it’s a great case and heatsink, it would make a real cool retro looking modern amp. Your biggest expense in a new build is the case and transformer, if you could reuse those half of your cost is covered.

Bill

Yes, it would but I wouldn’t recommend it here. They DO fetch enough on the used market to make selling it worthwhile and if you won’t or can’t do anything with it, sell it to someone who will. There is no shortage of us out there. The 700 is quite difficult to “re use” to make a super power retro looking reliable PA amp due to the high supply voltage. It’s too high for ICEpower modules, and no other extensive modifications would be both worthwhile and easy. Class H or TD type circuits could be used BUT modifying the transformer becomes necessary, as well as needing all new output transistors. FAR easier with the 400, because +/-75V is just about right for the biggest ICEpower modules, and the heat sink is probably big enough to use as a regular class AB amp with a modern circuit. I did that with a Dyna400 that was otherwise beyond hope. That has a HUGE heat sink so now it will do 2 ohms (with a circuit that looks awfully similar to a PL400 series 2, with the stability mods of course).

One of my “bucket list” projects is to re-build my largest PA from my early DJ years, with all reliable amplification and speakers, as true to the original as possible. Still looking for a PL 700 series 2 that is beyond hope of restoration, toput a class H circuit in. I already have the necessary transformers, so finding one with a burnt out transformer and 24 dead transistors would be ideal (and inexpensive, compared to one that’s actually rebuildable). I have suitable 400 (that is actually working but would undergo similar mods). The project is way beyond the scope of what most people restoring PLs would ever do, not inexpensive without a very well equipped lab, and only a project for a very experienced builder. I could just build a custom case, but having original cases with the red dot meters is part of the appeal of the project. I can pick up original CS800’s any day of the week for the sub amps.
 
My suggestion was to add buck converters to create the lower rails for class-H. It would probably be cheaper than a new transformer. Unfortunately the buck converters on Amazon are only good for about 70V so you have to roll your own or find them elsewhere.

I would probably re-use (most of) the existing OPS and just DIY a new IPS. But of course that would continue to use BJTs at silly voltages. Perhaps a class-G for better SOA?
 
If you ran even 2-step class G or H the voltage would no longer be silly, and 5 output transistors in parallel would be more than sufficient, even the original ones. If I found one with a burnt out trafo I’d see about rewinding it to get the extra voltages. Failing that I happen to have a pair of 640 VA 20-40-80 trafos that could be used, if things can be made to fit. If you ran buck converters you might as well make them track and go class TD. But that’s a more ambitious development project.
 
If it were mine, & I wanted to use it, I would
1 find & repair the popping beeping problem.
2 Replace the power transformer with a 800 va or higher 170 vct transformer. +-100 v is for low gain 1975 transistors, obsolete by 1980.
3 replace the PL909 with competent modern transistors, probably MJ21194
4 replace drivers with something with more gain
5 add a thermal shutoff or add a fan.
6 Check for oscillation or rail sticking by testing with square wave & clipped input signals.
7 Fix problems found by 6.
Original boards should be salvageable.
New boards come from a country I don't deal with, and require software running on op systems I don't have.
The supply of Crown, Peavey, Yamaha and who knows what else class AB hulks should be the cheapest way to acquire a transformer. Shipping is the biggest cost of those >70 pounds that USPS won't handle. Until 9/20 I had a spare transformer from a PV-1.3k that was carried off to the copper scrapper by the burglar. Who knows what the used AUS market is like, but we are drowning in class AB opportunties. Bar bands don't like to carry transformers anymore.
 
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Reading this forum is interesting.... at some point you have to wonder what is the value of this amp?

IMHO, the case looks nice... imposing in those nostalgia 70s/80s kind of way. There's one on eBay right now that they're trying to sell for $2100 OBO.

I would think that such an amp might make more sense for parts or just the cabinet. Take the chassis, gut it out, put in a nice SMPS and NCore or Purify amp, enjoy the meters and live long and prosper. Nevermind how much lighter it will be. I mean, even if a person where to spend the time fixing it, the fundamental design has several shortcomings.

Truly, googling this amp I was surprised the Harry Pearson actually liked this amp. As a rule of thumb he hated solid state designs from the 70s and most of the 80s.
 
Another potential mod for making one “a lot ore reliable” is to reconfigure the output stage as a stacked “cascode” like the Super-Leach. One could configure all 24 as outputs, with 2 groups of 3 parallel 21194 in series. Mount some flat packs as drivers some place convenient. It wouldn’t do anything about the *heat*, but you could always use a fan (actually you would need two) if you intend on playing it loud. But that output transistor arrangement is good enough for +/-127V with 4 ohm loads. It wasn’t inside a PL chassis, but I have done this, and gigged with it without any fireworks.

Just using a PL700 in a “normal” way with MJ21194 for both driver and output is plenty reliable for regular hi-if use, if you don’t run it too hot and keep the speaker impedance reasonable. These things do benefit from some airflow on the heat sinks when cranking it.
 
IMO, PL 700s have some historical significance, so I don't like seeing them scrapped. Modding for reliability is fine in my book, but ripping them apart to put class D modules in the box... kind of sad, IMO. Plenty of people who will pay decent money for one, even if it's dead.

These things powered countless rock shows back in the 70s, 80s and even into the 90s. They were one of the first really big solid-state amplifiers, at least that didn't cost a fortune. They always put fans on them in PA use, of course.

A good chassis and transformer, plus some VU meters, shouldn't run more than a couple hundred bucks. The ones on the DiyAudio store are a better place to start (with better heatsinks) and you can get a decent toroid for less than a hundred bucks. You can get a lot more than that for a PL700, even if it's blown.
 
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IMO, PL 700s have some historical significance, so I don't like seeing them scrapped. Modding for reliability is fine in my book, but ripping them apart to put class D modules in the box... kind of sad, IMO. Plenty of people who will pay decent money for one, even if it's dead.

These things powered countless rock shows back in the 70s, 80s and even into the 90s. They were one of the first really big solid-state amplifiers, at least that didn't cost a fortune. They always put fans on them in PA use, of course.

A good chassis and transformer, plus some VU meters, shouldn't run more than a couple hundred bucks. The ones on the DiyAudio store are a better place to start (with better heatsinks) and you can get a decent toroid for less than a hundred bucks. You can get a lot more than that for a PL700, even if it's blown.

What price nostalgia?

If you look at the parts for these amps, VU meters, front panel, chassis, they charge quite a bit and it does look very good. The amp "looks" very good but it is not like gutting an ARC amp or even a big Levinson.

And they are not rare. I mean, they are a big solid state amp from the 70s. Not something that sounds particularly good. Indeed, I remember a friend of mine who had one of these amps, back in '84, and his amp fried. He had a whole stack of PL components. Thank God it didn't fry his speakers.

IMHO, they are plenty of them around and not enough people who are going to fix them. Some people might get taken by the nostalgia and pay through the nose for an amp that "sounds good".

And then they will lose their speakers...

Maybe we should try to start a movement to "Save the speakers from DC".:)
 
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To me what makes them a little special (compared to a lot of other amps from that era) is that they were used as a PA amplifier for a lot of rock concerts. They were one of the first amps that could really make a lot of power.

I wouldn't gut it. Rebuild it and add a DC protection board, and you've got a reasonably cool and useful amp. I honestly never thought they sounded that bad, even if the design is a little primitive.
 
I suppose a “better” way of producing 350 watts per channel 8 ohms, 550 at 4 ohms with a hunk of iron and copper is the QSC USA1310. Same exact REAL power (not it’s inflated 400 watt EIA rating) as the PL700, but using a bigger trafo for better regulation on lower rails (+/-93V) and 8 output pairs per channel. Perfectly reliable, other than the usual minor QSC aging problems, which are easy to fix and never fatal. I’ve still got two of them - many are still in actual PA service although many are being scrapped because of weight and thus quite cheap. But they just don’t sound as good as a Phase Linear.
 
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There have been a lot of stories about these amps running into problems. Without an output L, and just relying on the speaker cable instead, there are bound to be problems.

A good way forward would be to build a model in LTspice and fix the compensation.

Unfortunately, back in the day when these were designed, there was no LTspice ( or other readily available simulator) so designers had to be very experienced and knowledgeable wrt feedback and compensation - my view is they were generally few and far between.

The slow output devices don’t make it an easy job either.

However, this is a piece of history, so you should try to save it IMV.

(I remember seeing this amp and is companion preamp with a joystick type control at a hi-Fi shop called ‘Sound Specialists’ back in 1970’s and as a 20 yr old lusting for them. My budget didn’t stretch that far!).