Suitable replacements for Quad405 output transistors

[Al (astral highway)]

Originally Posted by Radio Wrangler

Lightning strikes the aircraft and then jumps off somewhere else to complete its path. Ludicrous currents in the airframe and rather wild transients induced in all the wiring.

Anything taking the direct hit cannot survive, but the people in the plane sort of want the engines, radios and nave equipment to keep working.

Electronics can get quite lively.

Yes, I’ve often wondered how electronics survives when surges of many KiloAmps go through the airframe and as you say, giant transients in the wiring looms. How did you get round this ? It seems impossible, but obviously you did it!
I think Ed Dinning also designed up to 12-phase systems for aeronautics use.Ed?

[Radio Wrangler]

We're wandering a bit far from Quad 405s, but this involves a principle common to all electronics, Quads included.

In normal usage, a transistor sits there controlling the current through itself. That's their usual raison d'etre. The voltage across it and the current through it stay comfortably within its ratings. All is well. Along comes a surge on the power source and the transistor goes over its voltage spec and over its voltage capability. It fails, short circuit base to collector and the current goes to a value set by the source impedance of the power supply, the impedance of any decoupling capacitors and the impedance of the wiring. Other devices will likely have died too leaving a very low total impedance shunting the power rail. Eventually a fuse or breaker may pop but only after destruction has progressed dramatically.

I've seen caps blown off of TO-5 and TO-3 devices, plastic power devices blown apart.

Just because a power source is rated at XXX Amps doesn't mean that every piece of equipment powered from it takes that amount. Together they might, but if one item goes short, all those amps are ready and waiting for it alone.

In the humble 405, the reservoir capacitors are capable of many tens of amps into a low Z short, and they will dump their entire energy content into it, plus whatever comes from the mains via the transformer and rectifier while a fuse makes its mind up about blowing..

So destruction is a two-stage process. A voltage transient happens. A device responds by going (almost) short, then there is a large current spike. It's the latter that does the drama and collateral damage.

You handle induced lightning transients in multiple layers. An outer layer can take the huge current, but doesn't even try to keep the voltage within what the thing being protected can survive. A second layer doesn't have to handle all that current, but is needed to better limit the voltage going onwards. Add as many layers as it takes.

The aviation world has to take this to the extreme. If your complete electrical system fails, jet engines stay lit, piston engines chug away happily on their magnetos. Almost all your dials and gauges go off along with any fancy glass panel displays. You have a pressure altimeter and airspeed. Some instruments have gyros driven from a vacuum pump. No radio, no GPS. Navigate by the stars, follow railway lines, get thee to an airport, and fly in on a no-radio approach it's in their approach info. Scary, but you have to train and practice. Look up 'partial panel' if you want the full story.

Peter Baxandall said that the use of a stabilised supply in the Quad 303 was his input. He was worried about mains transients and surges over-voltaging the 2N3055, so the regulator and the amplifier devices would share the total voltage rather than the power amp having to take it all. He'd recognised that aiming for a 48v supply for the target power with nominal mains meant that high mains plus transient could trigger cascade destruction.

However, other designers brought out 2N3055 50-watters and had no trouble of note. But Messrs Walker and Baxandall were early into the field and had to find out things from scratch. So they played it carefully.

A guy called Bob Carver went the other way, he used higher voltage transistors and capacitors and set the power rails in his amplifier at unusually high voltages. His amp was designed to handle high power transients, though the charge on the reservoir capacitors and the weedy (well. relatively weedy) mains transformer couldn't sustain it very long. The idea was to engineer the amp to suit the fluctuating power of music and speech. It worked to an extent. It was a little giant-killer of an amp, but when it ran out of steam it degraded nastily, and it looked bad in sinewave tests.

Audio and audio-modulated RF (full AM, SSB) is a devil of a thing to handle. Valve amps tended to degrade gracefully and were a hard act to follow. The usual approach other than Carver, is to have a monster transistor amp and to under-run it. THe 405 is somewhat in this direction.

Anyway, the big message is to not just look at spot ratings when considering alternative transistors. Compare the graphs. These parts are not used at one fixed voltage and current. Gain changes with current (and voltage to a lesser extent)
Dough Self has written a lot of sense about current gain at low and high currents and the effects on crossover distortion.

The 405 has terrible crossover distortion from its power transistors. It makes no attempt to avoid it, but it does use a tiny class A amp in a bridge circuit to try to cancel it. This works, but it all comes unstuck into awkward impedances where the class A amp hasn't got enough umph to fill in for the dumpers. The base dive to the dumpers is a bit of a limitation, so you need good gain from them across the range of current they have to provide. You either see this circuit as very elegant, or as very inelegant as it all comes apart into nasty loads.

Everybody agrees the crowbar is a disaster, though. It saves your speakers by destroying your amplifier, but maybe amplifiers are easier to fix than speakers.

David

[bikerhifinut]

OK chaps,
Rest assured that the amp boards are in the most capable of hands.
They appear to have been refurbished in the past to a good standard including the resistors that feed the zeners and new Nichicon electrolytics.
I spoke to my mate today about them, he may comment too at some point, and he wont let them go until they are properly tested. We are thinking about disabling the original onboard clamp as a known weakness and I can retrofit an aftermarket protection circuit, the velleman kit seems very good value and is self contained with its own PSU.
ah yes the idea that the protection circuit protects the speakers at the cost of an amplifiers beating heart................. I think the poor amp didn't have a chance as it was driving a pair of Quad ESL989 that I subsequently discovered had a circuit fault on its own protection Clamp, I think it was probably when I first got the amps and plugged em in to the Quads and i suspect a spike or high transient signal overloaded the ESLs. These amps as monoblocs will deliver 180W and the ESL crowbar will energise at much lower outputs to protect the thin diaphragms. And thus the inevitable happened, the speakers went S/C as the amps were delivering a big signal and this in turn triggered the crowbar on the output board where the 2 amps are paralleled and also probably did the same on the onboard clamp circuits (early 1981 version). __That only one amp board vapourised its output transistors is a surprise in itself but both boards are getting a good going over.
So, the plan is while the boards are in the operating theatre, I have the case in bits and will remove the 410 mono pcbs on input and output and restore the amp to stereo, along with its stable mate. I bought em for a song anyway so i figure getting a pair of 405 stereo amps up and running will pay for any repairs by selling one later. 100W is still more than plenty for any domestic living room even with insensitive ESLs.
Live and learn and its an expensive lesson!
I do like the 405 all the same, and its a good amp for the ESLs which I reckon were the best thing I have done with the stereo system in years. some love em, some hate em. Since i got them (secondhand of course) I've been digging into my record collection and enjoying music again ,as well as spending too much pocket money on new LPs and CDs....................
Andy.

[Electronpusher0]

Thanks David, two fantastically informative posts in this thread!

Peter

[Diabolical Artificer]

I have some 405 boards or copies of boards someone gave me on here. I've bought most of components, ended up with 2N3773's as the OP devices. these were easier to find than driver replacements. I chose these after looking at Keith Snooks site and the minimum of research, which looks to be not enough in light of this thread.

It's on the rountuit pile, now I'm wondering if a lot more research is needed and a later current dumping amp design rather than the 405 would be a better build if it can fit on the PCB's.

Did other's build 405 clones that address some of these issues? Wasn't there an Elektor current dumping amp? Much as i'd like to design my own tranny amp I only have X time which I'm increasing aware is running out. Valve amp design is easier, valves don't die easily but tranny's are cheaper so i could have a go.... will think about it.

Andy.

[bikerhifinut]

give me a shout Andy, it's worth sorting them out.
They really are damn good amps.
You know how to find me.

The other Andy

[Tim]

I will confess to "popping" one or more final output tranny's on a 405 I had in for repair when the multimeter probe slipped. Mine was fitted with 2N3773's and a pair (which turned out to be Toshiba's) from Cricklewood went back in and it was away. I think I was very lucky.

[Radio Wrangler]

With the 405, the dumper transistors need to be gainy enough that the driver can drive them to enough output current, and quick enough so that the class-A amp doesn't have too big a hole to fill.

Problems would manifest as distortion appearing when the amp was working hard, high amplitude, higher frequency and into a lower impedance load. It's possible that borderline transistors might not be noticed.

David