QuoteYour love of complexity as a virtue is so great you are so obviously blinded to anything that threatens to take that beloved complexity away from you.

Like I say, LJ, you don't know me. To someone who does, this is more proof that you don't.

QuoteOf course there is no sound during a short,

But you were moaning about tone loss when short circuit protection is activated. Did you not see the problem? Short circuit protection activates... when... there... is... a... short.

Quotebut I seriously don't get how that proves there is no simpler solution. All you proved is you can't see one.

The politician is back. If you read what I said, I said that I didn't see a simpler solution. You're the one who's trying to make that be me saying that it proves there is no simpler solution. That came out of your keyboard, not mine. Me not seeing a simpler solution is not proof of anything - other than I'm honest about it when I don't know.

By the way - I do have the professional background, experience and training to make it significant when I say I don't know of a simpler way. I know a lot of ways, just no simpler ones. I've designed a lot of power electronics. What have you designed?

I kinda have this history of saying when I don't know something. I always did want to know not only what I know but what things I didn't know, too. It helps. You ought to try it some time. Practice saying to yourself in the mirror "I don't know [whatever]." It'll make a much bigger person out of you. Did you want me to provide some historical quotes about people who won't admit when they don't know something.

I didn't think you did.

QuoteMaybe one day someone will give you one with permission to use it.

Actually I got that permission a long, long time ago. But thanks for thinking of me.

QuoteSorry, but you rattle on about nothing too long to waste space reproducing it for you. But - ok, assume I don't.

I already assumed you didn't. In fact your posts showed it pretty plainly. The issue is whether you would admit it.

Quote"Yes", there are some situations where what you are calling "safe-area protection" can prevent an amp from dying.

ALL RIGHT!! LET'S HEAR IT FOR THE MAN!!  He's growing up!

QuoteBut that does not imply that what you are calling "safe-area protection" is necessary or desirable in all cases.

OK. That's fine. My contention was that sooner or later, depending on how you used the amp and how the loading was done, you would run into a situation which would damage the amp, requiring repair or junking the amp. You've fought that contention tooth and nail, only to finally admit that
(a) short circuit limiting does not damage amp tone
(b) there are situations where safe area protection protects an amp.

Only a fool does the same thing in all possible cases. You're trying to press the issue that there must exist at least some cases where safe area protection is not (a) necessary or (b) desireable. Well, duh. At least you've backed off the claim that no protection of any kind is ever necessary or desireable. But we did have to drag you there kicking and screaming.

QuoteWhat you are calling "safe-area protection" isn't because it only monitors one of the variables. Current limiting would be more accurate.

Like you finally admitted, you don't understand it. So how is it that you are qualified to act as a critic? If you don't understand it, how do you know what it monitors, what it doesn't, and how accurate it is, both in an absolute sense and a relative sense against other protection forms?

Safe area protection, if well designed, monitors many of the variables that happen to the output devices and start shutting down the output devices when they start getting to dangerous conditions. Well designed safe are protection watches absolute current, current-voltage product, time variance of CV product, and load impedance.

I could give you the lesson about why safe area protection is more accurate than current limiting, but as you said

Quoteyou rattle on about nothing too long

so if you ever want to know how it works, ask.

QuoteAnd you can say I was unsuccessful again, but the problem is with the hidden assumptions of your question. Qualify those assumptions and I have no problem with a yes or no answer.

No, you did OK. You admitted you didn't understand it. Bravo. You're getting better. Keep practicing.

QuoteRG said: What is your training and experience in analyzing power electronics for reliabilty? Is it just using them until they break or don't break yet?

QuoteLJ said:Yes.

OK. So you don't have any training or experience in analyzing power electronics for reliability. Well, no big surprise, but once again, I was just seeing if you were honest enough to admit it.

QuoteReality is fact - theory isn't. If it doesn't break in use, then it is reliable. Bridges built only on theory do fall down, and in doing so, theory changes.

And mothers kiss babies and bake their kids apple pies. Who said anything about theory? I've spent more than a few days peering at the burned-out guts of a power supply or amp that let the magic smoke out and designing something that replaced the hulk and didn't die. 

People with theory-envy want really badly to tar anyone with formal training with the brush that they have no real world experience. You misjudge it badly if you think that's the case here. I've known engineers who were useless. But I've also known guys with degrees that are the right ones to have on your side in crunch. It's another logical fallacy to assume that theory is always wrong, or that an engineer worth his salt will hold onto a useless theory.

Let me reproduce something that you seem to have missed from my last post:
Actually, my "hearsay" comes from people who practice the art, write textbooks, teach, make a living by getting things to work - over and over and over. I guess that yes, you could call that a better grade of hearsay. Call it informed hearsay. Who you gonna bet on - the guys who do it all the time or the uninformed?

The race is not always to the swift nor the contest to the strong - but that's the way to bet.

QuoteI doubt it, but you may have heard the saying: "when reality conflicts with theory, develop another theory".

Yeah, I've heard it. We had a few other sayings back in engineer-land that you probably haven't heard.
"You can't bullshit electrons."
"Put whatever you like in your design drawings, but Mother Nature waits for you at the end of the assembly line."

Your anti-tech bigotry is pretty dated (and herd-following, as I know that's near and dear to your heart) but it's not very accurate. Did you think it's original? Did you think it's clever? If you get right down to it, it's a form of pride in being ignorance; the idea that people who have training and experience in a technical field must be wrong. Save yourself a lot of money - don't get into a lot of wagering on technical issues.

QuoteI will never be INVOLUNTARILY forced to short the output of my amp. That is just one of your hidden assumptions.

OK, lessee here. You're never going to plug the wrong cord in. You're never going to get a whisker of wire or metalized foil in the wrong place in less-than-ideal conditions and time pressure. You will never, ever, no-how, no-where, no-time ever make a mistake.

If that's your contention, you are either not human or a liar. And yes, I guess it is fair to say that humans will make mistakes to be one of my assumptions. So tell me, and again, try to answer yes or no - are you trying to tell me that you will never make a mistake?

QuoteThe rule about plugging in the speaker before turning the amp on pertains equally to solid state - you won't short the output by plugging in the cable. Deny that one.

Why should I? If you are silly enough to use plugs that will short the output and also silly enough to demand that there be no protection of any kind in your amp, OF COURSE you have to make up rules to try to never make a mistake.

...uh-oh... there's that mistake word again. 

What I can tell you is that none of my amps, with the exception of my old Thomas Vox amps, care a whit whether you plug the speakers in first or not. You can plug the speakers in and short the output with a crowbar with the amps being driven to full power and there may be a momentary hiccup, but there's no smoke, no flames, hardly even a burp. More importantly, you can plug any speaker load you happen to find into them, and no combination of loads or lack thereof will have me muttering as I solder in new output devices. Or replace tubes. My tube amps are stable under no-load. I ...um... have that protection resistor in there that I told you about.

The TV amps are special. I keep them in a kind of sheltered workshop for sentimental reasons.

QuoteAnd common sense tells me not to use a long speaker cable, so it's physically impossible for it to be run over.

Kewl.
This "common sense"  you speak of - is that what you'd call "unstated assumptions" if I used it? Yeah, I thought so.

But back on topic, you're going to use nice, tidy, short speaker cables that will just ...barely... reach from amp to speaker, no trailing wire to roll over or step over here, uh-uh! That's great, until someone trips and knocks the amp off where it's sitting, or knocks the speakers over and breaks the plug in the jack, ...involuntarily... shorting your speakers.

QuoteI don't have a clue what tangent this is.

Yeah, not having a clue does seem to be a problem. Let me help.
You said:

QuoteMaybe a corporate engineer would lose his job by designing an SS amp for tone instead of short circuit protection "just in case".

The operant words being "for tone instead of short circuit protection". By that juxtaposition, you were trying to draw the picture that designing for tone was leaving short circuit protection out, and vice versa. You were attempting to present them as antagonists. So I said:

QuoteThere is no designing FOR tone and NOT FOR protection (again noting that "short circuit" has nothing to do with it, see above again). The two are not antagonistic concepts.

It may be possible that you see them as antagonistic because to you "tone" only happens on the edge of circuit failure. If that's the unstated condition in your mind, you need to do some more thinking.

Caught up yet, are we?

QuoteNo, tone doesn't happen only on the edge of failure.

OK, that's a step forward. I hear you saying "I can get tone without running my amp on the edge of failure." We'll come back to that.

QuoteBut with your mandatory protections, tone doesn't happen.

First of all, I never said they were mandatory. Smart to have, useful. Something I'd put in, yes. But mandatory? Gracious me, I'd never tell you what you could and could not have in an amp. I might snicker at the choices, but I'd try to do it in private to be polite.

QuoteAgain, since you keep avoiding the question, why don't you recommend these things for tube amps? Wouldn't having them produce a nice glichy sound be equally "great amplifier design?"

Avoiding the question? Not at all. I don't recommend them for tube amps because tube amps are not prone to the same kind of failures as SS amps. They are prone to a different class of failures, and didn't I already explain that one a few posts back?

But what you're trying to get me to admit is that I lump making glitches with protection circuits, and further that I think all amps should have glitches. Right? It won't work, because (a) I never said that, and if you misread it what way, go read again; (b) I have the opinion that well-designed protection circuits do not necessarily produce glitches in situations that are otherwise not damaging to the amp.

We have from you the following collection of statements:

QuoteHmm... yeah, maybe I'm lumping too much under the general term "protection circuit". It's the short circuit protection I primarily don't find very toneful.

QuoteOf course there is no sound during a short,

Quote... But - ok, assume I don't(know how safe area protection circuits work).

Quote"Yes", there are some situations where what you are calling "safe-area protection" can prevent an amp from dying.

Let me synopsize some things you've written:
You've used SS amps for a long time.
You are OK with the failure rate you get with them.
You have heard some SS amps with protection circuits in them that you don't like the sound of.
You generalize from that last statement that all protection circuits are bad and should be avoided.
You do not understand short circuit protection circuits or safe area protection circuits.
You admit there are situations where short circuit protection and safe area protection can save your amp from being damaged.

I think the rest of this is not liking being called on it. You'd sure like to paint me as a guy who has no real practical knowledge, is a follow-the-leader automaton, and is technically trained and therefore must be wrong. As you yourself said, reality is fact - theory isn't. The reality is that your theories don't match amplifiers or me.

When you can design a heatsink so the power devices don't burn up, when you can design a multi-slope safe area protection circuit so a set of speakers won't activate it, when you can design and build a transformer driven totem pole output stage will be stable over initial component variance, thermal drift and aging, when you can count off some of the problems with believing a circuit simulator is telling the truth, when... oh, never mind. When you get to any of these and want to know more, drop me an email. I'd be glad to help you with some of the fine points.

QuoteI have no problems understanding how short circuit protection works and have no doubt that it will prevent excessive current flow during a short of the output.

Kewl. OK, we have one point. You will accept that there are situations where protection circuitry works and can save an amp.

But you say you understand how it works. Tell us. 

QuoteBut you're missing my point - obviously blinded by the delusion that more complexity makes you a better engineer. I'm sure you can recommend much simpler alternatives to solve the "problem" of the user shorting the output of an SS amp - without affecting the sound while providing protection. And I'm just as sure you have a "technically superior" reason why they won't work as well as those that do affect the sound.

But I think you have a problem there. First, you don't know me, and so making the leap that I'm blinded by anything, even delusion, into a love of complexity shows that you're babbling again. Fact is, the simple overcurrent clamp is very, very difficult to improve on for short circuit protection. I don't have a solution to protecting from shorts that is simpler. And I don't have a solution to protecting from short circuits while not affecting the sound.

I think, in fact, that we can logically prove that's impossible. If an output is shorted, there is no sound, by definition. The short has seen to that. So once the short starts, sound ceases. What short circuit protection does is keep the amp from dying during the duration of the silence.

And this points up another issue with your argument. You do not realize that short circuit protection and safe area protection are not the same thing. I deliberately didn't correct that gap in your background for a few posts now.

Safe area protection is what keeps your amp from melting down under conditions where there is not a short, just too high a current-voltage product on an output device. The more reactive a speaker load, the more that output current and voltage diverge from being in-phase, and the more transient power which can be seen by the output device. For bipolar transistors - but not MOSFETs, that's another story - there is an are of voltage-current operation where the V*I product causes breakdowns at lower powers than any of the power rating, voltage rating, or current rating. This is the "second breakdown" region, and it's a fundamental limit on bipolar transistor longevity.

Safe area protection is what trips your amp's protection when it's not shorted. Notice that just like short circuit protection, your amp is already operating in a condition which will lead it to break if this goes on for a few microseconds too long. This, by the way, is as much fact as short circuit protection.

So let's go with question #2: Do you believe that there are situations where safe-area protection can prevent your amp from dying? Again, please try to limit yourself to yes or no. I noticed you were unsuccessful last time, just as I predicted. I'm guessing you'll do a LOT of dancing to avoid saying yes or no to this one.

QuoteJust like the only real reason to call a transformer driven totem pole design "unreliable" is to claim that it is an old design and not technically superior to modern designs.

There's that lack of a technical background thing again.

If we're going to discuss reliability on some basis other than "Wull, I used 'em atta way for thutty yars an' they nevah failed me yet!" we need to get to some basis for discussing reliability. I did power supply design for five years, field failure analysis for a year, and design-for-reliabilty reviews for power electronics for another year. What is your training and experience in analyzing power electronics for reliabilty? Is it just using them until they break or don't break yet?

Quote
Kind of like claiming a model T Ford is unreliable because it wouldn't survive a crash into a brick wall as well as a modern vehicle. The obvious solution is "don't do that!!"

The obvious answer is that very few people do that out of choice. "Just say no." went out with Nancy Reagan. It never worked with auto crashes. So given that you will INVOLUNTARILY be forced to crash into a brick wall, which one do you want, the model T or the modern vehicle. I know which choice I'd make. Your choice is what we used to call an "IQ test decision".

Besides, what you SEEM to be arguing for is your right to keep driving Model T's around in a large open field in which are a couple of brick walls while blindfolded, insisting that they will survive just fine because you haven't hit a brick wall yet.

I have to tell you - it's kinda fun to watch.   

Quote
Maybe a corporate engineer would lose his job by designing an SS amp for tone instead of short circuit protection "just in case".

I just have to go review my list of logical fallacies. The cleverness in that kind of statement is that if you get the answer without the person pointing out that NONE of the previous discussions talked about the hypothetical engineer designing an amp with or without protection circuits (note my correct use of protection circuits, not "short circuit") choosing to design NOT for tone. The point of shaping comments like that is to goad the other side into accepting the premise that if an engineer is designing for longevity under abusive conditions, he is designing tone out. It's one of the sneaky ways that politicians tar the opposition, and a standard trick.

There is no designing FOR tone and NOT FOR protection (again noting that "short circuit" has nothing to do with it, see above again). The two are not antagonistic concepts.

It may be possible that you see them as antagonistic because to you "tone" only happens on the edge of circuit failure. If that's the unstated condition in your mind, you need to do some more thinking.

QuoteAfter all, the corporate machine couldn't save money by using cheaper devices, couldn't downgrade the construction, and the amp might not make it through the warranty period before failing. The engineer also wouldn't be living up to his job description by failing to be "able to design and implement complex circuits that meet or exceed all specified cost constraints". Definite reasons for being fired - no doubt.

That's correct. How do you define doing your job? Do you want to damn the engineer for wanting the job, the company for wanting to make a profit (which, if you'll think it through, is the only reason you can have any amplifier at all), or both of them for not asking you first? At which of these stops do you get off?

QuoteHerd members in "good standing" just simply don't like mavericks - mavericks interfere with their erroneous and delusional perceptions about themselves.

So you're a member of the maverick herd, who pride themselves on not being in a herd, and all uniformly going their uniformly separate ways, right? 

I love it - rebel without a clue.

QuoteBut despite your well justified reasons to help protect my amp from me (and all I have to do is give up on good sound making another myth true); there are plenty of vintage SS amps out there, without short circuit protection, still providing great sound despite any opinion of unreliability by you.That sir is a FACT - but one you will deny cause I can't give you an all inclusive list of all the well satisfied users?

Nobody ever said that all amps without protection died instantly. I certainly didn't.

There are even a few of your model T Fords around that haven't been driven into brick walls. However, I would wager a considerable sum that if you drove one of the remaining Model Ts into a brick wall, either it would be scrapped, cannibalized, or there would be a hefty repair bill, which is what you're trying to deny. Sometimes people can walk across minefields and not get killed. It depends on the density of mines and what for lack of another word we'll call luck.

QuoteEven if I could, you would just chalk it up to a collection of opinions and not a "technical justification" - only because it doesn't match your opinion. You only accept hearsay as fact when it matches your opinion.

Actually, my "hearsay" comes from people who practice the art, write textbooks, teach, make a living by getting things to work - over and over and over. I guess that yes, you could call that a better grade of hearsay. Call it informed hearsay. Who you gonna bet on - the guys who do it all the time or the uninformed?

The race is not always to the swift nor the contest to the strong - but that's the way to bet.

QuoteI picked up one this weekend - for only $25 bucks. An EH dirt road special (now I don't have to build my own). I couldn't believe it and it was all an accident. It was in a pawn shop, in great cosmetic condition and the only problem was it didn't work. Only a faint hum sound when it was turned on. The guy at the store claimed his electronics tech estimated a $200 repair bill minimum and he just wanted to get rid of it.

About 5 minutes of debugging found the problem. No, it wasn't the output transistors or anything in the power amp section. Since I'll have to remove the board to replace the defective component, I'll post a picture of the output transistors and the heat sink - you'll just love it. Yes, despite no short circuit protection of any kind (I posted a schematic in another thread), AND it still has the original 1979 vintage output transistors.

The problem is the 4558 opamp in the preamplifier. The second side (pins 5,6,7) is hosed. It's run off a single polarity +15 supply with the non-inverting input biased at half. I get 7 1/2 volts at that input, but only 2 volts at the output and inverting input.

Don't these things have protection circuitry? Oh well, I guess it DID survive roughly 28 years with it.

So - how many others of those you got? They're kinda rare, aren't they? Not a whole lot of them survived?   

Quote

QuoteIf your definition of the designer not knowing his subject is that the sound made when the protection activates is unpleasant, then yes, I guess that you might say that. But then, how do you know that he didn't know his subject and saved you a trip to the techs? (Yes, be patient, we'll get to techs in a minute.)

Yes, that is my definition - ignorant or not caring about the tone his customer is after.

It amazes me that you can ignore the obvious - if the amp dies, there is no tone for the customer to care about. I think you must work in some elected office. You're fairly nimble at ducking an issue while making it seem like you answered. Most people like that have lots of practice.

I can tell you with some certainty that an audio design engineer that chose letting the amp destruct versus making an unpleasant sound for a moment would not be employed for long. You seem to think that designers with grudges against "real people" customers get all the work. It doesn't work that way.

But you don't want to talk about the technical issue that the unpleasant sound and not melting down are related do you? You'd like to think that any protection circuits whatsoever are useless, would you not?

So let's get this clearly
1. LJ, do you understand that a protection circuit activating can save your amp from being destroyed?
Please do your best to answer with yes or no. I realize you will not, but it'll be fun seeing you try not to.

If you answer yes, why not just own up to you'd rather pay repair bills?

If you answer no, can you give some *technical* justification for not believing it? It can be demonstrated, you know. When I say "technical" I mean "repeatably measurable or observable facts which can be verified by someone other than you". So far, your technical justification seems to be "well, I used to abuse SS amps all the time and they didn't die very often." That's not a technical justification. 

If it's just what you'd like to believe, that's OK too. But believing doesn't make it fact.

QuoteI say it again: If the short circuit protection causes some noises it is either designed poorly (most of them are) or it just saved your amp from blowing.

You're dead correct, teemuk. But I think you're wasting your time. I suspect that LJ will not turn up any backup info to support his opinion. He's long on opinion and short on fact. He'll never give you that last "or...".

You got some good advice early on that you haven't taken yet.

You can't debug fuse blowing problems very effectively if you only get 35 milliseconds of test time before the next one blows.

QuoteFirst: Before you turn the amp ON again build yourself a current limiter out of a light bulb that plugs between the amp and the main receptacle. It will prevent any further destruction.

http://repairfaq.ece.drexel.edu/sam/tvfaq.htm#tvtslbt
http://www.diyguitarist.com/PDF_Files/AmpCurrentLimiter.pdf

Go do it. There's a cleaned-up version of this at GEO (http://www.geofex.com) that is a little safer wiring as well.

But do it. That will let you quit tossing in fresh parts to wherever someone thinks you MIGHT have a problem.

There is a disciplined way to go about this.
1. Get the fuse blowing to stop so you can test voltages.
2. Test the power supply and get it running correctly, not blowing fuses
3. Remove the limiter and test individual pieces.

What you're doing in asking "Anybody know what it might be?" is asking to be told to replace a whole chain of expensive parts in hopes of eventually replacing the one or two failing ones by random luck.

Do the limiter, then get back here and we'll go to the next step. I've helped lots of people get running again with remote debugging. Maybe we can help you too.

QuoteDon't know.  Mine won't turn on because it blows the 500 mA fuse about as quick as a flash bulb when I try to power it up.  I know of several of these failed Frontman 15's.  I'm convinced they're truly a POS amp.

Well, there are several obvious design flaws I see in the schemo.

QuoteI never collected a list of them but, for example, Pritchard Amps website had some technical info about "Audio Myths & Truths"
http://www.pritchardamps.com/audio_myths.cfm
I think that one is the best of the ones I could provide if I thought it for a while...

I went and read it. Actually, I don't see any debunking or corrections there. Pritchard points out where Hamm was misinterpreted and where Hamm was talking about microphone amplifiers and may have missed some things like output stages, but I didn't see where he said Hamm was wrong.

Then too, Pritchard is in a frenzy to sell his own patented circuits and Pritchard Amps, so there's some marketing going on there as well.

So could you provide more pointers? I don't see any debunking there. In fact, I don't even see that any bunk was uncovered to de- .

The really pertinent question is - do you have the skills to connect up the AC power line wiring and also the low voltage power safely, so it will stay safe over the amp's life. If you do not already know how to do this, no forum is going to teach you.

If you are a newbie at electronics, it may be a mistake to start with such an ambitious project. You should perhaps do some smaller things first to develop your skills. I say this not to discourage you, but to be realistic - I don't want you to get killed trying to do this.

Usenet forums are in general useless. There are very few people on them that really do understand what they're talking about.

To answer your question, yes, it is entirely possible to wire up an amplifier on perforated electronics board - "perf-board" - and have it work reliably, but doing so is a skilled craft. You must build the skills first to be successful. Doing such an amplifier as a first project is not a good way to start.

Crackle on pots is often a sign that they have DC across them. If signal pots have DC on them, that's almost always a sign that something is wrong and letting DC through. Check the pots for DC voltage with a voltmeter.

QuoteR.G. - Have you always this paranoid about SS amps?

I might reply "LJ - Have you always been this paranoid about protection circuits and tube amp?" with just as much basis as you ask that. But the short answer is no. It took fixing several that had been smoked to make me appreciate an amp that not only works when things are good, but works when things are bad, too.

QuoteObviously your logic has a strong bias towards tube amps.

I don't think that follows at all. But your logic certainly has a strong bias towards SS amps. So maybe we're both dirty, eh?

QuoteWhy pretend that a "no plug in jack" resistor is the same as "open circuit protection".

Well, for one thing, that's what it is, so I guess I feel justified in my "pretense".

I don't know anyone who puts a "no plug in jack" resistor in because they like the colors. It's there for open circuit protection, no other reason. Or can you propose another reason for it to be there?

QuoteFor example, isn't it just as likely to have your tube amp speaker cord run over by a guy with a metal wheeled cart? And wouldn't that accident have roughly the same if not higher chance of producing an open as a short?

Actually, it will most likely produce first a short, then sometimes an open as well. But that's neither here nor there.

QuoteWhy would you accept the odd sound of protection circuits? Because when they start operating it means I'm abusing the amp? Sorry, but that definition of abuse rests with the designer that didn't know his subject - Amps of any type are routinely "abused" by overdriving them since the sixtys.

Let's rephrase that just slightly.

Because (a) you have been routinely abusing SS amps and/or seen others do so since the 60s and (b) some of the amps you abused didn't have protection circuits and lived through it and also (c) you also don't like the sound when a protection circuit activates, you conclude that (d) all amps would work fine and not go up in smoke when abused even if they did not have the protection circuits you don't like the sound of. It doesn't take much formal logic training to see that D is not implied by A and B and C.

Why would you accept an amp that just might go into meltdown if the output is shorted and live with paying for that if the amp can be economically repaired? Do you really mean that when an amp self protects and you only heard an ugliness, you think it would both not have sounded ugly and also not have died if only the protection circuits were not there?

How do you know that the only reason that amp you didn't like the sound of didn't suffer a meltdown is because the protection activated and made an ugly "squark" instead of a puff of doped silicon?

If your definition of the designer not knowing his subject is that the sound made when the protection activates is unpleasant, then yes, I guess that you might say that. But then, how do you know that he didn't know his subject and saved you a trip to the techs? (Yes, be patient, we'll get to techs in a minute.)

And then while we're working on not knowing the subject, there's that small matter of SS amps not caring if a speaker is connected or not and tube amps often not living through it. So maybe we oughta not toss around "knowing the subject" so much, eh? After all we're both "professional grade" people, right, not those beknighted, naive, bumpkins who pass as comsumers, right?

QuoteSince tube amps don't have protection circuits that corrupt the sound, does that imply that tube amps are not abused when overdriven?

No it doesn't. What it does imply is that SS amps are both (a) less able to withstand prolonged overloads and (b) cheap enough to make it reasonable to put in protection. There's a long history in the technical literature extolling just how few milliseconds it takes to melt holes in a silicon die if you get the right combination of wrong circumstances.

Once again, you're missing that logic thing.

Tube amps don't have protection circuits, sound corrupting or not, because (a) it was too expensive to design them in back when design with tubes was current and (b) tubes take larger overloads to melt down than transistors do. There's a long trail of technical literature on that one.

The long lag on overload and the cost of protection meant that no one could economically produce tube amps that way. But tubes do melt down when abused enough. Somewhere in my boxes of memorabilia I have a 6L6 with a conical indentation in the glass over part of the plate and a tiny, tiny circular hole at the bottom of the cone. The guy got it hot enough to melt the glass and let the air in. It just took a while.

The most common comment that tube amp techs hear when people bring in blow amps is "It sounded really great just before it blew out. Can you make it sound that way all the time?"

But it was a good try at backwards logic again. Does that work for you a lot?

Quote
If a blown tube can be accepted in persuit of tone, no reason that a blown transistor can't be equally accepted.

Of course it can. But you know, when I was fixing amps between semesters, and through a long association with repair guys over the decades, not one of them has told me that customer said that a SS amp sounded really great just before it blew out. Guys go to SS amps because they want them to be more reliable than tube amps. You see, I heard this story...

Quote
My favorite "tube tone that's not" story is Frank Morino of Mahogany Rush. An interview came out sometime during the early 1970s and one of the questions was "What amps do you use". His response was two Acoustic 260's. The next question from the obviously clueless interviewer was "Do you use any special tubes to help you get your sound?" I remember exactly how the response was written: "<pause> Uh... no. The 260's are solid state amps. Tubes are too unpredictable for me."

And while we're on that, I've seen guys retube between sets after losing one. Did you ever see someone replace his output transistors between sets?

I didn't think so.   

QuoteWhat's a tech? Oh, you mean those delusional self appointed guru's of tone that won't even look at an SS amp.

Had a bad time getting service, have we?   

QuoteI know engineers will tell me I need these things, but heck, I have been overdriving SS poweramps without them for over 35 years with great results - so nothing is going to convince me I need their tone defeating qualities this late in the game.

I think you're right - nothing will convince you. There is nothing as absolute as a closed mind.

Protection in overdrive is not what current and power limits are about. They're about avoiding repair bills. You ever have a repair bill? 

Before you get all up in arms, the Thomas Vox Beatle is one of my favorite amps. I have a section of my web page devoted to how to fix them and make them more reliable.

Quoteo Protection circuits are necessary for the ignorant consumer and manufacturers that cater to them. Professional level equipment do not need them.

You didn't play with some of the folks that were in my bands, I guess.

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I guess this is more of a statement than a question. But as an example, take short circuit protection. When is it really necessary? When the output is shorted. Ok, what kind of idiot would short the output? An ignorant consumer.

Or my old bass player. Or the guy who ran a metal wheeled cart over my speaker cable.

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Most professionals follow a simple rule that makes short circuit protection totally unnecessary: "Never power on an amp without a speaker load attached."

SS amps in general don't give a fig whether there's a load between rated and infinity (open circuit) attached.  And attaching a speaker to a SS amp does not protect it if there's a shorted cable, or jack, or if you get a beer spill.

QuoteCatering to people that ignore this rule (that pertains equally to solid state and tube amplifiers)

Actually, it's a tube amp only rule. Many tube amps have internal resistors that are auto connected as a protection against turning on without a speaker plugged in. Is that a tone sucker, too? Remember it only acts when the speaker is NOT plugged in. 

Quoteisn't necessary for yet another reason: output transistors are cheap. Tubes on the other hand are at least an order of magnitude more expensive, not to mention the output transformer - but I've yet to see a tube amp with "open circuit protection" which is more damaging to a tube amp than a short circuit (other than low wattage plate and screen resistors acting as fuses).

I've seen a number of tube amps with open circuit protectors in them. Usually this takes the form of a 100-270 ohm 2W resistor soldered onto the output jack, but there are others.

And here's a more important issue. Transistors are cheap these days. But leaving your amp with a tech will cost you a minimum of $50 plus parts, could easily go higher, and it will also cost you the amp not being yours to play with for a week or two while being fixed. Protection circuits may make the amp sound odd if you make it do things it should not be doing, but at least when you quit abusing it, it will start playing again without the week in the shop.

QuoteSo maybe the real question is: why use circuits designed to protect a device that is cheap to replace? Especially when they can adversely affect the sound.

Because it's expensive to have any professional equipment fail, both in terms of repairs and time, and neither of those are directly related to parts cost. 

QuoteMy favourite pick concerning tube tone myth would be Russell O. Hamm's famous "Tubes vs. Transistors" convention paper from 1974. Even today people like to refer to it as gosbel although most of it's content has been debunked or otherwise corrected.

Could you supply pointers to the debunking and corrections, please?

Just curious - why didn't you just use an LM3886 for the power amp? That's what I've done when running current feedback setups. It works fine. In this iteration of the current feedback where you're using the current feedback to in effect predistort the signal into the power amp you should have even less problems than normal with it.

It is very hard to come up with good reasons for making amps under 60W out of other than the LM3876 or 3886.

I've had a number of requests for this, and they're finally in.

Small Bear Electronics has hard copies of "PCB Layout For Musical Effects" in stock.

If your amp is making the faint hiss and it responds to the volume control, chances are the amp's fine.

Do you know that your guitar cord is good?

Is the volume knob on the guitar turned up to max?

QuoteHow can I find out about the parameters of any FET? I have seen datasheet information for the J201 somewhere on the internet, but how about other FETs such as the ones recommended by the shop?

Go to google.com and enter " (part number) datasheet". The (part number) should be the full part number of the FETs; what you were told is the shortened version. Those numbers should have "2S" in front of them, so look for 2SK170, 2SK246, and 2SK30.

The 2SK30 is a good choice for buffers in a 9V effects box.

QuoteOr, more importantly, if I do find datasheets for different FETs how to I compare them? What are the important parameters? (Perhaps I just have to spend more time with that theory document you linked to.)

Look for Vgsoff or "gate cutoff voltage", Idss, and Yds. Vgsoff is the voltage that when applied to the gate will cut off conduction completely. The J201 is unusual because it has a Vgsoff of only 0.1 to 0.5V, the smallest of any depletion mode JFET I've ever seen. The 2SK30 is 0.5 to 4.0 as I remember, and it's a good choice for self bias in a 9V environment.

The Idss is the biggest current the JFET will conduct without destructive voltages. It needs to be big enough to set up your buffer correctly. The J201 has a tiny Idss, really too small for good input buffer situations. The 201 is a good amplifier, less so a buffer. Again, the 30 is a good choice. I don't know about the others.

Yfs is the forward transconductance, the change in drain current per volt of change on the gate. It is the number which determines voltage gain in a gain circuit - which you're not doing - and how close to a gain of 1 you get in a source follower. It's not very critical in this application.

The parameters of JFETs vary about 5:1 to 10:1, so they are quite difficult to get exact biasing points on and exact gains. This is one reason they never got the wide application of bipolar transistors. In a source follower, though, the actual JFET is not as critical. You want a Vgsoff that's a significant fraction of the 9V battery, but not most of it or bigger than 9V. One around 4.5V is good for a 9V source follower.

Try the 2SK30A.

What you can do depends on your skills.

What amp repair skills do you have?

Signal ground and AC safety ground are different things. AC safety ground (on the AC input jack) should be connected to the metal chassis you have this thing in. Signal ground should be connected to the input jacks and to other signal ground places in the circuit and to the "ground" point in your power supply. There should be 1.00000 connections between the signal ground and safety ground. Construct your chassis, hook AC safety ground to it using proper safety grounding techniques, then run a wire from your signal ground star point to the AC safety ground.

The 10kuF caps are charged to about 30V each, so they're not really a hazardous voltage. They are a hazardous *current* though. No rings or watches on your hands when you reach in. Bridge a 10kuF cap charged to 30V with a gold ring and you just might get a third degree burn all around your finger, not to mention welding the ring to the cap.

If it were mine, I'd put about a half-watt of DC bleeder resistor across each one, then you could rely on the resistor to always run the caps down when the AC power is off. Then you can just turn off the power, wait a minute, and the caps are drained down, even if the circuit didn't drain them down - which it probably did.

Stay away from that AC power line voltage, though. That will kill you dead.