high powered PA amps.

[Mars Amp Repair]

I have a observation about the High Powered Transistor PA amps. I wonder if anyone else shares this 'feeling'.

First of all, I haven't found anyone who enjoys guitar amps & all the wonderful nuances that go with them (esp tube amps) who really 'wants' to work on the high powered transistor PA amps. Personally, I do them more or less under duress as part of the whole picture of providing as much of a one stop service facility as I can.

Also as anyone who works on them knows, you can charge decent labor as they sell for much more than your average guitar amp. However, the downside being if they're really blown up badly with the type of driver stages (like Yorkville) that have a quarter of a million little fusible resistors & driver transistors, you can really spend tons of time troubleshooting them. In that case I'd much rather repair a few guitar amps.

To me fundementally to me, they 'feel cold'. ..there's no warmth to a transistorized PA amp...not just in sound, but I guess their personality. They have no personality...other than a 'cold' unforgiving practical nature. Just a big heavy chunk of transformer, transistors, & metal, kinda like an iceberg.

God forbid if they have a switch mode power supply..then the devil himself may have spauned it.

just my 2 cents...glen

[Satamax]

Hey Glen, i don't do electronics for a living, friends have insisted that i take a Valvestate 8080, two or three years later, it's still not repaired and gathering dust. And a Mars G15, which i have opened once, and even old, i don't want to touch transistors! Yuck! I'm pretty lame when it comes to "scientific" or orderly testing, and you can't see a blown transistor or IC! Even PCB gives me shivers

[Mark Black]

My repair background is mostly in those PA amps (and lighting consoles/dimmers, and sound mixers, etc). There's something to be said for getting one up and running well again - some satisfaction in finding ALL the problems and bad parts. There is also satisfaction in knowing I am helping to help the customer to earn a living in many cases.

It's only in recent years I have been doing tube-based repair work and it does seem more fun generally.

On the other hand the PA amp crowd isn't as likely to re-visit you with requests for mods, complaints about how you ruined the mojo, that kind of thing. And it seems more often that the tube-amp repair isn't a make-or-break income-producing thing but more a matter of taste and desire. Pros usually have spares.

So yeah - different vibes for sure, but both rewarding in their own ways.

My $0.02...

Mark

[Enzo]

No the PA crowd doesn't do Mojo. WHat they do is blame ANYTHING that goes wrong on the power amp. One channel comes and goes, it can't possibly be the mixer, crossover, limiter, graphic or anything else in the system. Especially not those cheap signal cables they are using. Can't be the crossover in the cab with the parts coming off.

I am OK with SS powr amps. Me and PV amps are now a Zen thing. I am ONE with the amp. Now WHERE do I hurt?

I have been fixing switching power supplies for close to 30 years, though not in audio for most of that. I don't mind them. I still rebuild the ones in the little GK amps. Most switcher powered amps will just shut down if a channel blows rather than burn alive.

I like working on them frankly. Used to think I was pretty good on the Yamahas, but I haven't seen one in years now. Yorkvilles are a pain with their 11.2 and 247 ohm resistors and stuff. But I fly through PV amps. Once I had a blown up XR600 power amp, and it was 8:45PM - and we closed at 9. I thought to myself Oh why not open it up and see what it needs... 11 minutes later it was running on the burn-in bench.

SS power amps don't sound like tube guitar amps - or any guitar amps for that matter - but then a pork chop don't taste like an egg. EAch has its place.

[drewl]

When I got out of school all I knew was SS, I had to learn tube stuff on my own, so SS came alot easier...
The problem I see (especially on RF amps and equipment that I do in the real world) is alot of the newer stuff is so cheaply made that if something does blow up, it's almost not woth it to repair.
The mfg's usually just throw in a new board, it's cheaper than fixing a blown one.
Alot of the switching power supplies I fix at my real job are made ( or used to be) alot more robust than newer stuff.

We'll have to wait for Enzo to chime in, because it seems he's the master at this stuff.

[Mars Amp Repair]

Yeah I hear ya all,
I am speaking specifically of the 200 watt plus PA power amps...not the the ususal SS 100watt or so guitar amps.
I agree Enzo, the xr600's are pretty straight forward with very few driver components compared to some of the other stuff out there. Still any of these hi powered amps can be a challenge, esp with some of the weird bias issues you can have ie:with one leaky diode amongst the many...The really tenuous thing is that if something goes wrong while your trying to repair them, they can spontaneously self destruct & put you back at step one (esp with a switchmode that you can't variac down. As I said, I do them, but only because I want to offer one stop.
I also agree that there is a sense of accomplishment to solving any issue, but sometimes it can come at a tremendous time price.
Recently, I actually repaired an old Onkyo stereo amp from the early 80's . Brought back fond memories of the hayday of consumer audio when they really made the stuff to last. It had a separate power transformer & supply for each channel. It was really fun back then when quality seemed to be the foremost concern.
Funny how all those Japanese transistor numbers all came back in my memory, eventho I'm lucky to remember what I had for dinner yesterday!

As for the high powered switcher pwr supplies, we used to fix the little ones in VCR's years ago...pretty simple at that size. But the ones like the one in the Yorkville A4.4....Hell, Yorkville won't even fix them.

Interesting take on everyone's part....glen

[stokes]

I hate the thought of working on SS stuff at all,and like Max said,even PCB's give me the heebie jeebies,although if it is connected to tubes I will.I do this as a sideline,so I am able to pick and choose to some degree,although I got involved with a local music store here and it is starting to get like a full time job,and he is always pushing me to do SS stuff he gets in,I guess one day I should get my head out of the '50's and take the plunge,but not now.

[Enzo]

Someone dumped a drink into the switcher on a Mega Crest amp - I forget the model. Instant charcoal. I sent it to them and they laughed at it. SO I bought a new power supply for the amp at $400 and some. I sold the amp for a couple grand so it was OK.

SOme of the power FETs they use in high power switchers are darned expensive, so it pays to know what you are doing. Not always sure that I do.


When I was a kid learning electronics it was all tubes. Popular Electronics and other similar magzines had some transistor projects, but they were always simple little things like COde Practice Oscillator (for learnign your Morse COde) or maybe a simple one transistor RF booster for the short wave.

That was the mid 1950s. By the late 1960s and early 1970s I was rock and rolling but most of our stuff was tubes. I bumped heads with the PA mixer a little. Then I went to work for a coin-op amusement company, working on pinballs, jukeboxes, and Arcade video games - at the time Pong was a hot game. I had to learn logic, TTL and CMOS, and then a couple years later microprocessors came into use. SO I went from knowing tubes right into knowing logic - I skipped over transistors. I had to go back and fill in on them. Seems funny in retrospect.

[Mars Amp Repair]

Yep,
My experience as Enzo & I have spoken about was pretty linear. I'm 54, so my experience really started in the mid 60's with heathkit stuff, building radios & stuff. Used to pull tube radios out of neighbors trash & try to fix them...somehow I did not really knowing what the hell I was doing. It was good that my folks really didn't know the kind of voltages I was dealing with..they probably would have stopped me. It was ok, tho....learned a healthy respect for the juice at an early age!

Then came the 70's w/rock & roll fixing my buddies Marshalls while working at a TV repair shop from age 15 1/2 to about 19 while ...then an audio repair shop thru the 70's & then audio/VCR repair from the early 80's thru about 1998.
The early VCR's were very much discrete multivibrator ckts as well as all the discrete flip-flop IC built logic ckts along with microprocessors. Of course in the 90's, VCR's were getting to be one big board with about 3 flatpack processors & very little support circuitry. Throw-away time came quickly.

Working presently for AT&T doing a totally non-repair job while building my own shop up to go full time soon. Gotta do something for myself now. g

[Steve Conner]

Sure, SS amps sound and feel cold and unforgiving, or whatever the original poster said. That's what they're designed to do. They're just interchangeable commodity units of power that take a small signal and make it bigger. Kind of like the amp equivalent of a Happy Meal. If you want warmth and forgiveness, put a tube preamp in front of it.

If you want quirks, funky sounding distortion, tweed, hot smells, mojo, and other kinds of personality, of course tubes are better. However, solid-state circuits can be made to emulate all of this to a fair extent.

But when you're plugging together a big PA with active crossovers, do you really want your power amps to compress and distort? Probably not, because the theory of crossovers is based on linear amps and linear drivers. Bottom line IMO: a tube amp is part of your instrument, a SS amp is part of the PA, and so those high powered SS amps are built the way PA guys want them.

I built a solid-state bass amp, a hybrid guitar amp with tube preamp and transistor output stage, and a MOSFET hi-fi amp before I started homebrewing tube amps. Not because I didn't like tubes, but because I got into the hobby as a student and couldn't afford output transformers I currently design lab equipment for a living, and yes, it is stuffed full of microcontrollers and MOSFETs and powered by switched-mode supplies with not a tube in sight. But I still love tubes for their unique characteristics.

I'm currently thinking of building a SVT-sized tube bass amp with a switched-mode power supply just to annoy purists ;-)

[Enzo]

Oh come on, go all the way. Make the power stage digital as well. Don't stop at the power suply.

[Steve Conner]

Point taken It is pretty tempting though, when you can get a 400W Class-D power amp with matching switched mode power supply that all fits into something the size of a VHS tape:

http://www.coldamp.com/opencms/openc...html?idioma=en

I think it would be a real challenge to get the mojo of a tube preamp to pass through one of these, though...

[drewl]

Enzo, a little side track here.....
I know you know your stuff with the GK's, have you seen any of the new output board retro-fit go bad yet?
Got one in the other day...haven't got to it yet. (It's a 400RB)

[Arthur B.]

Originally Posted by Mars Amp Repair

To me fundementally to me, they 'feel cold'. ..there's no warmth to a transistorized PA amp...not just in sound, but I guess their personality. They have no personality...other than a 'cold' unforgiving practical nature. Just a big heavy chunk of transformer, transistors, & metal, kinda like an iceberg.

Part of that could be attibuted to the use of cold biased output stages with lots of NFB to keep the distortion in check.

[Steve Conner]

Yes, I heard bad things about that kind of distortion too. I don't know what they do in PA amps, but I biased my hi-fi amp to about 30 watts per channel idle dissipation. (250mA of idle current on +/-50V rails.) It runs hot enough that it needs a fan, but I can't measure any of that nasty high-order crossover distortion.

http://scopeboy.com/amp1.html

[Enzo]

The ONLY GK retrofit I have had any contact with was the switching power supply replacement with older linear style when they stopped supporting the switcher. COnsidering the cost,we opted to continue fixing the switchers. If they added a couple output xstrs, they would not be the first. Even Carver updated the PM1.5 by adding more xstrs. ANd often as not lower power models in a given brand line are the same boards minus some out the power components.

PA amps are like soldiers or marching bands marching in step. You want to see row after row moving in unison. You do NOT want to see individuals each with their own little swagger and nuanced step. What goes in comes out the same, just louder.

In SS PA amps, we don't gain any benefit by running things a little hotter. We want to run it right to the point where xover notch disappears

[drewl]

On the GK's they take out the entire power supply/output amp board and replace it with a a totally different design that uses high power TO3 transistors.
I should take a pic of it.

[R.G.]

Originally Posted by Mars Amp Repair

I have a observation about the High Powered Transistor PA amps. I wonder if anyone else shares this 'feeling'.

I've never met a guitar amp tech that liked working on them.

Originally Posted by Mars Amp Repair

First of all, I haven't found anyone who enjoys guitar amps & all the wonderful nuances that go with them (esp tube amps) who really 'wants' to work on the high powered transistor PA amps. Personally, I do them more or less under duress as part of the whole picture of providing as much of a one stop service facility as I can.

Two different personality sub-types, I suspect.

Originally Posted by Mars Amp Repair

Also as anyone who works on them knows, you can charge decent labor as they sell for much more than your average guitar amp. However, the downside being if they're really blown up badly with the type of driver stages (like Yorkville) that have a quarter of a million little fusible resistors & driver transistors, you can really spend tons of time troubleshooting them. In that case I'd much rather repair a few guitar amps.

High powered SS PA amps are a fundamentally different beast. You average tube guitar amp is forgiving. It'll live through momentary abuse just fine. High powered SS PA amps are precision machinery, and usually designed right out to the edge to wring all the watts/$ out that can be had. They're unforgiving in that they work almost perfectly right up to the line. Push them one millimeter over the line and they start a chain of destruction from the outputs back into the amp. Hair-triggered.

When a SS power amp comes in, it's safe to assume that unless it's something really trivial, you're going to replace most of the semiconductors from the output stage back to the voltage amp transistor.

Originally Posted by Mars Amp Repair

To me fundementally to me, they 'feel cold'. ..there's no warmth to a transistorized PA amp...not just in sound, but I guess their personality. They have no personality...other than a 'cold' unforgiving practical nature. Just a big heavy chunk of transformer, transistors, & metal, kinda like an iceberg.

That's an interesting anthropomorphism, and I can see how you get there. SS PAs are accurate. Again, I think of them as precision machinery. They reproduce whatever you put in very cleanly indeed. At least until you push them one iota further than they can go. They don't do the second and third harmonic triode thing to warm up the sound.

I think of tube guitar amps as replying to your input with "Here- I warmed that up for you a bit. How's this sound?"

I think of SS PA amps as replying "OK. You asked for it, you got it. I hope you knew what you wanted."

Originally Posted by Mars Amp Repair

God forbid if they have a switch mode power supply..then the devil himself may have spauned it.

When I was designing switching power supplies, we did always call them power surprises.

[Mars Amp Repair]

yeah,
I agree with you on most levels. When I say they 'feel cold', I'm not referring to the sound...I mean how you 'feel' working on them..as you pointed out..they're very unforgiving..take my wife...someone please...DAH DUMP!....rim shot.

I guess it's like you said how they're very unforgiving...Seems like most of us would rather work on a guitar amp...more flexibility involved & also a chance for us to play engineer from time to time.

I have found many occassions where I'll have the one or 2 bad O transistors bad & everything else being fine...of course I like those. You still typically have to unsolder each one to determine which one is the shorted culprit.

I have found with some amps that I can connect my junction tester & scope & gain the scope waaayy up & determine what is shorted without unsoldering...not completely reliable, but does help isolate the bad eggs sometimes. It's great help finding the bad drivers & other stage xistors with a quick check. You don't have to be concerned with the polarity of your meter. Then once you've isolated a bad one, you can give it the cursory meter diode check.

The junciton tester is basically a transformer stepped down to about 24v with a current limiting resistor is series to one of the scope probes. The other probe just connects to the other secondary (as I recall). The scope is set to X/Y mode. Perhaps you're familiar with this simple tester. I think some scopes at one point incorporated them internally.

glen

[Enzo]

The guitar amp gives a tech the opportunity to put his own fingerprint on the thing. "I modded it thus and so and now it sounds..." The high powered PA amp offers no such. "I fixed it, I made it sound like all the rest."

If you have a row of power xstrs and ONE is shorted, then here is the trick. Well two tricks. If the short is from base to collector, most likely ALL of them are shot. If the typical emitter to collector short is the problem, usually only one is bad, then the confusion is from the fact they are in parallel, but they are not really. Each has an emitter resistor in series, and if they are .33 ohm then there is a .66 ohm resistance between emitters, not a dead short. The collectors are all wired together, but the emitters are wired through the resistors. Put one meter lead on the collectors, and now go down the row and measure resistance at each emitter pin. If they all read like 1 ohm, but one shows 0.2 ohms, THAT is the bad one. The good ones measure open to the meter, but the emitter lead has a false path out the emitter resistor there, and back up the emitter resistor to the shorted one. The actual shorted transitor does not have the false path, it is just shorted.

[Mars Amp Repair]

Enzo,
Couldn't agree more about the 'fingerprint'. Guitarists are also very appreciative when you get their mojo amp back up & working, cause they're probably using the 2nd choice while ur repairing the one.
No one ever seems to get excited about getting one of their power amps back up & working. It's merely a utility to their needs.

Great suggestion on figuring the bad out transistor in a row of them. I'll be using that one. glen