The Cold Clipping Stage

[Chuck H]

Thanks Enzo. I have considered just jumping in since I have reasonable trouble shooting and diagnostic skills. I just thought it would be a time saver to know a little more about common circuits so I can trace faster and recognise symptoms of specific system failures. Even here on the forum, when I follow along on the SS threads I can't remote diagnose the way others here can. That's going to make it hard to give bids and slower to do the troubleshooting. Though I figure it shouldn't be too lucrative for a guy like me with very little experience in repair. That would be insulting to the pros. My best guess is that I'll take a couple of scab jobs to see how it goes and decide from there. Also... I do have the forum here as my ace in the hole I know other techs are regulars here and use the forum to diagnose jobs commonly. I would hate to become an even bigger pain in the a$$ than I already am though

[diagrammatiks]

there's no magic.

the cold clipper is a very well understood stage.

pentodepress has everything you need to know about it.

the turn off characteristic of a transistor isn't the same as a tube during cold clipping.

the waveforms will look almost the same since they are both heavy asymmetrical clippers.

running a sinewave or a function isn't going to give you any useful information about how the 2 topologies differ.

[Enzo]

I have a running argument with a fellow. I tell him all tube amps are the same, and he says, "Nooooo, they are all different." I of course am a technician, and looking at an old Marshall or an old Fender Bassman, the differences are maybe a few component values. If I draw a 6L6 push pull output stage on a napkin, could anyone say what brand it might represent? He is focused on how one sounds different. But the circuits are all about the same, and all work alike.

Point being that if you know your way around a Fender Bassman, other than maybe specific model specific weak points, you;d approach ANY 6L6 push pull amp the same way to fix it. If a Traynor Bassmate came through your door, you wouldn;t say, "Oh, gee, I don;t know how to work on Traynors." Transistors are no different. The first time you try to figure out why that PV 400BH is blowing fuses, it may be confusing, but the next time you see one, you'll have better familiarity. Further, you will get a 400B/G one day and realize, HEY, this is almost identical to the 400BH. And soon ALL the Peavey power amps start to look familiar, because they are all designed the same way. Then you encounter a Crate something. Well the drawings look a little different, but it is kinda similar, and whatever, it works the same way. SO you fix it.

Then the next six things through the door are dumb little practice amp repairs - free money.


Do the first few hard ones take four hours to do an hour and a half repair? The customer's alternative was to ship /drive it to the mainland? SO charge them 2 hours, they're happy, and you write off the excess as education expense. And what took you two hours to learn is now learnt.


Geez, do I have to stand there yelling at you while you do pushups, cadet?

[Chuck H]

Sir. No Sir.

[J M Fahey]

I´m sure those mom and pop stores will have a couple broken beginner´s amps , 15W chipamp ones which do not deserve to be sent to Seattle for repair and won´t be serviced locally, whose certain destiny is the junk bin at the corner.
You know the type, a TDA something, a couple TL072/4558, a couple clipping diodes, maybe a reverb tank, *very* simple switching.
Ask for them, and start troubleshooting.
No pressure involved, nobody's waiting for them, you can take a month to repair them in your free time, who cares?
You won´t even need to know "transistors", consider them a series of "building blocks" (input stage/distortion/tone control/reverb/power amp/PSU/speaker and headphone connection/jacks/pots/switches) each of which can work ... or not.
Only when one of said blocks stops working you´ll have to look inside.
You are welcome to clear any doubts here.
Meanwhile keep doing what Sarge Enzo ordered

[Enzo]

Whole lot of SS amps have no transistors, just ICs.

[Steve Conner]

Yeah, transistors are unfortunately old hat now Our little 18 legged friends the TL072, LM3886, TDA2050 and so on, are taking over. Give it another 10 years and the standard practice amp will contain an A/D converter, a DSP and an integrated Class-D output stage. Probably all on the same chip...

The "building blocks" approach is very common in electronic design. Usually each block is an op-amp or some other kind of IC, and you arrange things so that the block behaves like how your EE textbook says. Then you can just chain a bunch of them together to achieve the desired result.

That doesn't work nearly so well with tube amps, where all of the "blocks" bleed into each other and interact in subtle ways.

[loudthud]

Chuck H, the biggest problem you will have is obtaining obscure parts. Some of them will have to be ordered and you will have to be disciplined enough to find somewhere that has the parts and get the orders placed in a timely fashion. If you order a few extras, you will have to be organized enough that you can find them the next time you need one.

[Chuck H]

Thanks guys. I didn't really want to hijack the thread but this is a great confidence booster. I mean, I've never done professional electronics repair (I did run an airless sprayer repair shop out of a paint store though) and I'm not nearly as familiar with the circuits as I could be. But I'm pretty sure I CAN do it. So if the option is me on a time lag or drive to Seattle, well... I'd still be offering a "service".

It's precicely because of the building block mentality that digital doesn't intimidate too much. A module works or it doesn't. The amplifiers fore and aft are just amplifiers. I'm sure I'm in for lot's of head scratching, but at 43 my body is telling me to find a way to earn a living that doesn't involve big ladders, rooftops and so much heavy labor. I don't mind painting but I know I can't do it forever. The opportunity is NOW whether I'm actually ready or not.

[Tom Phillips]

...The opportunity is NOW whether I'm actually ready or not.

I'd say that you are ready. Certainly "more ready" than some others who present themselves as a pro repair shop. I remember a thread in which the OP said something along the lines of "A customer brought me an amp that was doing X. I already replaced the output transformer but it didn't fix the problem. What should I replace next." It was obvious from the post that the OP did not understand the basics of circuit operation or the principles of troubleshooting and I felt sorry for the customer. I think that customer would have been well taked care of in your shop.

[Enzo]

Chuck, I'd send customers your way. It's just that, well, it's just that you're SUCH a thread hijacker. Y'know?

[soundmasterg]

Where in the NW are you Chuck H? I gather its probably up near Seattle, but was wondering where it is that there are no local repair shops?

I'm in Portland, Oregon area myself. We do have a local shop down here. He does good work, but is very expensive. There used to be another local shop that was cheaper and also did very good work, but when he left town for LA area, the other one raised his prices. There are the usual repair benches in some of the music stores, but those are hit or miss. There are a couple warranty repair places who work on tube as well as solid state and most of them aren't worth the print on their signs. Then there are some local amp gurus who fix things here and there. I don't advertise my repair services at all since I'm very busy with EE school, yet just from going to some local jams here and there and getting overflow from some of the others and word of mouth, I get a couple amp repairs and/or mods a month and sometimes have to turn repairs down due to school.

I'm sure you could work to get the word out before you actually decide to make the jump. Get some cards and flyers made up and hand them out at stores and other places where musicians congregate. Advertise in some local magazines or publications....down here we have Blues Notes for the blues scene and some people advertise in there for various things. Go down to some local blues jams and get to know people....basically get your name out there and if you repair some things for a reasonable price, you will get a reputation and it will just grow and pretty soon before you know it, you will have a business.

Greg

[Chuck H]

I'm on Whidbey island. I'll make up some cards. I'm probably not going to be a "shop" at first. Just a guy who fixes things. I'm taking JM's suggestion and I'll go to the couple of instrument retailers and fix some of the broken stuff they have on hand and then give them some cards.

[Gingertube]

All you need to know about transistors. They come in 3 types, NPN, PNP and NBG (No Bloody Good). The first 2 types are readily converted to the 3rd type.

Here is the culprit John Bardeen getting his just deserts for inventing the 3 legged fuse (picture from the Triodemafia).

Cheers,
Ian

[Steve Conner]

Oh, yeah those things. In our lab they get filed in the round cabinet along with the disintegrated circuits and smoke-emitting diodes. Some people Widlarize them to make sure they won't creep into the stock of good parts.

Most NBGs we encounter in the audio field are blown-up output transistors in low-end solid-state amps. This isn't because transistors are fundamentally unreliable, but because there are powerful commercial pressures towards using fewer transistors, smaller heatsinks and less protection circuits than needed to get the job done properly.

One example of this is in the "Hartley Peavey White Papers" available for free download from the Peavey site. Peavey got into solid-state early on, and Hartley just couldn't get his early amps to work reliably. Jack Sondermeyer from RCA's transistor division came to work for him, and his advice was: "Surely you don't believe our datasheets! They're full of B.S. and I know because I wrote them." He persuaded Hartley to use twice the number of transistors he thought he needed, and all was well. As far as I know, to their credit, Peavey have kept this approach to amp design ever since.

So, the guys who write transistor datasheets don't help, as they want to make the device look as rugged and capable as they can get away with. A classic gotcha for those used to designing with tubes is: A tube's rated power dissipation is what it can actually dissipate. But the figure on a transistor's datasheet is a theoretical one, if the transistor were attached to an infinite heatsink with a joint of zero thermal resistance. In practice, as the heatsink is finite sized and those insulating washers aren't particularly good conductors of heat, you can only get 1/2 to 1/3 of it.

So, in the end it works out that a good quality audio power transistor has about the same dissipation as a good audio power tube.