Amp internals

[Danny1969]

"Never seen any balanced in's or outs on any guitar amp so not sure what you mean there ?"

Exactly! Gittamp ins and outs are never balanced and so if they all emerged from the front there is every possibility of I/O coupling and the consequent screamies.

Even more insidious is ultrasonic instability which you cannot hear but is doing your valves in (actually wouldn't need to be that high since few guitar speakers chuck much out above 8-10kHz) .

For PA setups I have used a 90V neon across 100volt lines then I got hold of a really nice dB scaled meter movement and with OA91s in it that read into the megabicycles. Of course we were running 1/2 mile mic and 100yard + speaker lines, in the wet!
And that kit was balanced.

Dave.

Dave
I never suggested I used XLR connectors for signal because they can be wired balanced, I only use them because they are a far more reliable connector for audio than a 1\4 jack .....

As guitarist we just accept that amps use jacks for everything but after 30 years of gigging I've found more reliable and easier ways of doing things .

[ecc83]

I don't like kettle leads for gigging equipment because there's nothing to lock it in ... which means it might not be pushed all the way home which can cause arch'ing, or it might fall out. The Neutrik Powercon locking mains connector is much better and fits a standard 30mm cutout.

My ideal amp would have XLR connectors for all signal, Speakon for speakers, Powercon for mains and no connectors on rear at all .... as we almost always gig with the amp close to a wall so you can't see behind the amp let alone get to anything : )

It was that point about all the connectors on the front that I was refering to. And you can get right angled IEC leads as well as jack plugs you know?

Dave.

[ICBM]

And you can get right angled IEC leads as well as jack plugs you know?

Which with perfect conformity to Sod's Law, always point the wrong way!

[ManWithoutSkin]

Not a stupid question at all and the answer is yes, it is not only possible it is/has been done since time immoral! Called "reverse engineering" if you want the polite phrase but unless you intend to rip off and market the "Blenkisop metal ballcrusher mkll" there is little point. In fact the broadbrush circuitry of valve guitar amps is so well known and similar* that there really is nothing much new under the sun.
Well, not at first glance perhaps but the devil, as they say, is in the detail. Within the handful of triodes (and A pentode!) that you find in gitamps there is a vast range of voicings and choices as to how you arrange gain blocks. Getting these details how the designer wants them (I won't say "right" because the definitive guitar amplifier does not exist) is the result of many hours toil and cut/play/solder/curse/tea/fag/cut/solder.....

*There is probably more variation in the circuitry of valved hi fi amps. But then the aims are different. A much wider power bandwidth is required especially below 100Hz, need big, expensive traffs for that. Low distortion demands a lot of feed back and so stabiltiy becomes an issue and noise should be inaudible in normal operation, can't say that about many guitar amps!

Dave.

Pretty much, if you have both a layout picture and a circuit diagram. There are a few components that are hard to reverse-engineer from that unless the specs are written on the schematic (eg output transformers), but as Dave said it's not rocket science, and most of the building blocks are well-enough known that you could make at the very least a well-educated guess and be in the same ballpark.

Obviously the more complicated an amp the harder it is - eg trying to reverse-engineer something like a Mesa from photos and even the schematic would be quite hard, since the PCB layout is not all that obvious in many places without stripping all the components off. Traditional designs are quite easy.

There are a *lot* of small-builder amps which are either exact of very near copies of classic simple circuits like the Fender 5E3 Deluxe or the Fender 5F6-A Bassman/Marshall JTM45 - which are the same thing, in an early example of reverse-engineering! Although Marshall used a very different output transformer, resulting in quite a different-sounding amp.

It's pretty crappy in a way, as I'd imagine companies spend so much money on developing new products that anyone can just then look at the circuit and then rip off, but I guess then I guess it also keeps companies on their toes as it leaves them open to others coming along and improving on any shoddy designs. Thanks for answering my question though!

[misterg]

Indeed.

Amps modded like this are, in view, the worst to work on as you first have to trace the circuit out before you embark on any repairs.

And unlike a production amp, you can't even assume it was ever working at all.

You wouldn't like this, then:



Luckily, there's only one of them

Andy

[jpfamps]

You wouldn't like this, then:



Luckily, there's only one of them

Andy

That doesn't fill me with dread; looks like it would be fairly straight forward to work on.

The amp I find a real pain are where components are soldered directly together so that changing one component requires that several others are removed.

By the way are you using diode biasing?

[misterg]

By the way are you using diode biasing?

Only on the tremolo oscillator (the other LED is an amplitude limiter for the trem).

[cacofonix]

And here's inside one of ours.

Nice.

[cacofonix]

More ampage.

Some of those PCB amps look really well designed.

http://www.rig-talk.com/forum/viewto...4211&p=1452784

[ICBM]

More ampage.

Some of those PCB amps look really well designed.

http://www.rig-talk.com/forum/viewto...4211&p=1452784

And some less so.

Some of the modded amps are truly nasty hack jobs too, including a few with famous names on them...