Amp internals

[loverocker]

Sorry if this is a stupid question, but could someone with knowledge of circuits like ecc83 or IBCM just take the chassis off an amp or look at a picture in detail, look at the circuit and then copy it

As mentioned above, it's a standard part of amp building practice. A lot easier on some amps than others.

Here's one that would need a LOT of hard and tiresome work. Made by one of the most respected builders of modern amps, this is a mid-70s Marshall with three extra preamp valves squeezed into the space between the original preamp valves, with a mind-boggling 3D arrangement of point-to-point components lashed into place.

[ICBM]

"Lashed" is the right description. That's a mess, no matter who did it. Components tacked together or to wires with no proper support is just bad workmanship.

It would have been much better to make a small circuit board to mount all that on next to the valves. Not particularly impressed with the hacking of the original circuit board either. I know "it's the sound that counts", but good workmanship and care matters too, in my opinion... you can have both, they aren't mutually exclusive.

[musophilr]

I've never plugged a speaker outlet into something expecting lower level signals. I can read the labelling alongside the socket that says "Output" or "Speaker" However I'm sure it can happen if you're in the dark, with the amplfier backed up against the wall. I usually check very carefully with my Bluesbreaker not to put the footswitch into a speaker outlet ...

I have learned to HATE connections at the back. Of Computers, HiFi, instrument amplifiers, you name it. It's ALWAYS a PITA.

Oh, and by the way - decriminalise Bulgin connectors! I've still got some on old Plexis and I don't need some Elfin Safety Jobsworth telling me the amp isn't fit to use

[jpfamps]

"Lashed" is the right description. That's a mess, no matter who did it. Components tacked together or to wires with no proper support is just bad workmanship.

It would have been much better to make a small circuit board to mount all that on next to the valves. Not particularly impressed with the hacking of the original circuit board either. I know "it's the sound that counts", but good workmanship and care matters too, in my opinion... you can have both, they aren't mutually exclusive.

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.

[Dadave]

Wienbrock?

correct

[jpfamps]

correct

Do I win a prize?

[ICBM]

Oh, and by the way - decriminalise Bulgin connectors! I've still got some on old Plexis and I don't need some Elfin Safety Jobsworth telling me the amp isn't fit to use

They were always crap and unreliable as well as dangerous though. They often make a poor contact to the pins so the amp can crackle or cut out, describing the cable retention as poor would be a compliment so the cord can easily get torn out, and the bakelite is brittle so they break if crushed by something, like a cabinet or a drummer .

And they really are dangerous - because they can easily break or come undone while the other end is plugged in, leaving exposed live connections.

I haven't tried these yet but they look like they'll do the job without needing to file the chassis (the usual problem with standard IEC sockets):

http://www.tubeampdoctor.com/product...dBLIYfK5xURed2

[jpfamps]

I haven't tried these yet but they look like they'll do the job without needing to file the chassis (the usual problem with standard IEC sockets):

http://www.tubeampdoctor.com/product...dBLIYfK5xURed2

I got some the other day.

Didn't fit directly in 1975 Marshall, so still required some filing, although not so much that you couldn't retrofit a Bulgin socket if you so wished.

It could be the cut out are different in other amps, so I will reserve judgement on them.

[ICBM]

Thanks. That was my guess as well since a standard IEC just will not go in without two notches for the 'square' corners, and I couldn't see how a socket could be made that would unless the wall thickness was incredibly thin.

It's got to be a better start than some of the hack jobs you see on them though.

[ecc83]

"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.