my mg100dfx problem

[Enzo]

The speaker return to ground is through that fractional ohm resistor. The output signal current flows through it, causing a small voltage drop across it that follows the signal. SO a small portion of the signal is sampled off across it and sent back to the inverting input of the power amp. Not really much different from a resistor off the output transformer secondary running back to the phase inverter tube in a tube amp. In short, it is your NFB.


Here I am much more concerned about his R14, than his R15, but it is worth checking. R14 is his stability network.

[Jazz P Bass]

R14

Thanks Enzo. Now it is clear.
I cannot seem to locate R14 on the schematic.

[Enzo]

Upper right of the power supply page - where the 15 pin CON3 is labeled TO POWER AMPLIFIER P.C.B.

R14 is just to the left of pins 10,11,12 on that connector.

[Jazz P Bass]

R14

Thanks again Enzo.
Is that whole circuit for stabilizing the output?
The 7293 shows a 22K R going back to the negative input, using the + for the signal.
Honestly, it looks Greek to me.

[Enzo]

Look at the top of that section, the input. See it is labeled PAIN. Then over on the 15 pin connector pin 4 you see STBY. That stands for Stab You. The whole point of this circuit is you get a stabbing pain if you think about it too much.

I'd have to sit down and think out what each part is doing there, and I am not up to it at the moment, I am digesting my chicken pot pie and bisquits from Bob Evans. R14 is the main stability resistor here. Yes, I would say the rest of that bunch of stuff there is also for stability and response shaping and gain settling, but an open R14 can result in the 7293 becoming a power oscillator which will burn its little self out trying too hard.

[Jazz P Bass]

R 14

Thanks again Enzo.
Might want to get my glasses checked.
Is that whole circuit the Stability Network. Looks alien to me.
The 7293 datasheet shows a 22k back to the - input, using the + for signal.
Not having the amp in front of me, what is switch 4 all about.

[Steve Conner]

R15 and associated components are for current feedback. This raises the output impedance of the amp, which supposedly makes it sound more "tubey".

As Enzo or someone said, if it burns open, all hell breaks loose. Current feedback is a standard trick used by most SS guitar amp makers, but Marshall came up with a clever new self-destructing implementation that reached its peak of perfection in the MF350.

[teemuk]

Thanks again Enzo.
Is that whole circuit for stabilizing the output?
The 7293 shows a 22K R going back to the negative input, using the + for the signal.
Honestly, it looks Greek to me.

R15 and R20 = current feedback path

R14 and C48 = “stability circuit” better known as Zobel network. This will compensate the inductance of a speaker load.

Rest = voltage feedback and Marshall’s FDD circuit. R11, R13 and C13 are likely there to maximise AC voltage gain and consequently to minimise DC voltage gain as the whole amplifier circuit is inverting and you can’t use the trick of capacitively coupling the negative feedback shunt path. R9 is a mandatory DC feedback path that tries to stabilise the DC gain.

The FDD is a circuit that tries to compensate the non-linear frequency response of a current-feedback amplifier driving a speaker load. When FDD is off, the amp’s frequency response is shaped by the circuit so that it becomes rather linear. When switched off, the current feedback will have more effect and the amplifier will have a higher voltage gain at higher load impedances.

[Jazz P Bass]

7293 -V on heatsink

My apologies for suggesting the thermal pad should be checked.
I did not have an amplifier in front of me.
I do now.
The back plane of the 7293 (-V voltage) is smack dab on the heat sink.
While this is excellent for heat transfer, the only thing keeping the -V from 0 volts is the isolators on the fan screws. The screws underneath the toilet seat lid caps.
Good thing to watch out for.
What an awesome amplifier.
I think the FDD switch pushed in gives it a Fendery sound.