Marshall MG Series?

[nickb]

Not same architecture:

Guv'nor, as good as it sounds, has nothing like that, and can easily be found unbearably buzzy , specially in so called "pedal unfriendly" amps.

EDIT: never learnt to simulate, can somebody please do and post resulting curves?
It's built around IC5 , everything between R13 and C48 .
Two active 12 dB/oct filters with a third passive 6dB/oct one sandwiched between them.
If this does not *drastically* alter squarewaves passing through it, don't know what will .
Thanks

I'll bite. I can't believe that there is something I can do that Juan can't!

Freq Resp:


Transient, Square Wave 440Hz


and 4.4Khz:




The schematic I used. I'm not too sure about some of those cap values but what I chose seem consistent for a sharp roll off around 10Khz:


It rolls of at a much higher frequency than I would have expected. There's a lesson there.

[J M Fahey]

Thanks nickb

Weeeiiiirddd!!!! , way higher than I would have expected

2 to 4 kHz would have been logic, maybe as low as 1500Hz, but ... 10kHz? ... guitar speakers roll off at about 24dB/oct above their typical high peak, which usually lives between 2kHz and 3.5kHz , in any case there's practically nothing left at 10kHz .

Maybe olddawg can check that the schematic values match what's actually on the PCB.

So far, a mystery

[teemuk]

"Single-channel" design so I guess a few compromises had to be made.

Yes, hi-cut filtering will probably remove nasty buzziness and improve -overdriven- tones...

...But in this "single-channel" design also -clean- tones pass through that filter and they, on the other hand, just easily "muddy" up.

Additionally, also reverb circuit is fed post that filter and I would think the effect would not sound that good if driven with too "middy" signal.

And then we have the optinal "EQ" tailored around the power amp and its "mixed-mode" feedback. Plus "EQ" introduced by the speaker system. We need to think how these interplay with the "preamp" response.

[nickb]

"Single-channel" design so I guess a few compromises had to be made.

Yes, hi-cut filtering will probably remove nasty buzziness and improve -overdriven- tones...

...But in this "single-channel" design also -clean- tones pass through that filter and they, on the other hand, just easily "muddy" up.

Additionally, also reverb circuit is fed post that filter and I would think the effect would not sound that good if driven with too "middy" signal.

And then we have the optinal "EQ" tailored around the power amp and its "mixed-mode" feedback. Plus "EQ" introduced by the speaker system. We need to think how these interplay with the "preamp" response.

Here it is with both the pre and power amps combined.

The effect of the top boost "FDD" switch is shown as is the current feedback. The current feedback adds quite a lot of low frequency boost and I suggest that is an important contribution to the sound.

The choice of switch "FDD" name has IMHO more to do with marketing than science.

The same caveats apply on the capacitance values as before.

[olddawg]

So .... I have been using this little beast a lot at practices because it is easy for my old aching ass to drag around and it sound pretty damn good for what it is. The only problem I have is that it doesn't have a clean channel/dirty channel foot switch. It does have a front panel switch. I've been thinking about an easy way to mod it for a foot switch, but the only way I can see is with complex relay. Any simpler, less time consuming, and less expensive suggestions? I'm just about ready to rig a solenoid to punch the switch with a rubber bumper, lol! There's a schematic in this thread.

[tedmich]

Some MG15 owners have mentioned this switch looks to be a DPDT with unused lugs, so maybe there is an easy way to remote actuate if you look closely at the switch?
Others suggest gluing a cork on the button and kicking it...

or this setup

no pesky relays...

[nickb]

So .... I have been using this little beast a lot at practices because it is easy for my old aching ass to drag around and it sound pretty damn good for what it is. The only problem I have is that it doesn't have a clean channel/dirty channel foot switch. It does have a front panel switch. I've been thinking about an easy way to mod it for a foot switch, but the only way I can see is with complex relay. Any simpler, less time consuming, and less expensive suggestions? I'm just about ready to rig a solenoid to punch the switch with a rubber bumper, lol! There's a schematic in this thread.

If you were to remove the switch and replace it with a DPDT footswitch at the end of two decent (i.e. good shield coverage) twin shielded cables that are not too long and inside a metal box also connected to one of the shields, it must just work. The worry is hum pickup but it would not be too hard to try it out and see.

[olddawg]

If you were to remove the switch and replace it with a DPDT footswitch at the end of two decent (i.e. good shield coverage) twin shielded cables that are not too long and inside a metal box also connected to one of the shields, it must just work. The worry is hum pickup but it would not be too hard to try it out and see.

I really don't think that would work. The switching is much more complex than that. It's not a simple SPST switch.

[g1]

It's shown on the schematic (see post #7) as a DPDT, 2 halves both are called SW1B. I think one of them should be called SW1A, so there seems to be a typo. One half shown between IC1A & IC1B, the other half at VR3 pot.
So I think what nickb suggested should work.

[olddawg]

Yeah .... Thanks for looking at it Nick and G1. I printed the schematic and took a fresh look at it. I have some perf board, a small 12vdc DPDT relay, a diode, and a 1/4" jack laying around in my junk. I may just see if it I can energize it from one of the 15v rails and use a standard footswitch as a project on a slow night. Not much more work than messing with all of the shielding and stuff. But I would have to take the front panel switch completely out of the circuit. Maybe put a status LED in the hole.

[nickb]

Yeah .... Thanks for looking at it Nick and G1. I printed the schematic and took a fresh look at it. I have some perf board, a small 12vdc DPDT relay, a diode, and a 1/4" jack laying around in my junk. I may just see if it I can energize it from one of the 15v rails and use a standard footswitch as a project on a slow night. Not much more work than messing with all of the shielding and stuff. But I would have to take the front panel switch completely out of the circuit. Maybe put a status LED in the hole.

Using the +15 to drive the relay will cause problems. The relay will draw so much current, around 41mA @ 15V for a 47W sub min type, that the +15V rail will no longer be regulated, it's max capability is ~25mA. It's better to use the main +V rail - around 24V I think. If you haven't got a 24V relay drop the excess using a 330 (or 390) ohm 1 watt resistor in series with the coil.

[tedmich]

TE's MT2 series of 2 form C relays with 150mW coils are quite useful where current is limited, only about $3 and are stocked at Mouser

C93401 5VDC
C93405 9VDC
C93402 12VDC
C93403 24VDC

They are only rated to 220VDC/250VAC but work well in that range

[olddawg]

Thanks guys... I meant to say the rail supplies before the regulation. It still might pop or load it down. If nothing else I can power the relay off of one of the 50 wall worts I have laying around and and be totally isolated. I have a box of relays. Many from VCR applications in the old days. I'm sure I can cobble something up.

[Roi]

Can the mg15cdr be moded for a JFet mute circuit?
Will such mode solve/reduce the power off thump?

[nickb]

Can the mg15cdr be moded for a JFet mute circuit?
Will such mode solve/reduce the power off thump?

Anything is possible but this may have a complex answer. I think you'll get more a more meaningful discussion if you start your own thread about it.