50 Plus Master Volume

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50 Plus Master Volume

Postby Dick Dackray on Thu Feb 20, 2014 6:09 pm

I have one of these that I purchase new in the early 70's. I also have an original and purchase new 6/10 cabinet that came with the head when I bought them both together.

For years now, I been using a Tom Sholz Power Soak so I can use this rig in smaller venues.

My question is, is there a good master volume mod that can be done that works similar to the one I have on my 1978 Marshall 2203. If there is, does someone have a schematic?
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Re: 50 Plus Master Volume

Postby Mr. Foxen on Thu Feb 20, 2014 7:22 pm

If it has an effects/echo loop, might be easiest getting one that inserts there.
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Re: 50 Plus Master Volume

Postby Dick Dackray on Thu Feb 20, 2014 9:46 pm

Nope, don't have a loop.
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Re: 50 Plus Master Volume

Postby Papa Dog on Thu Feb 20, 2014 11:29 pm

I do not know of any schematics showing the mod you are after.

However, there are a couple ways that can be accomplished.
Does you armp have the sensitivity switch? Does it have a reverb loop?
Those features came and went over the course of the Mk4 series, depending on the sub-series.

At any rate, with a bit more information I can tell you exactly how to do a master volume similar to a 2203. PPIMV is a whole nother kettle o'fish.
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Re: 50 Plus Master Volume

Postby Dick Dackray on Fri Feb 21, 2014 4:50 pm

I don't have a reverb loop but I do have the sensitivity switch on the back.

Sorry but what's PPIMV?
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Re: 50 Plus Master Volume

Postby Baron Von Machinenmann on Fri Feb 21, 2014 5:15 pm

Post Phase Inverter Master Volume.

There are a few different versions of that type that could be installed into your amp by a qualified tech. :thumbsup:
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Re: 50 Plus Master Volume

Postby Papa Dog on Fri Feb 21, 2014 5:54 pm

BVM is correct...there are several post phase inverter master volume mods, but not specifically for the Sound City Mk4 series.
Mk4s have a slightly different arrangement of the phase inverter which makes the effect of such a master behave different to what it would on a Fender or Marshall.

The mnain thing to remember is that pre-phase inverter master will give you more pre-amp overdrive (simetrical) and posrt phase inverter will overdrive the phase inverter giving what is often called "power tube" distortion, but that is not exactly accurate. It crreates balanced overdrive, which gives one more odd harmonics.

So...depending on what you are looking for you cna choose either pre or post phase inverter.
If you are using the sensitivity switch you could do the mod I have suggested to others for repalcing the switch with a 22k - 25k pot for variable attenuation vs two selections. OTOH you could remove s4 and 4r6, put r32 and c22 to ground, and replace R28 with a 250kA pot. Wiper to grid, pin one to wire coming from r27, pin 3 to ground.
Put that pot where the sensitivity switch is now and you wont have to punch holes in the chassis. Keep the old parts and it can be put back "stock" if you want to sell it.

All references to part designations are from this schematic:
Image
sc_lb50_plus_schem
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Re: 50 Plus Master Volume

Postby Dick Dackray on Fri Feb 21, 2014 8:44 pm

So you are saying that I can just replace the two position switch with a pot and use that as a master volume. That sound quite simple
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Re: 50 Plus Master Volume

Postby Papa Dog on Fri Feb 21, 2014 9:07 pm

Dick Dackray wrote:So you are saying that I can just replace the two position switch with a pot and use that as a master volume. That sound quite simple


not exactly.
you have to remove the trimmer in that circuit as well. s4 and vr6. put the 22k-25k pot between the tail of c22/r32 and ground, wired as a rheostat. It works but I dunno if that will give you the effect you want....it makes the amp quieter fer sure though.
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