Master volume in my '71 SB

[Guv'nor]

OK, I have decided that I need a master volume in my '71 super bass to make it more "realistic".

Nedless to say I am taking it to my tech but any recommendation that I can forward to him? He's a good friend so he takes recommendations pretty well.

On the other hand I want to make it 100% removable and I can't figure where to put the pot. I am even considering something external.... don't know....

 

[superleadfixer]

I have probably the most dynamic MV for you, BUT there is many other ways to do them.....

You'll have to try and see which you prefer...

here it goes:

1) get a dual 250k audio pot (24mm alpha are way ok for this)
2) two 2.2Mohms 1/4w resistors
3) Some shield wire and regular 22gauge wire (just a few inches to a foot approx)

If you don't need you 2nd output jack (the one that goes to the speakers), remove one and install the pot there, that way you don't have to drill a hole in that very precious chassis....

What you need to do is remove completly the two 220k resistors from the board (bias) under the phase inverter, the ones between the two output coupling caps, then put a 2.2M resistor from each wiper (on the pot)to each output (where the bias voltage is supplied, pins 5 of the power tubes, one for each side only, use shield wire for that section, and connect the shielding to the pot...), then you avoid tube death after a possible wiper failure.

The coupling caps are connected to the input lug's of the pot (use shield here also with the shielding to the pot..)
The bias supply voltage (which usually feeds the both 220K) is connected to the output lug's of the pot (of course to both)
The pot's wipers are connected to the power tube's grids (or to their swamp resistors)
Now simply solder each a 2.2M on each section of the pot from the wiper to the output lug - a small 1/4W resistor will do it

These are only for safety, because if the pot's wiper fails, then the output tube's grid is w/o bias voltage and will immediately jump over the Jordan.

But with the resistor applied and a failing wiper, although this side can't be regulated anymore, but nevertheless there's bias voltage on the output's grid - and the tube(s) will survive.....

Make sure you connect the shieding of the shield wire at the pot side and run an unshield wire from there to were the two 220k met on the board...

When you'll play fully dimed it will act like there is no MV, you'll keep all the dinamics of the amp with that one also, very nice....

I think I have a diagram of that somewhere, I'll post it if I find it....

let me know if you need more details.....your tech should understand!! I hope...lol

 

[superleadfixer]

here is one exemple I found for it:

http://www.putfile.com/pic.php?img=3953067

 

[Guv'nor]

Thanks a lot!!! I will do it this way

 

[superleadfixer]

Thanks a lot!!! I will do it this way

Great, let me know how you like it!!

There is other ways also.........

 

[Guv'nor]

My friend told me this is a nice mod that can be improved by adding a couple of 250-500pf caps in the phase inverter to compensate for loss of bass generated when introducing the mv.

Sounds cool

 

[superleadfixer]

My friend told me this is a nice mod that can be improved by adding a couple of 250-500pf caps in the phase inverter to compensate for loss of bass generated when introducing the mv.

Sounds cool

Really? Like where???

 

[SWLABR]

Late to the party,I hope you didn't mod it yet.

What speakers are you using?

 

[coldsteal2]

I had two of my Plexi's taken into the shop in San Fran
back in the day for a master volume

it totaly ruined thier sound

 

[sincitymusic]

Is there a way to add an extra gain on a 1980 JMP MK II Lead 50?
Thanks,
Joe
sincitymusic