Post phase inverter master volume

[dude]

Can I use this master volume on this Ampeg schematic?

[sluckey]

yes

[stratele52]

This won't shut amp volume . A volume / master vol need one side of the pot connected to ground.

[sluckey]

This won't shut amp volume . A volume / master vol need one side of the pot connected to ground.

It certainly will. Been done many times. The pot is connected to the bias voltage supply which is at AC ground due to the filter caps.

[stratele52]

Sluckey , very strange circuit , I learn someting if you say it work.

But IMO there are better master volume circuit

[sluckey]

Sluckey , very strange circuit , I learn someting if you say it work.

But IMO there are better master volume circuit

It's as good as it gets. Been around a long time. Read this...

http://metroamp.com/wiki/index.php/Lar/Mar_PPI-MV

[dude]

Thanks,

Can I use a 1 meg A ganged pot? No 500KA on hand


al

[HotBluePlates]

Yes, that should still work fine.

[PRR]

> need one side of the pot connected to ground.

8uFd in bias supply is audio-ground enough.

Ground is not essential to killing a balanced amplifier. Strapping the two sides together will do it. Usually with side effects we don't like, but it works.

[Willabe]

Sluckey , very strange circuit

Think of it as a Vox cross line tone cut, but with out the tone cut cap so full frequency and a dual pot instead of a single pot. Feeding signal from both sides of the PP lines to each other kills the opposing signal.


              Brad        

[HotBluePlates]

No, don't think that because it's still a voltage divider like every other volume control.

It's just that the ground end of the pots are not connected to d.c. ground but to a cap which is itself connected to ground. For a.c., that's the same thing as if the pots had a connection to the chassis.

[Willabe]

Maybe I'm not seeing this right, but I was not talking about any signal going to ground whether it's ac or dc. The Vox cross line tone cut has no direct path to ground.

I was talking about out of phase cancellation of the ac signal caused by mixing the 2 opposing PP signals together. Just like the Vox cross line tone cut except no cap to limit the bottom end of the cut across the 2 signal lines. So full frequency cut = MV.

Even if you cut the connection from the 3 terminal (x end of the pots) to the junction of the -bias/220k grid leaks it would still phase cancel the signal to the output tubes.


                  Brad      

[SILVERGUN]

http://metroamp.com/wiki/index.php/Lar/Mar_PPI-MV

Watch out for that link....my Vipre active protection picked it up as a known bad exploit

[HotBluePlates]

I was talking about out of phase cancellation of the ac signal caused by mixing the 2 opposing PP signals together. Just like the Vox cross line tone cut ...

Redraw the entire circuit. Notice that at the minimum-volume setting, there exists the same resistance between the two drive signals as at the maximum volume setting (more really because the 220k resistors aren't shunting the pots).

In fact, redraw just one side of the push-pull circuit. Then I think you'll see how this is like any other volume control, except there are 2 of them and it's connected to the bias circuit. So it is not functionally like the Vox Cut control or Matchless master volume.

[darryl]

Even if you cut the connection from the 3 terminal (x end of the pots) to the junction of the -bias/220k grid leaks it would still phase cancel the signal to the out put tubes.

If you do cut the connection from the pot terminals labeled "3" to the bias circuit, then the pots will act as a cross-line master volume.

If you leave the connection in place, then AC signals at the "3" terminals are effectively connected to ground via an 8mfd electrolytic capacitor. There is no cross line phase cancellation, because the out-of-phase signals are isolated from each other by the shorting effect of the 8mfd cap.

[Willabe]

If you leave the connection in place, then AC signals at the "3" terminals are effectively connected to ground via an 8mfd electrolytic capacitor. There is no cross line phase cancellation, because the out-of-phase signals are isolated from each other by the shorting effect of the 8mfd cap.

Ahhh... Thanks Darryl, now I see it. I missed that.



                Brad        

[kagliostro]

It's just that the ground end of the pots are not connected to d.c. ground but to a cap which is itself connected to ground. For a.c., that's the same thing as if the pots had a connection to the chassis.

Is that architecture and cap the reason for which isn't necessary a second couple of capacitors between the 220k resistors and the MV pots as to avoid scratch due to DC, correct ?

K

[HotBluePlates]

No, because the output tube bias voltage is also coming through the master volume pot.

Seems like the existing 220kΩ resistors could be more like 2.2MΩ, as they're just there in case the wiper lifts from the resistive track in the pot.

Now I would normally wire up a master volume like what you showed... but then again I don't use master volumes much and don't know which is "best sounding" or not...