What's the best way to remove master volume from this build?

[markmalin]

Hi all
I'm working on my "Dumb-Lux" build, doing some tweaks.  The schematic is attached. 


For the sake of this question and simplifying things...in your mind think of this as simply a 2 channel AB763, NO Vibrato, and using Robinettes mod for adding reverb to both channels.  Both channels now pass their output signals to the reverb circuit via 220k mixing resistors, then into the Reverb Recovery (see picture of RobRob's reverb channel mod).  Now I add a Master Volume (100kA between Reverb Recovery stage and the input to the phase inverter.).  This leaves me with a coupling cap after the plate of the Reverb Recovery stage, and one after the Master at the input to the Phase Inverter.


Here's my question.  If I want to REMOVE the Master Volume from my circuit...I now have 2 caps in series.  A 0.1uf and a 0.01uf.  This may sound stupid, but can I simply remove one of those caps, or should there be a 220k "mixing resistor", for lack of a better word, in-between those two caps?


My confusion comes from having an AB763 NON Vibrato template, as it were, mapping a Dumble preamp into one of the channels, mixing both channels via RobRob's mod, and sticking a Master Volume after the Reverb Recovery.  This all works fine now, but when I remove the Master those 2 caps are sort of remnants of the original "mapping of the topology like an AB763 with no vibrato" and I'm out of my element as to whether "something" is still needed where the 100kA Master Volume pot is currently...


Can someone look at this and help me understand?


Humbly
Mark

[nandrewjackson]

I believe you still need 2 caps. The first to decouple the DC of the previous anode, then your master volume pot, then the 2nd cap will decouple any DC from the input of your P.I.

[shooter]

this MV????




if so, just put a jumper where the blue line is
EDIT: 
AND disconnect the pot

[markmalin]

this MV????
if so, just put a jumper where the blue line is
EDIT: 
AND disconnect the pot

Yes.  So I'd leave the 2 caps in series, correct?
I can do that.


FWIW I found a schematic I had gotten from Slucky a while ago where he had a MV like in my schematic.  He actually had a 220k resistor between the wiper of the master volume and the coupling cap from that node to the PI input.   

[SEL49]

Yes.  So I'd leave the 2 caps in series, correct?

That's correct. And there's one more thing to do since you don't have a 50K Tremolo Intensity pot. Connect a 47K resistor between the junction of those two caps and ground. This resistor mimics the missing 50K INT pot and will reduce the gain of the amp. Without it, the amp will sound much hairier than a real AB763 amp. Some people will actually put a switch in series with the 47K and use it for a gain boost.

PS... That 100K pot reduces the gain just like the 47K does. I chose to use a 100K MV pot rather than the 47K resistor in my amp.

[markmalin]

Yes.  So I'd leave the 2 caps in series, correct?

That's correct. And there's one more thing to do since you don't have a 50K Tremolo Intensity pot. Connect a 47K resistor between the junction of those two caps and ground. This resistor mimics the missing 50K INT pot and will reduce the gain of the amp. Without it, the amp will sound much hairier than a real AB763 amp. Some people will actually put a switch in series with the 47K and use it for a gain boost.

PS... That 100K pot reduces the gain just like the 47K does. I chose to use a 100K MV pot rather than the 47K resistor in my amp.

Thanks, this is super helpful.  I was wondering about the missing load from the vibrato.  That clears that up!


Mark