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Post PI, Master Volume control.


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4/30/2006 6:08 PM
Humbucker
Post PI, Master Volume control.
I was wondering if this might be the best way to wire a master volume control on a Fender Bassman clone? From what I understand this method works quite well. I found this schematic over at The Blue Guitar and it's drawn by Bruce Collins.  
 
Linky:  
www.blueguitar.org/new/schem/misc_mod/pi-m-vol.pdf  
 
I don't have a 1 Meg dual-gang pot but I do have a 500K dual-gang pot in my parts bin. According to Bruce Collins, you can use either a 1 Meg or a 500K for this purpose. What would be the difference in sound/performance by using a 500K rather than a 1 Meg pot?  
 
Also, since there's actually two 0.1uF/600V caps in series after the PI, doesn't that roll-off a lot of bass, or does that 33K-100K resistor on the MV wiper keep everything on an even keel?  
 
One more question, when this Master Volume pot is set at maximum, does it pretty much preserve the overall, original, un-modded tone of the amp?
 
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4/30/2006 5:50 PM
Satamax
You should be fine with a dual 500. I know that there's suposedely better post PI masters, i cna't remember real well, but Bruce Collins was saying i think that he was using a dual 100K nowadays. You have the crossline master volume too, where a pot insulates between the two post PI legs, and as you lower the resistance of this pot, volumes drops due to the cancelation of out of phase signals. Actualy i have started a discution about this subject which i'd like to adapt to tone controls!  
 
Bye.  
 
Max.
 
4/30/2006 6:30 PM
Bruce / Mission Amps

I still use the 500K and 250K pots.  
The reason I use the larger .1uF caps in my dual gang MV setups is that when the wiper and the top lugs of the pots are connected (as they would be with the MV turned up all the way loud and nearly no attenuation) the two .1uF caps are actually in series.  
These means the total coupling capacitance is actually .05uF or less depending on what value of caps you use.  
You can use a .1uF cap and a .047uF cap on the wiper if you want for less bass response.  
The resistors on the wiper help negate that effect and I only illustrated the resistors in that artwork as it was years and years ago while Steve Ahola and I were emailing back and forth on how I was tuning my amps with a MV after the phase inverter.  
It's still my favorite MV for boosting preamp distortion.  
My Aurora amps have a MV too but in front of the PI and it is not really to gain preamp distortion.  
I mostly use that MV just to lighten the signal up a bit as the audio hits the phase inverter input which can be driven into distrtion if not cared for.  
 
Bruce
 
4/30/2006 8:06 PM
Humbucker

Thanks Satamax!  
 
And thanks also to you Bruce, speak of the devil and he shall appear! Excellent!!!  
8^)  
 
The amp that I'm planning on adding this master volume control to is a 1969-1970 Traynor YBA-1 BassMaster amp, basically a Fender Bassman design but with a pair of 6CA7 power tubes.  
 
On this amp, there's a 0.1uF/600V cap between the PI and the power tubes. So, if I go ahead and put another 0.1uF cap in series with it like on your drawing, I'm going to lose some low end, right? To keep the same amount of low end after doing this MV mod, I'd have to put two 0.2uF caps in the place of the 0.1uF caps, correct? But maybe that bit of bass roll-off won't even be detectable?  
 
I've noticed that Marshall often used a 0.022uF cap in this location, so I'm probably okay, since I'll have the equivalent of 0.05uF once this mod is done, still twice as much as a lot of Marshalls. There shouldn't be a deficit of low end after this mod, would you agree?  
 
BTW Bruce, nice job on that drawing! I was also having a look around your website, very neat, you've got some really nice looking amps and the kits look great too.  
 
Thanks for your help guys!
 
4/30/2006 8:09 PM
Humbucker

Oh yeah,  
 
I also wanted to ask you about the 33K-100K resistors on the pot wipers. When using a 500K dual-gang pot, would you tend to go with the 100K resistors or would the lower value resistors work better?
 
5/1/2006 4:45 AM
Humbucker
Is a linear pot okay for the MV control?
It turns out that the 500K dual pot that I have on hand is a Linear type, does the MV pot really need to be an Audio pot?
 
5/2/2006 2:56 AM
Bruce / Mission Amps
Re: Post PI, Master Volume control.
I'd use 33K to 56K max.  
The stage can very very slightly dull the overall tone if you use too large of value of resistor on the wipers.  
The larger the resistor, the more isolated the two caps in series are and the less they will sound like two caps in series when the MV is not being used so try it with no resistor too for max effect of the two series caps.  
 
Bruce
 

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