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Master Volume ideas

1/9/2006 7:42 AM
Steve A.
Master Volume ideas
    As an alternate to a MV using a dual-ganged pot, I think that a fixed 1/4 power circuit would handle many of the needs of a master volume control. But instead of reducing the B+ voltage or adding a cathode resistor to the power tubes, why not just adapt one of the MV designs for a fixed low power switch? In particular I was wondering about the one in the Trainwreck Papers that uses a dual-ganged pot instead of the two 220k bias feed resistors. For 1/4 power would I replace the two 220k resistors with two 56k resistors? Or go even lower than that because as I recall KF recommended using a 100k dual ganged pot (which would make it something like 25k).  
    Along these lines, maybe use a rotary switch with maybe 3 or 4 different sets of resistors for different volume/power levels.  
    If the bias feed/MV design wouldn't work right, then how about the basic post-PI MV that uses a second set of coupling caps and a 1M dual ganged pot? Only with a few fixed resistors...  
    Any suggestions would be appreciated!  
Steve Ahola
1/10/2006 12:31 AM
Steve Dallman

The simplest post PI master is the cross line, which only takes one pot. Although for some reason, the dual ganged pot type sounds a little better in large amps to my ears, the cross line is effective and simple.  
I tried different values for pots over the years and settled on a modded 100k audio taper pot. I scraped a little of the carbon trace off the pot at the high end of the rotation...just like making a no-load tone pot. 100kA gave me the most useful and consistent control. I do know others who do similar only with a 250k pot.  
Cross line with resistors would be a snap.  
Fixed resistors in place of a dual ganged pot (not the bias feed replacement design) followed by coupling caps should do nicely. KOC suggests using coupling caps after the master that are 10 times the value of the stock coupling caps.
1/10/2006 2:45 AM
Rick Erickson

"Cross line with resistors would be a snap. "
I like the circuit Vox is using, adding a 10K resistor in series with each of the coupling caps from the PI before the cross-coupled Master. This circuit sounds smoother at low settings than without the resistors to my aging ears. It also keeps the PI from working into a direct short when the master is turned all the way down.  
1/10/2006 8:53 PM
Steve A.

Steve said:  
KOC suggests using coupling caps after the master that are 10 times the value of the stock coupling caps.  
    If you do the math you will see that adding a large cap like that will not change the total capacitance much at all (when added to the original capacitor). However, with a lot of the old Fenders using a 0.1uF coupling cap to the power tubes you would need to add a 1uF cap. :(  
    I look at the total capacitance of the two capacitors to make sure that it adds up to a value that is acceptable... like two 0.047uF caps in series which will add up to roughly 0.022uF, a good value for the coupling caps.  
    Question: one purpose of the added coupling caps is to keep dc voltage off the pot. However if you were to use switched resistors (and only when the amp was off or in standby mode) would you still need the extra coupling caps? Also if the amp was cathode biased, there should not be DC on the output tube grids so I'm not sure if you would need the extra set of coupling caps.  
    This is for a Deluxe Reverb Reissue, and there is not a lot of room inside to add a full Post-PI MV setup so I'm trying to find something that will fit in and work.  
Steve Ahola
1/14/2006 8:47 PM
Steve A.
Question on the Cross Line master...
    Can you add the leads to a cross line MV right at the tube socket? That would sure make it a lot easier to add to a Deluxe Reverb Reissue since I wouldn't have to screw around with the circuit boards...  
    I was thinking of adding a resistor in series with the modded 100k audio taper pot you recommended, since the low settings sound pretty bad. Something like an 18k resistor for a 100k pot.  
Although for some reason, the dual ganged pot type sounds a little better in large amps to my ears, the cross line is effective and simple.  
    The cross line MV shorts the two phases together, so at the lower settings the clean signal cancels out more than the distorted signal, so it can get kinda buzzy. The dual-ganged post-PI MV (with the extra pair of caps) shunts the two phases to ground, so you don't get the extra buzziness of the cross line MV.  
    I don't think that a MV has to be continually variable all of the way down to complete silence; I think that if you can dial back a 22 watt Deluxe Reverb to 2 watts or even 5 watts you are doing fine.  
Steve Ahola
2/18/2006 9:37 AM
Steve A.
Cross line MV didn't work in my PR!
    After putting two of these in Deluxe Reverb Reissues I figured I'd put one in my 1965 Princeton Reverb in the hole for the ground switch I no longer needed. I had already done the John Stokes, Paul C. and Rick Erickson mods, so the cathodyne PI was converted to fixed bias, and at a higher B+ voltage.  
    I added a 15k resistor in series with the 100k audio taper pot, but the control has very little effect on the playing volume of the amp. My guess is that the two phases are dissimilar enough that they don't cancel out as they normally do in guitar amp circuits. I've used cross line MV's with cathodyne PI's so it can't be just that. I even tried swapping the power tubes out, thinking that maybe one of them wasn't working.  
    FWIW at this point there are no grid stoppers on the 6V6's, although I'm not sure if that would make a difference. I figure that I'll probably add grid stoppers and the replace the cross line MV with a dual-ganged pot like the one Bruce Collins drew up.  
Steve Ahola
2/18/2006 12:31 PM
Weird, I have a Princeton (no reverb) and I've done the same mods and a crossline didn't work in that amp either.
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