5E3 pre-PI Master Volume and Grid Stopper

[Fresh_Start]

I've come across multiple discussions regarding "nasty" distortion from a cathodyne PI. Saw it on scope with my Princeton Reverb several years ago. Solutions include a big grid stopper on the PI and/or fixed bias for the cathodyne.


Merlin Bledsoe advocates a 100K - 470K grid stopper Valve Wizard. Others report good results with this.


Guys also comment that the 5E3 circuit isn't "pedal friendly" because the PI can distort in a bad way before the power tubes when you put a dirt or boost pedal out front. So a Pre-PI master volume seems like a good way to balance preamp and power amp break-up points. Rob Robinette offers this approach:





Looks like a simple voltage divider. However, as the volume is rolled down, the grid return resistor value goes down while a larger grid stopper is introduced. IOW at max volume, you have the stock circuit but there are multiple things happening while the volume is turned down.


Looking at this drawing, does Option A retain the stock bias for the cathodyne or is the grid return resistor now effectively 1.5 meg?





Option B leaves the bias alone and inserts a 500K grid stopper.  But does it also attenuate the signal going into the cathodyne grid by 1/3?  (1meg / 500K + 1 meg)


I thought that a master volume control which maintained stock bias, at least at max volume, and had the maximum grid stopper value at max volume would be good. So I came up with this idea:





I do not know whether or not the resistor between the pot wiper and the cathodyne's cathode network should be 1 meg or 500K to maintain stock bias of the PI at max volume (wiper to the left - Option A above).


Guys over on the Music Electronics forum tell me that this control would have very little effect on volume. Unfortunately, I still do not understand WHY.


I greatly appreciate any insights you all can offer.


Respectfully,
Chip

[PRR]

> Option A  ...is the grid return resistor now effectively 1.5 meg?

Yes, and so?

There is no max resistance on 12AX7 sheets.

The "max" on 12AT7 12AU7 can be grossly violated when you "know" the tube can't hurt itself (such as 113K in series and no obscene-high supply voltage).

Just do it.

> Option B  ...does it also attenuate the signal going into the cathodyne grid by 1/3?

No.

The bottom of the 1Meg is boot-strapped by the cathode-follower gain. Assuming gain is >0.9, the 1Meg looks-like >10Meg.

But dinking with the numbers doesn't say much about the "sound". I suggest you put a few extra turrets in that area, grab your box-o-resistors and a hot iron, and try stuff. Unless you grab the wrong end of the iron, you can try this and that and the other faster than you can set-up the math, and then you have "the sound" at hand for study. (Your neighbors may not appreciate overdrive tests, but too bad for them. Or you can drag it up here and compete with Jimmy's hot-rods.)

[Fresh_Start]

Thanks PRR!

It was impact of the bootstrap on the effective value of the 1 meg resistor that I didn't understand. Still don't entirely, but I'll just add a few turrets to try several approaches and find out what sounds best.


The importance of Option A is that I can use a 500K fixed resistor attached to the pot wiper. Won't affect the voltage divider much but it may go from useless to marginal.

I'm waiting on parts to arrive and trying to narrow down the list of experiments while finishing the layout. 

Closest neighbor is 1/4 mile away and she can't hear anything without hearing aids anyway.
   


Cheers,
Chip

[eleventeen]

"Closest neighbor is 1/4 mile away and she can't hear anything without hearing aids anyway. "


Think of it as a challenge. 

[Fresh_Start]

"Closest neighbor is 1/4 mile away and she can't hear anything without hearing aids anyway. "


Think of it as a challenge. 

I think if I borrowed the Super Reverb I built for my buddy in Falmouth, put it on the back door stoop and turned it all the way up, she could hear it. If she were outside and there are no leaves on the trees. FuzzFace might help...

Challenge accepted!   


Chip