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previous: Ray Ivers Gil,Sounds like you... -- 7/12/2000 6:10 PM View Thread

Re: Trip + Presence + Why Dumble's Way?

7/12/2000 8:50 PM
Gil AyanRe: Trip + Presence + Why Dumble's Way?
Sounds like you had a blast!"
Thanks, we sure did.  
About the 'scaling ratio' of the Dumble circuit; the actual effective Marshall pot value is roughly 4K to 5K, Gil, not 22K."
I was talking about the physical pot itself. As you will see, the whole feedback loop is scaled:  
1. Feedback resistor: (M) 100K, (D) 4.7K (off which tap in the Marshall? Important point, and I don't know!)  
2. Ground resistor on feedabck side: (M) 4.7K, (D 390  
3. Pot: (M) 25K, (D) 2K  
4. Cap: (M) .1uF, (D) 1uF  
Even if the Dumble has roughyl twice the negative feedback, I wonder why the choice of "scaled down" values. The PI's "tail" is (M) 10K, (D) 24K...  
" The older Marshalls used the 'scratchy' circuit with the 5K pot passing the PI DC to ground, while the newer ones use a 22K variable resistance/cap series combination in parallel with a 4.7K resistor, to eliminate the DC thru the pot. So the Dumble circuit is using a lot more feedback, and the frequency breakpoint is much lower, than the Marshall 100K/5K/.1 uF circuit."
Right. But the breakpoint is not that different... In the extreme case, i.e. with the presence wide open, the Marshall sees 4.7K//.1uF, and the Dumble 390//1uF, the Dumble's frequency breakpoint being HIGHER, the way I see it...  
"Well, agreed. But look at the overall scheme, and the Marshall has a total of 10K I would think that you would set your PI static conditions and then adjust feedback loop values depending on desired presence control action/damping factor/etc. I would personally prefer to keep feedback loop resistances as low as possible, to minimize the chance of oscillation in the output stage."
I guess I don't see that quite in the same way. The feedback loop is negative, so having a higher impedance wouldn't result in more noise in this particular case, would it? I guess if it was a regular gain stage, sure, lower impedance would definitely mean more stability.  
As I said, I am not sure I readily see a good "reason" for doing things that way. :) Probably nothing more than Fender Blackface legacy, where the feedback resistors were all "scaled down" with respect to the Bassman, as well.  

Ray Ivers Gil,Yes, you're abs... -- 7/12/2000 9:55 PM
Steve Ahola Gil:  &nb... -- 7/13/2000 2:18 PM