Soul control

[Fred G.]

I would like to put a variable negative feedback control in my Bandmaster - I believe this is called a "soul" control, right?

I want it to sweep from NO negative feedback up to the default feedback I have set up now (100K feedback resistor, 4.7k shunt ala Marshall).

Can somebody tell me what pot value, linear or audio, and hookup instructions?

Thanks!

Fred G.

[Regis]

I fooled around with the SC in my Bassman 100 rebuild and it was seriously underwhelming. Here is the schematic.

At first I thought there was something wrong because I couldn't hear any difference, but after a while there was a difference, but only slightly.

I recommend doing the Resonance Control mod, which I got from the old Ampage site. I put a page about it on my website, check it out here.

Except don't use a 1 meg pot, use something a lot smaller like a 50k. If you use the 1 meg pot all the change will be in the first little bit once you start turning the pot.

[Mouthurst]

Try a 5K for the soul control.

[loudthud]

For a resonance control, try a pot value about 10 times that of the exsisting feedback resistor. For the blackface era 820/100 combo, I suggest a 5K or 10K. Experiment with the cap value but something in the 0.1uF to 1uF range should work there. The problem with the 100K/4.7K setup in a Marshall is that there is significant DC across the 4.7K. In the Peavey 5150, the problem is solved with a 22uF cap to isolate the DC. Check online for a schematic. You might try a combination soul/resonance control. Just put a switch in series with the resonance control's cap or use a center off switch and two caps.

[Fred G.]

Thanks for all the input, fellers, BUT...

I'm NOT looking for info on a resonance control!

I want a pot to sweep the NFB in my Bandmaster, varying from NONE to the default 100k/4.7k which I now have it wired for.

Where did anybody get the notion I was wanting a resonance control????

[Regis]

I'm NOT looking for info on a resonance control!

I want a pot to sweep the NFB in my Bandmaster, varying from NONE to the default 100k/4.7k which I now have it wired for.

Where did anybody get the notion I was wanting a resonance control????

Gosh, pardon me for bothering you.

[Fred G.]

Regis,

You weren't bothering me, and I appreciate the good info - it's just not what I was asking.

Thanks,

Fred G.

[rathmann]

just put a variable resistor in series with the current feedback loop. When it's at zero you'll have the stock feedback. when maxed you will be reducing the amount of feedback. there is no way to truly open the NFB loop here unless you want to mod the pot and scratch off the semiconductor to create a nonconductive section. anyway experiment with different pot values. you'll notice that a some point there is enough resistance in the loop that it effectives behaves as if it were open.

the reason a resonance control was mentioned was likely due to it's similarily to the control just mentioned. it is exactly like the "soul" control only with a cap in parallel with the pot. if you use a push/pull pot you can even switch between the two modes.

steve

[Alex/Tubewonder]

Soul or as it is known by its technically correct name "damping factor ctrl" can not be properly implemented just by changing value of feedback resistor.
Have a look at this Heathkit schematic where it is implemented correctly.

[rathmann]

Hmm. Fair enough, but the W9 is not a guitar amp and the feedback loop incorporates the whole amp - not just the power section as is common in virtually every guitar amp.

The original poster wanted to go progressively from default NFB value to essentially "open".

Can you explain why this cannot be properly implemented in the way I've described? (Note, again, guitar amp != hi fi) Also am I correct in reading that as a dual ganged control with one 10ohm element and one 100ohm? That's not a very common item. Not too practical a recommendation.

[Alex/Tubewonder]

The schematic I referred to illustrates the principle. It doesn't matter if you apply it to "whole amp" or just power amp i.e. PI+Output stage. The distinction between Hi-Fi and guitar amps is in how different sections of a circuit are (mis)used not in design principles.

A ganged 10/100 ohm pot is easy to D.I.Y, get a 100/100 and a 10/10, take them apart and put together two 10/100 pots.

The idea is to keep input sensitivity roughly constant so that changing the damping factor (reducing/increasing NFB = increasing/decreasing closed loop gain) doesn't cause output volume to change significantly with same input signal level.

The Heathkit does what the original poster asked for: from no NFB to maximum designed NFB. This principle can be applied to ANY amplifier with NFB.

[rathmann]

The idea is to keep input sensitivity roughly constant so that changing the damping factor (reducing/increasing NFB = increasing/decreasing closed loop gain) doesn't cause output volume to change significantly with same input signal level.

Thanks very much for the explanation.