Negative Feedback Question

  • Thread starter Thread starter Cap217
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Cap217 said:
I have an amp that doesn’t really have the aggressiveness to it that I expect. It has a depth .0047 on it. 47k resistor nfb. But the depth is wired off the impedance common, not the 4ohm.

The depth knob is also sensitive. I can only turn it up between 2 and 4 (out of 10, not o’clock) before it gets muddy and woffy.


Would this be the first place to start before worrying about values?
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If you want more aggression, I would increase the NFB resistor in series with the depth. I’ve done this in several Ceriatones for people in 5 minutes and it’s a night and day difference. Raising it. For me, I don’t like anything lower than 68k with a depth circuit. You could also lower the .0047 to .0022uf
 
scottosan said:
If you want more aggression, I would increase the NFB resistor in series with the depth. I’ve done this in several Ceriatones for people in 5 minutes and it’s a night and day difference. Raising it. For me, I don’t like anything lower than 68k with a depth circuit.
Click to expand...

What about Moving the nfb off 4ohm tap?
 
Cap217 said:
When adding a depth knob, how does that make a difference in filtering? Does all the filtering happen on the pot with the .0047uf cap? I see a difference of no depth knob and it goes 4ohm tap to 47k with 220/.0047 to 5k presence knob. But with a depth it goes 4ohm tap, depth knob pot 1 meg with a .0047 to a 47k resistor to a 5k presence knob.

So is the .0047 on the depth knob making a 220k/.0047 not needed?


And should a depth knob come off the 4ohm tap vs the common? I have an amp with a depth knob that comes off the common of the impedance selector. Id like to make it more aggressive sounding so would moving it to 4ohm help?
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The pot replaces the 220k value and makes the RC filter adjustable. You’re shifting the 3dB or half power point around by adjusting the pot.
 
Cap217 said:
What about Moving the nfb off 4ohm tap?
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Every tap is 3dB less NFB with 4ohm being the least, 16 being the most. Think about it as how many winds each tap has on the secondary of the OT and how much of that tap passes signal back into the phase inverter. The more NFB the less gain you’ll have but the wider the amp will sound. Too much NFB can make an amp feel stiff and not as dynamic, there’s a balance.
 
glpg80 said:
Every tap is 3dB less NFB with 4ohm being the least, 16 being the most. Think about it as how many winds each tap has on the secondary of the OT and how much of that tap passes signal back into the phase inverter. The more NFB the less gain you’ll have but the wider the amp will sound. Too much NFB can make an amp feel stiff and not as dynamic, there’s a balance.
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So if it’s in the common (I say common but it’s adjusting that whatever ohm you selected) currently and I play on 8ohm cabs. I think it’s a little dark or muddy. Then putting it in the 4 will open it up slightly, right?


But if I played on 16 ohm cabs and then did the 4 ohm tap, that’s a big jump ?
 
You can really fine tune with a (resistor<->cap) in parallel with the resistor or pot.

 
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scottosan said:
If you want more aggression, I would increase the NFB resistor in series with the depth. I’ve done this in several Ceriatones for people in 5 minutes and it’s a night and day difference. Raising it. For me, I don’t like anything lower than 68k with a depth circuit. You could also lower the .0047 to .0022uf
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I will try this.

From 47k to 68k, it will be a little brighter and aggressive?

What does changing the .0047 to a .0022 on the pot Do in terms of sound and feel?
 
Cap217 said:
I will try this.

From 47k to 68k, it will be a little brighter and aggressive?

What does changing the .0047 to a .0022 on the pot Do in terms of sound and feel?
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For a 100w, I personally like 100k, 68k would be lowest to go before getting that blanketed sound. The .0022 should shift the frequencies and make the flub a little more manageable
 
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scottosan said:
For a 100w, I personally like 100k, 68k would be lowest to go before getting that blanketed sound. The .022uf should shift the frequencies and make the flub a little more manageable
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0.0022 will act like a mid boost
0.022 will manage flub
 
Cap217 said:
I follow, thanks.

So 220k/.0047 woks on what frequency? Is that how you look at it? This must be a standard because a lot of circuits use this.


For depth circuit, the higher the nfb resistor, the brighter the tone. So is 47k standard? If I want brighter I go to 100k or darker 33k. Are those big jumps that are noticeable? If you really wanted to push the low mids and lows, what would I look to use if I have a 47k now?
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The higher you increase that .0047 cap, the lower the frequency. On my Jet City SLO-based modded amp, I used a .0068 because I liked the lower frequency. That is the value the 5150 uses, and seems to chug more. The stock .0047 was more of that cardboard thud type low end because it gets more up into that 350-450hz range.
 
I think by far the easiest route in your situation is to attach the NFB tap directly to the 8 ohm tap, add a 100k linear pot in series with the NFB resistor, then a 1 meg audio in series after that. Adjust the 100kl pot to where you like it the best. Read the resistance and add that to the NFB resistor value to come up with the the preferred value. Replace the NFB resistor with that value. With the 1 meg pot, alligator clip capacitors across it and try .0022, .0033. 0047 and .0068. Those are common values but you can in theory try anything you want. For example, some guys use .001 to tighten things considerably. I don't like that value, but it's been used.

I think you asked this earlier....you would never use a 220k/.0047 and a 1 meg pot with a .0047 at the same time. The 220k/.0047 replaces the depth pot as a fixed value, approximating a 1 meg audio pot at around 1 o clock. You can use a 470k/.0047 for more fixed depth. It's all subjective and nothing is concrete.
 
Shask said:
The higher you increase that .0047 cap, the lower the frequency. On my Jet City SLO-based modded amp, I used a .0068 because I liked the lower frequency. That is the value the 5150 uses, and seems to chug more. The stock .0047 was more of that cardboard thud type low end because it gets more up into that 350-450hz range.
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On the Friedman be100 deluxe the thump has a 3 way. Does anyone know the 3 frequencies there? I assume that’s just a cap value switch.
 
Cap217 said:
On the Friedman be100 deluxe the thump has a 3 way. Does anyone know the 3 frequencies there? I assume that’s just a cap value switch.
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.0022/ .0047/ .0068

Get a 3 way toggle. .0022 to the two center lugs, one more .0022 on one outer set of lugs and a .0047 on the other side. You'll have .0022 in center, .0047 one way and .0068 the other. You're just parralleling caps.

If you want the switch to be .0022 on one end and not in the middle, use an on/on/on and use the corner lugs. That way the smallest value won't be in the middle of the switch but far left and the values will go .0022/.0047/.0068 as you you switch to the right. Or you can do it right to left. In this scenario you aren't paralleling so you'd use the actual 3 cap values on the switch.
 
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RedPlated said:
.0022/ .0047/ .0068

Get a 3 way toggle. .0022 to the two center lugs, one more .0022 on one outer set of lugs and a .0047 on the other side. You'll have .0022 in center, .0047 one way and .0068 the other. You're just parralleling caps.

If you want the switch to be .0022 on one end and not in the middle, use an on/on/on and use the corner lugs. That way the smallest value won't be in the middle of the switch but far left and the values will go .0022/.0047/.0068 as you you switch to the right. Or you can do it right to left. In this scenario you aren't paralleling so you'd use the actual 3 cap values on the switch.
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I should pull out a deluxe and see what I like best maybe and just live with that. I know when I used the deluxe it was to the left and I never really came off of it. But I like the idea of a 3 way. I just don’t want to drill holes on this amp. I’d like it to be as original as possible.

What is the voice Switch on a be100? There is not a depth knob
 
You can use a 1 meg Alpha push pull bass pot. Switch built into the pot. Can only put two values on it. .0047 and .0068 for example. No holes drilled.
 
RedPlated said:
You can use a 1 meg Alpha push pull bass pot. Switch built into the pot. Can only put two values on it. .0047 and .0068 for example. No holes drilled.
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Of course. Thanks. I need to think about what I want, get the parts, and then talk with you a little more.
 
Cap217 said:
Of course. Thanks. I need to think about what I want, get the parts, and then talk with you a little more.
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Anytime.

In that scenario, I'd install the fixed depth mod, 220k/.0047, inline with the NFB wire. Put a .0022 cap on the switch, run ground shielded 2-conductor cable from switch to the 220k/.0047 and attach the two wires to the capacitor. This way you really shouldn't have any unusual noise or popping. Of course any of these values can be adjusted to taste.
 
RedPlated said:
Anytime.

In that scenario, I'd install the fixed depth mod, 220k/.0047, inline with the NFB wire. Put a .0022 cap on the switch, run ground shielded 2-conductor cable from switch to the 220k/.0047 and attach the two wires to the capacitor. This way you really shouldn't have any unusual noise or popping. Of course any of these values can be adjusted to taste.
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The depth control pot is already installed. I like the idea of leaving everything but maybe putting a push pull with a different value there to choose. Then see how it is on the 4ohm tap and stock 47k. Then switch to a 100k maybe if I’m still not happy. Would be nice to have a switchable resistor switch there without holes. Not sure I can do that.
 
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