Negative feedback on 50W variants.

[Dr d]

Some advice on this would be greatly appreciated. Should the value of the negative feedback resistor (output stage) on a 50W variant be greater ie. roughly double that of a 4K7 for say 100W #102 design. How do you decide on its value?

[ayan]

Some advice on this would be greatly appreciated. Should the value of the negative feedback resistor (output stage) on a 50W variant be greater ie. roughly double that of a 4K7 for say 100W #102 design. How do you decide on its value?

I guess we could talk about how the voltage appearing at the OT's secondary in a 50W amp is 1.4 times smaller than that which appears in a 100W amp, so you'd scale the feedback resistor accordingly (roughly making it 1.4 times smaller in the case of a 50W amp) to achieve the same level of feedback at the PI. Truth is that most people, e.g. Marshall and Mesa/Boogie that I could check, don't bother and what ends up happening is that, inherently, their 50W amps run with less feedback than their 100W counterparts.

Cheers,

Gil

[67plexi]

On the early 50 watt Dumble amps what I have seen is 320r-820r negative feedback resistor.
I would use a 4 watt rheostat @ 5k and let your ears decide, No presence control.
Only have one 50 watt build (#183) and used a 4k7 but it had a 2k presence control.
I guess a 2k2 would work fine with presence.

Steve.

[ayan]

On the early 50 watt Dumble amps what I have seen is 320r-820r negative feedback resistor.
I would use a 4 watt rheostat @ 5k and let your ears decide, No presence control.
Only have one 50 watt build (#183) and used a 4k7 but it had a 2k presence control.
I guess a 2k2 would work fine with presence.

Steve.

I guess going strictly by the math, a 3.3K would work "the same" in a 50W amp as a 4.7K in a 100W one, all other things being equal.

Gil

[martin manning]

Or, if you like, just move the NFB connection at the OT secondary "up" one tap from the 4 to the 8-ohm.

[ayan]

Or, if you like, just move the NFB connection at the OT secondary "up" one tap from the 4 to the 8-ohm.

True that.

G.

[Dr d]

Ok, thanks for that guys. I am using a bassman output transformer with a 4ohm tap and using a 3k3 resistor. I was confused by the info on the 101 schematic that recommended @ 8K! Thanks again for your help.

[sonicmojo]

I am curious to how the NFB resistor on the Bluesmaster(100k) with a 25K pot computes in the scheme of things. What would changing that value up or down do? Any insight for that amp design would be appreciated too. Mine is 50W and has the 100k.

Thanks

[odourboy]

I use 8.2K in the feedback loop and 1K in the presence circuit (with a 1 uF cap and 2K pot) tapping off the 4 ohm OT connection for my 50W builds. It's the ratio of the two resistors that matters.

[ayan]

I am curious to how the NFB resistor on the Bluesmaster(100k) with a 25K pot computes in the scheme of things. What would changing that value up or down do? Any insight for that amp design would be appreciated too. Mine is 50W and has the 100k.

Thanks

You need to look at the voltage divider in the feedback loop, which in the case of the BM is 100K with a 4.7K to ground. The voltage that appears in the OT's secondary is multiplied by 4.7/(100+4.7) = .045 and fed back to the PI. That's the crux of the feedback loop. Now, the 25K pot acts with the .1uF cap and creates a high pass filter around the 4.7 resistor. That means that highs are dumped to ground and therefore will not be fed back to the PI, and that increase in highs is what we call "presence."

Compare that to most other Dumbles, and you will have a 4.7K feedback resistor with a 390 ohm resistor to ground. The voltage fedback in this case will the the OT's secondary multiplied by 390/(390+4700) = .071. What this means is that he BM (with a .045 feedback " coefficient") has less NFB, which in part imparts the chartacter to that design.

Gil

[Dr d]

Thanks again for your help, guys...it makes better sense to me. I can see now that the 8K2 / 1K combo would be the same as 3K3 / 390R. Thanks again.