How does the OT 5K Primary Affect NFB?

Discussion in 'Shock Brother's DIY Amps' started by James Knox, Sep 24, 2020.

1. James KnoxTele-HolicSilver Supporter

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For example, on a 5F1 or 5F2A the stock NFB resistor is 22k for a 4ohm Speaker Load.

For the equivalent outcome with an 8ohm Speaker load, one would substitute a 33k (22k squared).

But if using the 5k OT tap rather than the 8K, what does the standard, stock NFB resistor need to be?

My ear likes the NFB around 56k, so I currently have a 33k and a 50k pot in series. This is a 5F2A with a 5881 using the 5K OT tap.

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4. andrewRneumannTele-Holic

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Think of the OT as having a “gain” just like any other stage of the amp. The gain is the winding ratio. The OT steps down voltage, so the gain is less than one. Actually quite a bit less than one. The voltage on the different taps is different because the winding ratio is different and therefor the gain is different.

Winding ratio is the SQRT(Impedance Ratio). So for what you call “stock” the winding ratio is SQRT(8000/4) = 44.7. The gain is the inverse of this. What I mean is, for every 44.7 volts across the primary, we only get 1 volt across the secondary. 44.7 volts in, 1 volt out. That 1 volt is also what shows up in the NFB ready to do business.

Ok, so if you change the impedance ratio, either by changing secondary impedance or by changing the primary impedance, you are changing the “gain” of the OT. So the new gain in your question is SQRT(5000/4) = 35.4. So for every 35.4V in, we get 1V out. That’s a better ratio—for the same input voltage we will get more output voltage compared to that 8K:4 ratio. How much more? 44.7/35.4 = 1.26 times more. So that’s 1.26 times more voltage ready to do business. We better increase the NFB resistor by 1.26 times to compensate for the higher voltage. How about an E24 value of 27K?

Edit: @Ten Over points out correctly in later post that I forgot to account for the change in gain of the power tube when its plate load is reduced. The power tube gain is reduced by a factor of approximately 5000/8000 or 62.5%. This is partially offset by the 26% increase in gain from the OT. The total effect on gain is about 78%, or a 22% reduction. Thus a smaller feedback resistor would be required. 17K looks right.

Last edited: Sep 25, 2020
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5. James KnoxTele-HolicSilver Supporter

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So, if I replaced the 22k with a 28k resistor, added I series with my 50k pot, I would have a NFB range of 28k to 78k on the sweep of the pot? Or, if I used a 25k pot, the sweep would be from 28k to 53k?

6. andrewRneumannTele-Holic

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Sounds about right. Or just keep the 22K and 50K pot... they are so close anyway!

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7. James KnoxTele-HolicSilver Supporter

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Ahh, good point! Thanks for your insight. Appreciated as always...

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James, aren’t we all “the/Sonics” people? (;^)
I have never messed with a Champ’s NFB loop values since the ones I have owned or worked on were vintage amps in original condition. I do like to install potentiometers 8n series with the existing series resistor so as to be able to move from the stock NFB factor to less cancellation in some amps. The tweed champ has never been one of those amps because they are hot little amps as they are, and I can manipulate the sound with the guitar’s output.....turn the amp up and play.
Aiken’s White Papers in his Tech section are valuable resources. The link above is just one article of many there.

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9. TimmayTele-Meister

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Thanks for the heads up on that resource Wally.

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10. andrewRneumannTele-Holic

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@Wally, you are trying to be modest, but you are the Tone King of Llano Estacado.

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11. Ten OverTele-Holic

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For a 6V6 and an 8K:4r OT with -4dB of NFB: 22K

For a 6V6 and a 5K:4r OT with -4dB of NFB: 17.9K

For a 5881 and an 8K:4r OT with -4dB of NFB: 31.7K

For a 5881 and a 5K:4r OT with -4dB of NFB: 25.6K

Using a transconductance of 5000umhos for 5881.

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12. elpicoTele-Afflicted

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There's nothing technically "correct" about the 22k value in the first place, it's just an arbitrary number, but also it was chosen based on the gain of a 6V6 power amp. You're running a 5881 instead, so even if you used the champs 8K load and 22k feedback resistor you would not have the same amount of feedback as the champ.

Given all that I'd say the only useful answer is "use a pot to find the value that sounds good", but maybe you're interested in a calculated result just for curiosity? Nobody has quite nailed that yet ... never mind Ten Over just did while I was writing this

I think the stock champ's nfb is closer to -6db though

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13. Ten OverTele-Holic

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Yeah, but the power tube gain takes a bigger hit when going from an 8K primary to a 5K. The open loop gain winds up taking a 21% reduction overall.

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14. andrewRneumannTele-Holic

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You’re exactly right! I didn’t take that into account. Thanks for the correction.

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