difference between 4, or 16 oh taps for NFB?

[12AX7]

When updating  my schematic i had a thought that became a question. I have experimented with this in the past and i *basically* know the difference.  If you the 8 ohm tap it's generally smoother then the 4. The 4 is more cutting and crunchy. But thats with all else being equal such as the NFB resistor and the presence setting. But the question i would like to pose is this. If you use that smoother 8 ohm tap then change to the 4, is the tone and the feel going to be exactly the same IF you adjust the feedback resistor and presence setting to try and achieve that? What i'm getting at is whether or not there is a sound/feel that each tap has that can only be achieved with that tap no matter how you adjust the feedback resistor and presence cap and setting? Hope thats clear, as it's a bit hard to explian.

[thetragichero]

https://www.aikenamps.com/index.php/designing-for-global-negative-feedback

[bmccowan]

The difference between 4 and 16 ohm taps is 12 ohms.

[12AX7]

https://www.aikenamps.com/index.php/designing-for-global-negative-feedback

I'm not tech so most of that is over my head. But my takeway from that which may well be wrong is that unless both the feedback resistor and shunt resistor are changed there WOULD be a difference of some sort that would make one tap sound or feel different. I ask because i DO have a variable NFB pot so i i could compensate to some degree but w/o changing the shunt resistor (presence pot) i could not get it to sound exactly the same from tap to tap. and mine is a 5k pot where some are what...35k? It's really hard to tell how close it can get just tweaking the presence and NFB pot after changing taps but it seems as tho they do sound and./or feel to a degree i can't adjust for. Can't really A/B this kind of thing instantly either. I'm leaning towards just leaving it on the 8 ohm tap but it seems the 4 does allow a less compressed sound/feel that if i try and cop on the 8 gets too thin/bright before it can achieve that.

[sluckey]

The only difference between the 4Ω, 8Ω, and 16Ω taps is the feedback voltage will be greater for the higher taps. You can make the 16Ω tap behave exactly like the 4Ω tap simply by changing the value of the voltage divider resistors. And since you are using a pot as a variable resistor in the voltage divider you can get the same amount of NFB voltage by turning the pot. You could use a scope and set the NFB voltage to the exact level you desire, but it's much more practical to just use your ears and not worry about how much NFB you actually have. There is no rule that says the NFB must be some exact value, unless you are copying another circuit, in which case you would not use a pot.

[12AX7]

The only difference between the 4Ω, 8Ω, and 16Ω taps is the feedback voltage will be greater for the higher taps. You can make the 16Ω tap behave exactly like the 4Ω tap simply by changing the value of the voltage divider resistors. And since you are using a pot as a variable resistor in the voltage divider you can get the same amount of NFB voltage by turning the pot. You could use a scope and set the NFB voltage to the exact level you desire, but it's much more practical to just use your ears and not worry about how much NFB you actually have. There is no rule that says the NFB must be some exact value, unless you are copying another circuit, in which case you would not use a pot.

Thanks, thats what i was looking for. I did put the 4 and 8 taps on the switch i formerly had the presence caps on and did that last nite late so i will use it today just to see how close it gets. Then assuming i hear what you said i'll leave it on the 8 and use the switch for the presence caps or something else. (use the .1uf most of the time anyways)

[12AX7]

/Here's another question.  I built this amp years ago and don't remember what my reasoning was behind some of the choices i made. One of those is the 5k presence pot. I believe there were some marshalls that had a 5k if i recall,  but the vast majority had a 22k pot with a 5k resistor in parallel. In mine i have a 5k pot but no 5 k resistor in parallel. Can i assume the reason they used a 5k resistor in parallel with the 22k pot to approximate a 5k pot because 5k pots aren't that common and they probably had plenty of 22k pots o use a 5k resistor to approximate a 5k pot? Or is there some reason to parallel a resistor with the presence pot?

[sluckey]

I assure you, 5K pots are very common. The original presence circuit used a 5K pot, but the pot was part of the tail circuit for the LTP PI and had cathode current flowing through it. This caused the pot to sound scratchy as you turned it. The "improved" presence circuit replaced the 5K pot with a fixed 4.7K resistor and placed a 22K pot and series .1uF cap in parallel to the 4.7K resistor. The cap blocks dc current from flowing through the 22K pot. No more scratchiness. The 4.7K provides the current path for the LTP PI tail circuit. The dc current path is highlighted in the attachment.

Many people prefer the original circuit even though it has a scratchy pot.

[12AX7]

Thats strange. Mine is like the original yet the pot isn't scratchy at all.And yes, i know 5k pots are common but I was just thinking maybe not as much so back then when they designed those amps. But that clears it up.

[Willabe]

In TUT 3, Kevin O'Connor, there's a Marshall Plexi build chapter. In it he lays out what KOC calls a 5100 Plexi amp, fifty 100.

He talks about Marshall changing the -FB loop values in their 50w amp when they built their 100w amp. Saying the early 50w models had a 47K series -FB R off the 8 ohm tap, with a 4K7 ohm shunt R. And the 100w had 100K series -FB R off the 8 ohm tap, with a 4K7 ohm shunt R.

He has 2 switches to change the -FB loop values and tap points. He put in 2 x 47K R's in series instead of the 100w 100K R value for the -FB series R.   

On 1 he calls the 'vibe' switch, it grounds out bypasses 1 of the 47K series -FB R's. So you get either 47K or 94K for that series R. Switch 2, 'bounce', switches between the 8 ohm tap and the 16 ohm tap. You can set either switch how ever you want with no harm to the amp, just a different sound.