Adding presence, or something else

[überfuzz]

Hi!

I'm rebuilding an old amp, Carvin 33, and there's one unused pot/whole in the chassi. At the moment the idea is to add a presence control. Looking at Fender schemes I see that the presence control is altering the ground bypass, see 5f6a scheme. Other schemes I've looked at have the pot controlling the negative feedback... I'm not sure of wich way to go.

1. Replace the NFB resistor with a pot.
2. Bypass the ground with a pot ala-Bassman 5f6a and thinker with the value of the NFB resistor until I have a value that rings nicely.

Or maybe if someone has an other simple mod I can add, to my build. This build is based on the carvin 33 circuit.

[Wilder Amplification]

The 5F6-A Presence circuit essentially is shelving high frequencies out of the negative feedback loop. Anything you shelve out of the NFB loop gets boosted at the amp's output, which causes the boost in the highs. In reality they're not really getting "boosted" per se, you're just cancelling less of the highs out due to shelving them out of the NFB loop which is exactly what the 5F6-A Presence circuit does.

[überfuzz]

Wilder - I know, I think... With the pot where the nfb resistor is sitting, I'd boost all frequencies. Or? I'm not sure, but wouldn't that be funnier than just boosting highs, or unshelving highs.

[JoeM]

A variable FB control. You might like that.

[Gaz]

Or, if the amp ends up being too bright, you could add a vox AC30 style "cut control" to smooth out the top. JM2C.

[überfuzz]

Joe - That would be a pot in series with a nfb resistor, or?

Gaz - I'll look into it. Thanks!

[Gaz]

Well, I'll answer for Joe. it would be a pot (I think linear type works best) in series with the feedback resistor. The resistor sets the maximum feedback, say a 47k off the 8 ohm taps (depends on the amp, of course), and a 100kB pot increases the value of the feedback resistor up to 147k effectively. You can also add a large cap in series with the pot as well, say 1uf or higher to keep DC off the pot, which prevents a scratchy sound when the control is adjusted.

I should note that you have to be somewhat careful with lead dress if you're running the feedback wires back and forth across the chassis, and you may need to experiment with lead dress and shielding to avoid noise issues. Depends on the amp too, of course.

[überfuzz]

Gaz - Doesn't the negative feedback have to be in phase..?

[JoeM]

Gaz - Doesn't the negative feedback have to be in phase..?

The feedback loop is negative, hence the term "negative feedback loop", otherwise you have positive feedback and most likely it will oscillate. So no, it's not 'in phase'.

[überfuzz]

Ahh... Oki.

There's a 220k NFB resistor in the feedback circuit of my amp. If I got this right I can replace this resistor with a resistor in the reelms of 100k in series with a 100kB pot. And off course, the big cap. One question though. about the pot, is 100kB a suitable range or should I keep it smaller? I'm not looking for überdrive...

Edit, I got it all backwards... Lol. Sorry! The 220k resistor could stay and then a pot in series. Right..?

[JoeM]

Using a 100k pot with a 100k resistor will let you dial in more feedback, not less. I'm not familiar with this amp's circuit, but you could use a 220k (as it has now) and a 100k pot to have less feedback, which usually is what you want. However, just playing around with my 5F6A clone, I found that I liked a certain value of feedback. Too much and the amp was kind of sterile and too little was kind of 'grainy' to my ears. I used a 1meg pot and found a value I liked (in this case 33k), and just put that fixed resistor value in the amp. Sometimes the fewer knobs to turn the better, IMO.

[daz]

Yeah, look at it like this....the smaller value resistor the more feedback and of course the bigger the less. Once you get very small the tone will become very soft and lose a lot of much sparkle and cut and all that. Just mush. Too big a resistor and you're heading toward no feedback and a more bright cutting tone which will get intensely cutting and bright at very high values. I'm not sure where the sweet spot will be on that amp, but on marshalls i usually find the extreme ends of usefulness for me any ways are between around 33k and 100k. So when i had a pot to adjust how much NFB i wanted i used a 47k resistor in series with a 50K pot, tho i went thru a bunch and found one closer to 60k. That gave me a range where i never needed to go either way further than the mod world allow. Works great, tho i must admit that i eventually came to realize that i always had the pot set at the same point and liked it there best, so i removed it and just stuck a set resistor of the measured value there. turned out to be right about 47k so thats what i used. the more i tweak the more i come to believe the better the circuit is the less features in the way of adjustability you need. the old less is more theory. when things are truly right you don't want to have to many adjustable things available to you or you may just screw yourself out of the perfect tone at times.

[Wilder Amplification]

Yeah, look at it like this....the smaller value resistor the more feedback and of course the bigger the less. Once you get very small the tone will become very soft and lose a lot of much sparkle and cut and all that. Just mush. Too big a resistor and you're heading toward no feedback and a more bright cutting tone which will get intensely cutting and bright at very high values. I'm not sure where the sweet spot will be on that amp, but on marshalls i usually find the extreme ends of usefulness for me any ways are between around 33k and 100k. So when i had a pot to adjust how much NFB i wanted i used a 47k resistor in series with a 50K pot, tho i went thru a bunch and found one closer to 60k. That gave me a range where i never needed to go either way further than the mod world allow. Works great, tho i must admit that i eventually came to realize that i always had the pot set at the same point and liked it there best, so i removed it and just stuck a set resistor of the measured value there. turned out to be right about 47k so thats what i used. the more i tweak the more i come to believe the better the circuit is the less features in the way of adjustability you need. the old less is more theory. when things are truly right you don't want to have to many adjustable things available to you or you may just screw yourself out of the perfect tone at times.

Of course this depends on which secondary tap on the OT that you take NFB from. Given the same value resistor and all else the same, the 4 ohm tap will yield less NFB than the 16 ohm tap simply because it puts out 1/2 the voltage of the 16 ohm tap (but at double the current so the output power remains the same).

A 50K resistor on the 4 ohm tap is the equivalent of a 100K resistor on the 16 ohm tap, 47K being the closest "standard" value.

[JoeM]

Of course this depends on which secondary tap on the OT that you take NFB from. Given the same value resistor and all else the same, the 4 ohm tap will yield less NFB than the 16 ohm tap simply because it puts out 1/2 the voltage of the 16 ohm tap (but at double the current so the output power remains the same).

A 50K resistor on the 4 ohm tap is the equivalent of a 100K resistor on the 16 ohm tap, 47K being the closest "standard" value.

Right, but I think the point here is that it doesnt necessarily have to be an adjustable amount. Like a lot of options, I think most would find the 'sweet spot' they prefer and never touch the control again.

[JoeM]

....
the more i tweak the more i come to believe the better the circuit is the less features in the way of adjustability you need. the old less is more theory. when things are truly right you don't want to have to many adjustable things available to you or you may just screw yourself out of the perfect tone at times.

I agree. I was in the local music store a few weeks ago and they wanted me to try out a new Mesa MkV. Forget it! For me it's just a nightmare. Knobs and switches everywhere. I told them I justed wanted to turn one knob.