Any reason this wont work?

[daz]

I have 2 different caps for the NFB on my marshall style build. One is the typical .1uf which i really like because i can use it in conjunction with the treble pot to fine tune the treble to get the right blend of very hi treble and lower treble frequencies. But when i switch to the 2.2uf i love the way the tone gets thick. But then i miss the ability to fine tune the higher end with the .1uf. So i want to use both at the same time. right next to the presence pot is the switch i use to go between the 2 caps. What i want to do is replace that with another "presence" pot so that i have two presence pots, one for the .1uf and one for the 2.2uf. Is there any reaon this wouldn't work? i intend to do it as in the drawing below. If anyone can tell me whether that will work or if not how i should change it i'd appriciate it. Thanks .

[Chuck H]

It would work...Sorta.

One thing is that by using 5k pots you already have the percieved presence setting at about 7 for the "presence" and about 5 for the larger cap. Thats why if you look at the bypassed presence type circuit in the 800's they use a 25k pot. You would be better off with 50k pots for your application. But the controls would have most of their adjustment between about 7 and 10.

Other than that there is the problem of the larger cap swamping the response of the smaller one. In other words, the higher you set the control for the larger cap the less effect you would get from the adjustment for the smaller cap. With the larger caps pot set to 10 adjusting the pot for the smaller cap would do nothing.

You could use an inductor in the circuit for the larger cap so that it only controls the upper mids. Then you could get the same effect you have now by simply putting both controls on the same setting. Or you could bump only the upper mids or only the actual "presence" or any combination. I think that could be a neat circuit.

Not sure what size inductor you would need because I'm formula impaired. But if I figure it out I'll repost.

Chuck

[daz]

Chuck, are you saying the 5 k pot is like having presence at 7 when it's all the way down? The reason that has me scratching my head is that i went to 5 k in the first place because anything bigger had no effect till it was almost all the way up. I used 25k at first but noting changed till it was at about 8 or 9, so i used the 5 k to lengthen the usable throw, and it now just starts to make a difference about 10:00.

Anyways, it's a quick thing to try, tho i don't have another 5k i will use a 10k and try it today.

[Chuck H]

Chuck, are you saying the 5 k pot is like having presence at 7 when it's all the way down? The reason that has me scratching my head is that i went to 5 k in the first place because anything bigger had no effect till it was almost all the way up. I used 25k at first but noting changed till it was at about 8 or 9, so i used the 5 k to lengthen the usable throw, and it now just starts to make a difference about 10:00.

That's what I'm saying. With the 4.7k fixed R all frequencies "see" 4.7k from ground. In an old school type Marshall presence control when the coltrol is set to 0 the .1 cap is completely out of the circuit. With the control on 5 the .1 cap is 2.5k from ground. On an old Marshall you can certainly hear a difference between 0 and 5 on the presence control. I don't use the later 800 style presence so I can't say anything on it's adjustability. But I can say that with the fixed 4.7k R and the .1 cap going through a 5k pot that all the frequencies controlled by the .1 cap are 2.5k from ground with your presence set to 0 (4.7k in parallel with the 5k pot) so this is exactly the same circumstances as a standard Marshall presence control set to 5. With the second 5k pot and the bigger cap, that bigger cap will also allow the frequencies controlled by the .1 cap to reach ground. So now the frequencies controlled by the .1 cap have 3 paths to ground. And you end up at what would be about 7 on a standard Marshall presence control with all your knobs set to 0.

That isn't to say it's wrong. I personally would want to be able to balance my feedback loop closer to flat. But thats just me being finicky. If it works for you, it works. There are no rules in this game. But I am brainstorming a circuit you could try for what you want to do. I'm trying to get around using an inductor because by my findings you would need about 1 Henry. And thats a hard inductor to find if you only want one of them. I'll post a drawing when I have something.

Chuck

[daz]

I see now, thanks Chuck. The 4.7k and the pot together in parallel make for 2.5k to ground. So what you're saying makes sense, but for some reason my NFB loop doesn't sonically seem to reflect that. In other words, when all the way down the amp is muddy untill i turn the treble up quite a bit. As tho the presence has no influence. And it takes till around 10:00 before i can tell theres a difference, and thats subtle. Not till noon does it have a decent effect. But i will try a 25k pot again and see what i get. But as i recall it did nothing till it was most of the way up. however, i want to try it anyways and see how the tone controls work when the presence pot is all the way down because i have noticed the treble and mid controls are very much affected by the presence. One thing i've never been totally happy with is the tone section. I have tried every possible value everywhere in the tone stack and used the duncan TS calculator extensively. But i've always felt i did care for the treble and mid centers, and the bass has never worked the way i wish it would. i can get great tone out of this amp to be sure, but i feel if the tone section were as i imagine it would be phenomenal. And i have noticed a lot of interaction between the tone stack and the NFB.

what i was trying to accomplish with a 2nd pot in the NFB was to get the thick tone the amp has when i turn the pot up about 1:00 with a 2.2uf cap, then have a regular presence pot (.1uf) to dial back in some of the top end sizzle that using the 2.2uf takes away. But i think you're right and this just isn't going to do what i want. I think i will experiment more with other values in the NFB because i haven't done that for a long while, and the amp has been changed so much since i last did. If you have any suggestions for how i could change the NFB's voice somewhat radically form where it is now i'd like to hear what you might come up with. I feel like it's good as is, but i also feel like theres some magic there just waiting to be unlocked. I think the reason amp building and tweaking is so addictive is because all you have to do is make one change and it can throw everything else out of balance requiring you re-tweak many things you have previously tweaked to perfection ! so it's never ending.

[Chuck H]

I see now, thanks Chuck. The 4.7k and the pot together in parallel make for 2.5k to ground. So what you're saying makes sense, but for some reason my NFB loop doesn't sonically seem to reflect that. In other words, when all the way down the amp is muddy untill i turn the treble up quite a bit. As tho the presence has no influence. And it takes till around 10:00 before i can tell theres a difference, and thats subtle. Not till noon does it have a decent effect.

The response of your presence control could have to do with other differences in your circuit. It's all interactive. That's why we have different amp designs to choose from even if we want to clone. Tweak here, tweak there, bada bing, new amp.

But i will try a 25k pot again and see what i get. But as i recall it did nothing till it was most of the way up. however, i want to try it anyways and see how the tone controls work when the presence pot is all the way down because i have noticed the treble and mid controls are very much affected by the presence.

In your case I wouldn't trouble with the 25k pot again. Only because you've already been there. But you could try the older style presence circuit that uses a 5k pot with the cap mounted on it. The ONLY drawback to this circuit is that it can make scratchy sounds while it's being adjusted. And I don't see this as a problem because I don't do much adjusting of the presence control in the middle of a performance or recording.

And of course your tone controls are affected by the presence setting. As I said, it's all interactive. When you change your treble control (for example) you change the signal being "fed back" to the PI tail. This changes the overall response and dynamics of the amp. So the presence setting will effect the treble effect and vice versa.

One thing i've never been totally happy with is the tone section. I have tried every possible value everywhere in the tone stack and used the duncan TS calculator extensively. But i've always felt i did care for the treble and mid centers, and the bass has never worked the way i wish it would. i can get great tone out of this amp to be sure, but i feel if the tone section were as i imagine it would be phenomenal. And i have noticed a lot of interaction between the tone stack and the NFB.

I wouldn't sweat it too much if I were you. Guitar amp tone stacks are very limited in their operation (ie: passive means they only "cut" and never actually boost and they are usually limited in db swing). Lots of amp builders have tried to find unconventional ways around this. Remember those graphic EQ's on the old Boogie's? As long as you can hit a couple or a few sweet spots for the important usable tones you've done well. Adding more circuits just mucks up the tone sooner or later. It's a fine line.

what i was trying to accomplish with a 2nd pot in the NFB was to get the thick tone the amp has when i turn the pot up about 1:00 with a 2.2uf cap, then have a regular presence pot (.1uf) to dial back in some of the top end sizzle that using the 2.2uf takes away.

AAhh! There's where some of the misunderstanding is. It's important to get your head wrapped around what's happening with the NFB. The 2.2uf cap being in operation doesn't take away the effect of the .1 cap. It swamps it. When the 2.2uf cap is operating in the circuit it is not only increasing "presence" on the upper mids, but also those frequencies normaly effected by the .1 cap. with the 2.2uf cap in service there's nothing for the .1 cap to do. Those frequencies are already in play. Even though your perception is in the upper mid effect.

but i also feel like theres some magic there just waiting to be unlocked. I think the reason amp building and tweaking is so addictive is because all you have to do is make one change and it can throw everything else out of balance requiring you re-tweak many things you have previously tweaked to perfection ! so it's never ending.

Remember your first hot rod? Have you ever owned a boat? Either of these examples would serve. It's a never ending project that never lives up to an ideal. I guess I'm getting old but I use to imagine a perfect amp. Now I try to specialize so that I can be happy with each design. I've stopped expecting that I can build one channel that is great for everything (but I got pretty damn close IMHO ).

Chuck

[daz]

Wow, thats a book, not a post !

When you say...

But you could try the older style presence circuit that uses a 5k pot with the cap mounted on it.

I'm not sure what you mean. I DO have a 5k pot with the cap mounted on it. So i'm wondering what you are saying to do different? Whatever it is it'll try it if you describe it to me.

[Chuck H]

When you say...

I'm not sure what you mean. I DO have a 5k pot with the cap mounted on it. So i'm wondering what you are saying to do different? Whatever it is it'll try it if you describe it to me.

Yeh, That was a pretty poor description... See below. This circuit works better IMO if only because of it's eloquence. Good adjustment swing, fewer parts. easier to implement. My kind of circuit.

You could try the 2.2uf and .1uf on seperate pots with this design too. Just use 10k pots so that the NFB shunt total is still 5k. If you put a 1 or 1.5 henry inductor in series with the 2.2uf cap, each pot would operate completely independant of each other. I'm trying to locate that inductor.

Chuck

[Chuck H]

Just to be clear this time...This is how the new circuit would look.

Chuck

[daz]

Thanks Chuck. I'll try wiring in like the `1st jpg. Don't bother trying to find an inductor tho because if i can't find it locally i'll just pass, as i know i'll end up paying $10 for shipping. But i DO have some pretty good local resources. I just need to know what H value to look for. In particular i'd really like to be able to pump up the low mids like in the 400-800Hz area.

[daz]

I tried wiring it as you showed Chuck, but i think i like the throw as it was before. Maybe just because i'm used to it. However, you helped me out in one way....i found you are correct in that the presence IS indeed allowing some high end with the 5k pot. i had it switchable completely out of circuit and it does add a considerable bit of high end even all the way down. this is a nice find because i can now turn it down and then i'm able to turn the treble up more. this is something i do to get a more fenderish tone, while turning the treble down more and the presence up is more marshall like. So now i can go a bit further towards the fenderish side. Nice !

However, i can't understand how to take the cap out of circuit with a larger pot because when i use my electronics calculator to find what size pot would allow for 4.7k or higher to ground when paralleled with the 4.7k resistor thats there, it takes a 1 meg pot to be close to 4.7k when turned all the way down. yet i don't see presence pots over 25k usually. What am i missing here?

[daz]

Ok, heres the latest. After you (chuck) told me that 5k was not taking the cap out totally i now feel like i may no longer need that mid boost. It seems with the cap out of circuit i can turn the trble up to get enough high end, but it's a thicker tone. With the cap 2.5k away from ground i was getting enough presence to where turning the treble up got nasty and harsh. But w/o the cap the treble gives me the high end i need but at a lower treble freq where the tone is thicker w/o needing that mid boost. So in a roundabout way you helped me get what i was after, at least to a degree if not completely....time will tell as my ears get used to it and make thier decision.

That said, i now have a 25k pot, but the range is now all near the end. But if i go to the next lowest value, 10k, i know there will be enough to screw things up again when i want that thicker tone with the cap at a point where it's not contributing even the slightest presence. I may just go to a 10k tho anyways and cut the trace at one end so turned full counter clockwise it will be open. Chuck, i really appriciate all your help. Truly I do...thank you.

[black_labb]

jumping in a bit late here. what if you used a single pot (10 or 25k) and tied the wiper to one end of the 4k7 resitor, and the 2 different caps attached to either end of the pot, with the other ends connected to the other end of the 4k7 resistor.

this is assuming you use the 25k, which i think would be better.
in the middle there is 12.5k resistance to each cap (6.25k effectively for all highs, 12.5k for the real highs). when you turn it one way, the resistance drops for one cap and increases for the other, effectively giving you the effect of 2 pots. the only thing it doesnt do is let you turn them both up, but that isnt very useful anyway as chuck described above. also saves you putting any extra holes in for pots.

[Chuck H]

FWIW Daz, you could use the old style presence control and get at least as much adjustability as a 10k pot gives in your current arrangement. It would give you the ability to turn the presence down all the way. You noted how with the typical 800 type presence circuit you always have SOME presence activated. This is true. Your not off your rocker, you just saw it for what it is. Anyway, If you use the older style presece circuit it would allow you to turn your presence down all the way (like a Fender with flat NFB). You would then have the adjustability to use more treble (if it were there). It can be. Just change your slope resistor in the tone stack to 56k or 68k. This would give more upper mids which will thicken your top end and tighten your bottom end. And the presence would now be more like icing on the cake rather than a needed componant all the time.

It's easy to try. Just wire it the way you did before with the 5k pot as the NFB shunt with the cap on it, and change your tonestack slope R to 56 or 68k. 33k is a small slope R for a 50 watter anyhow. Even Marshall only used it on 100W models. And then I think it was to pull back on the top end. At 100W to much top end is like stabbing your ears with a shishkabob swewer.

Chuck

[daz]

Actually i've had a 47k slope in there for the last 3 weeks or so. I tried more but it seemed too thin, and i have made some major changes in the last few days going from a 220k V1A plate back to 100k as it first was and a few other things. that and a few other changes have been great but left my tone rather thin and i'm having a bit more trouble with flabby lows. I know that sounds crazy....made some changes and like it, yet now i have a thin sound and flabby lows !!! LOL! but really, the tone seems better in a way (richer harmonics) if i could just thicken it up and tighten the lows. I tried some .1uf couplers at the EL34's tonite but it was too late to turn it up to tell. Seems thicker at the low volumes i could try it at, but the flab in the lows also got worse i think. I'l know more when i can turn it up tomorrow.

what bothers me is that i fought flabby lows forever with this amp and finally got it under control well enough quite a while back. But now with the changes i've made it seems to have reared it's ugly face again. I may have to change the V1A plate back to 220k, as thats what i think may have done it. Anyways, i like the presence right now since at zero it's the same as switched out. I now need to either figure out this flab thing and the thin-ness which i think would be ok if i could add some low mids. If it's just too much to deal with i may just set everything back to how it was a week ago and had been for quite a while. I just can't stop trying to improve it no matter how good it is ! It's a freakin' disease !