.68uf presence cap. Help me read this schematic!

Egghair

Member
Messages
63
Hello everyone, i recently come across a mod that many players found useful, the .68 presence cap mod. I own a Bugera 1960 Infinium and i found this schematic:

https://ibb.co/GFcbPbw

What componet should i change based on this pic?

Can anyone tell me the tonal differences between the stock cap (which i think it's the 100nf one) and the .68uf one?

Thanks!
 
Last edited:

PierreL

Silver Supporting Member
Messages
2,806
Just curious about this as I just bought one, haven’t even played it yet.
 

PierreL

Silver Supporting Member
Messages
2,806
You'll be really surprised of how it sounds. Kinda hate with the master on, but without it sounds absolutely fantastic! Really love it.
Tried it through the Captor X and... it's really great, from plexi to modded Marshall, I think I'm going to have a lot of fun with this one...
 

Egghair

Member
Messages
63
Tried it through the Captor X and... it's really great, from plexi to modded Marshall, I think I'm going to have a lot of fun with this one...
Glad to hear you enjoy it! If you ever have the chance, try it without attenuation and no master. It gives that body "concussion" feel typical of a plexi!
 

PierreL

Silver Supporting Member
Messages
2,806
:) I know that feeling. In the mid Eighties I was playing an early 70’s 100W Superlead on 8... bone crushing feeling.
 

J M Fahey

Member
Messages
2,399
Weird.
I suppose you replace the .1uF presence cap by a .68uF one?

Not sure I´d like it because it defeats the Presence control effect , but fwiw and for those who want to know what it actually does:

* power amplifier has an "X" amount of gain by itself

* said gain, called "open loop gain" because it is the raw untamed one, gets tamed, lowered, by using a negative feedback loop, which takes part of speaker out voltage and injects it back at the input, *out* of phase, so it lowers overall gain.

What Presence does it attenuate some of the mid-high frequency NFB so it boosts such frequencies, adding ... you guessed it ... "Presence" to the sound.
In practice increased mids-highs.

With values shown, (0.1uF and 4k7) gain starts increasing above 340 Hz and is really boosted around 2kHz; above that it loses steam because there was not THAT much extra gain to begin with.

If you increase cap value to 0.68uF you will boost everything above 50 Hz, meaning you will boost *everything* .
Flat, no EQ, just a (modest) gain increase.

Personally don´t see much improvement in getting a *modest* gain increase (which you can get in 1000 other ways) and losing a tonal option, but hey, if it floats your boat ......

And not sure how it could improve Master Volume function or sound, since it is *after* it.
Oh well.
 

DrainBamage

Member
Messages
2,395
This mod is basically turning the presence circuit in to a late 70's Marshall JMP. Amp will have a thicker bottom end to it.
 

Egghair

Member
Messages
63
Weird.
I suppose you replace the .1uF presence cap by a .68uF one?

Not sure I´d like it because it defeats the Presence control effect , but fwiw and for those who want to know what it actually does:

* power amplifier has an "X" amount of gain by itself

* said gain, called "open loop gain" because it is the raw untamed one, gets tamed, lowered, by using a negative feedback loop, which takes part of speaker out voltage and injects it back at the input, *out* of phase, so it lowers overall gain.

What Presence does it attenuate some of the mid-high frequency NFB so it boosts such frequencies, adding ... you guessed it ... "Presence" to the sound.
In practice increased mids-highs.

With values shown, (0.1uF and 4k7) gain starts increasing above 340 Hz and is really boosted around 2kHz; above that it loses steam because there was not THAT much extra gain to begin with.

If you increase cap value to 0.68uF you will boost everything above 50 Hz, meaning you will boost *everything* .
Flat, no EQ, just a (modest) gain increase.

Personally don´t see much improvement in getting a *modest* gain increase (which you can get in 1000 other ways) and losing a tonal option, but hey, if it floats your boat ......

And not sure how it could improve Master Volume function or sound, since it is *after* it.
Oh well.
Well thanks for the detaliled explanation. I didn't know everything about it, i just knew that presence controls the negative feedback loop which increases or decreases highs. I read about .68 cap mod to make the presence knob control more of the uppermids giving a boost to it.
Anyway, if i ever decid to change the component, it will be C8 right?
 

Egghair

Member
Messages
63
This mod is basically turning the presence circuit in to a late 70's Marshall JMP. Amp will have a thicker bottom end to it.
That's what i've heard. I read that it gives more mids/uppermids boost and thats why i wanted to try it, but i didn't know about that gain increase thing.
 

communarchy

Silver Supporting Member
Messages
668
I have found this useful if you are playing the amp all the way up for a distorted sound. If you like clean/edge of breakup sounds I think the stock value is best for adding just the sheen of brilliance on top.
 

J M Fahey

Member
Messages
2,399
Well, if you want to have your cake and eat it, you can add a small unobtrusive toggle switch to the front panel (think a pickup phase switch type), leave the original 0.1 uF cap in its place and at will add a 0.47uF cap in parallel with it when you want to.

Original schematic:


So called ".68uF Mod", FAHEY version, actually ".57uF Mod" he he (0.47 + 0.1uF) which is more than enough.



hope it`s clear enough :)

EDIT: "but ... but ... it`s not .68!!!!"
Answer:
1) the one who originated this probably had a .68uF on the bench, it IS a value found inside Marshall amps, so he used it.
If you find it important use one, and rewire switch so it connects either original .1uF or new .68 one.
2) But check my math above , in an earlier post, .68 is already too big for the intended job, combined .47 + .1 are also more than enough , my wiring is as effective and simpler and .47uF caps are way more popular and easier to find than .68
Again, if you want to use it, feel free to do so.
 
Last edited:

communarchy

Silver Supporting Member
Messages
668
Well, if you want to have your cake and eat it, you can add a small unobtrusive toggle switch to the front panel (think a pickup phase switch type), leave the original 0.1 uF cap in its place and at will add a 0.47uF cap in parallel with it when you want to.

Original schematic:


So called ".68uF Mod", FAHEY version, actually ".57uF Mod" he he (0.47 + 0.1uF) which is more than enough.



hope it`s clear enough :)

EDIT: "but ... but ... it`s not .68!!!!"
Answer:
1) the one who originated this probably had a .68uF on the bench, it IS a value found inside Marshall amps, so he used it.
If you find it important use one, and rewire switch so it connects either original .1uF or new .68 one.
2) But check my math above , in an earlier post, .68 is already too big for the intended job, combined .47 + .1 are also more than enough , my wiring is as effective and simpler and .47uF caps are way more popular and easier to find than .68
Again, if you want to use it, feel free to do so.
Great point. If you are going to go the extra mile to wire a switch in parallel and have the room, a .56uF would get you .66uF, and tolerance-wise, it would likely spec to the tolerance of an actual .68uF. Not the most common value sitting around the bench but still obtainable if you order online.
 

Egghair

Member
Messages
63
Well, if you want to have your cake and eat it, you can add a small unobtrusive toggle switch to the front panel (think a pickup phase switch type), leave the original 0.1 uF cap in its place and at will add a 0.47uF cap in parallel with it when you want to.

Original schematic:


So called ".68uF Mod", FAHEY version, actually ".57uF Mod" he he (0.47 + 0.1uF) which is more than enough.



hope it`s clear enough :)

EDIT: "but ... but ... it`s not .68!!!!"
Answer:
1) the one who originated this probably had a .68uF on the bench, it IS a value found inside Marshall amps, so he used it.
If you find it important use one, and rewire switch so it connects either original .1uF or new .68 one.
2) But check my math above , in an earlier post, .68 is already too big for the intended job, combined .47 + .1 are also more than enough , my wiring is as effective and simpler and .47uF caps are way more popular and easier to find than .68
Again, if you want to use it, feel free to do so.
Seems a pretty cool deal, i'm kind of unsure about installing a switch since i don't really want to drill the chassis. Do you think that by removing the current .100uf swapping it with the .47uf will give me the same result you described?

And what tonal differences will it give me?
 

Egghair

Member
Messages
63
Great point. If you are going to go the extra mile to wire a switch in parallel and have the room, a .56uF would get you .66uF, and tolerance-wise, it would likely spec to the tolerance of an actual .68uF. Not the most common value sitting around the bench but still obtainable if you order online.
Can you give me a little math lesson about that?
 

J M Fahey

Member
Messages
2,399
In practice all 4 possibilities:
* .68uF
* .47uF + .1uF
* .56uF + .1uF
and plain
* .47uF
end up being the same, because respective "turnover" frequencies, meaning "where they start to work" are:
* 49.8 Hz
* 59.4 Hz
* 51.3 Hz
* 72 Hz
because guitar lowest frequency is about 82 Hz, so .....

Out of all those values ,.47uF is by far the easiest to find, which also means there`s more variety, so it tends to be cheaper in general, what`s not to like?

But those wanting to go the extra mile ... it`s a personal choice.

FWIW I think .68uF was used simply because "it was there" on somebody`s bench but really all work the same.
Your choice :)
 

DrainBamage

Member
Messages
2,395
In practice all 4 possibilities:
* .68uF
* .47uF + .1uF
* .56uF + .1uF
and plain
* .47uF
end up being the same, because respective "turnover" frequencies, meaning "where they start to work" are:
* 49.8 Hz
* 59.4 Hz
* 51.3 Hz
* 72 Hz
because guitar lowest frequency is about 82 Hz, so .....

Out of all those values ,.47uF is by far the easiest to find, which also means there`s more variety, so it tends to be cheaper in general, what`s not to like?

But those wanting to go the extra mile ... it`s a personal choice.

FWIW I think .68uF was used simply because "it was there" on somebody`s bench but really all work the same.
Your choice :)
Marshall had .1 and .68 already on hand as they used these values in all amps.

The reason Marshall used the .68 on the pot is likely due to the Celestion speakers got brighter from the Greenback to the Creamback and finally the Blackback. Each speaker got brighter. Come the low volt Marshall JMP era Blackbacks were in the cabs. Amps were chewy an darker but the Blackback was bright.

FWIW I have done the switch in the past with paralleling in a .56uf It sounds good not better or worse than the .1uf but if you paly a lot of single coils it may be a good mod.

Also I dont know what the rest of the Bugera shematic is? If its already bass spec then I wouldn't do it.
 

Egghair

Member
Messages
63
Marshall had .1 and .68 already on hand as they used these values in all amps.

The reason Marshall used the .68 on the pot is likely due to the Celestion speakers got brighter from the Greenback to the Creamback and finally the Blackback. Each speaker got brighter. Come the low volt Marshall JMP era Blackbacks were in the cabs. Amps were chewy an darker but the Blackback was bright.

FWIW I have done the switch in the past with paralleling in a .56uf It sounds good not better or worse than the .1uf but if you paly a lot of single coils it may be a good mod.

Also I dont know what the rest of the Bugera shematic is? If its already bass spec then I wouldn't do it.
I mostly wanted to try this mod because of the freedom it would give my in taming bright and sharp speakers. I got a cab with jensens but i can't bring it always around, so...
As far as i know, this amp is almost point to point a super lead circuit.
 

Egghair

Member
Messages
63
In practice all 4 possibilities:
* .68uF
* .47uF + .1uF
* .56uF + .1uF
and plain
* .47uF
end up being the same, because respective "turnover" frequencies, meaning "where they start to work" are:
* 49.8 Hz
* 59.4 Hz
* 51.3 Hz
* 72 Hz
because guitar lowest frequency is about 82 Hz, so .....

Out of all those values ,.47uF is by far the easiest to find, which also means there`s more variety, so it tends to be cheaper in general, what`s not to like?

But those wanting to go the extra mile ... it`s a personal choice.

FWIW I think .68uF was used simply because "it was there" on somebody`s bench but really all work the same.
Your choice :)
The parallel wiring of a .47 and a .1 would sum them up?
 




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