More presence out of the presence control...

[BluePat]

Hi all,

Still tweaking my JCM800 2204 and want to know how to squeeze more presence out of it. I figure altering the value of the cap on the pot, but open to suggestions on what value, or if there is something else I could do? :AOK

Not that the amp doesn't have enough presence, but to really get it to shine, I pretty much have to dime the pot, and even then I think I could go higher.

Any and all opinions or suggestions welcome.

Pat

[phsyconoodler]

Go see a doctor and get your hearing checked.

[BluePat]

Go see a doctor and get your hearing checked.

Wow, was that called for?! :eek: Nothing wrong with my hearing buddy, and to think getting a response like that from a fellow Calgarian? You can be certain I'll have nothing positive to say about Clara amps to anyone I converse with. :BITCH

[gldtp99]

More NFB should give more response from the Presence pot----- make sure that the NFB feed is coming from the 16 ohm OT tap----- install a smaller value NFB resistor, if needed, some use a .68uF presence cap instead of the .1uF.
Mucking with the NFB circuit will change the overall tone/feel of the output section of the amp, however. you may need to experiment around.
I'd also check to see if the "bright" cap has been removed from the vol pot or the treble peaking circuits have been modded to give the amp a fatter tone response.
Do you usually prefer a brighter sounding amp with your guitar/speaker cab ? If so, a "Fat" modded JCM 800 might not be to your liking and you might prefer the stock values here.................gldtp99

[phsyconoodler]

Boy,I was just joking pal! Try removing the negative feedback from the output jack.That will give full presence.Or try a differet preamp tube in V1.Or a smaller bypass cap on the V1 cathode.

Bluepat, you can call me and I'd be glad to help you with your JCM800.

[BluePat]

More NFB should give more response from the Presence pot----- make sure that the NFB feed is coming from the 16 ohm OT tap----- install a smaller value NFB resistor, if needed, some use a .68uF presence cap instead of the .1uF.
Mucking with the NFB circuit will change the overall tone/feel of the output section of the amp, however. you may need to experiment around.
I'd also check to see if the "bright" cap has been removed from the vol pot or the treble peaking circuits have been modded to give the amp a fatter tone response.
Do you usually prefer a brighter sounding amp with your guitar/speaker cab ? If so, a "Fat" modded JCM 800 might not be to your liking and you might prefer the stock values here.................gldtp99

Thanks for your reply. I thought of playing with the NFB resistor value in hopes of making the presence control range "wider", for lack of a better term. I did alter the front end of the amp in that I removed the usual items that make these amps so bright from the factory, including that bright cap on the volume pot, and now just want more "shimmer" if you will, as opposed to overall brightness, if that makes any sense. I like the sound of the amp with the presence set relatively high, but doing that doesn't seem to make it as brilliant/peaky as it was stock. My understanding of the presence control is that it affects a fairly narrow bandwidth with respect to frequency, and those seem to be the sweeter ones I want to bring out in the amp (almost like harmonics), without enhancing more midrange, gain, etc.

By decreasing the value of the cap on the presence pot, will that make the control "wider" as well?

Pat

[phsyconoodler]

Did you remove the treble peaking circuit?The 420ohm/500pf cap that goes to the low input jack?If so,put it back in.It doesn't add as much treble as the bright cap may add the 'presence' you are looking for.

[gldtp99]

Changing to a .68uF Presence cap is a common mod that adds a mid-range hump that may or may not be to your liking, try it and see.
There's often quite a bit of variance between individual JCM800 amps so to get the "shimmer" you want it may be necessary to mod the tone stack to highlight the freq's that you're looking to bring out----- this is if the Presence mods don't do it for you.
Have you already tried the easy things ?--- i mean like trying a bunch of different preamp tubes in this amp ? I'll often try out 25 or so different preamp tubes of different brands in an amp i've built or serviced----- preamp tubes can make or break an amp's tone response. The same tubes can sound different in different JCM800's--- you just have to try 'em and see.
If you do want to go as far as modding the tone stack then download this:http://www.duncanamps.com/tsc/index.html
You can alter values of caps, resistors, pots in the Marshall (and other) tonestacks and see what the changes do to the behavior of the tone stack before soldering anything.
It sounds like you've got specific tones in mind for your amp, but it might take some experimenting to get just what you want---- i like to tweak amps to my customers' individual tastes but we live many miles apart----- i'd contact phsyconoodler since it seems that you guys live near each other and take him up on his offer of help on a local scale---- he's not a bad guy despite the "hearing test " jab---- have him buy you a beer for the remark and let him help tweak out your JCM800.................gldtp99

[RedMan]

Thanks for your reply. I thought of playing with the NFB resistor value in hopes of making the presence control range "wider", for lack of a better term. I did alter the front end of the amp in that I removed the usual items that make these amps so bright from the factory, including that bright cap on the volume pot, and now just want more "shimmer" if you will, as opposed to overall brightness, if that makes any sense. I like the sound of the amp with the presence set relatively high, but doing that doesn't seem to make it as brilliant/peaky as it was stock. My understanding of the presence control is that it affects a fairly narrow bandwidth with respect to frequency, and those seem to be the sweeter ones I want to bring out in the amp (almost like harmonics), without enhancing more midrange, gain, etc.

By decreasing the value of the cap on the presence pot, will that make the control "wider" as well?

Pat


I would try reinstalling the bright cap across the volume control but go to a much smaller value. Somewhere between 47-250 pf might do it. Decreasing the value of the cap on the prescence pot below .1 will give you less effect in a narrower frequency range.

I got Psychonoodler's joke, those amps are really bright stock (without removing the bright cap) and the highs are the first thing to go when you start losing your hearing.

[nsureit]

You are all steely eyed missile men, or mad scientists. I don't even know what tube bias is...are some tubes deserving of scorn or disrespect? :(

[darkbluemurder]

The presence control decouples the feedback at higher frequencies. If it is not bright enough turned up full you need to adjust the treble frequency response through the preamp. Good suggestions have already been made such as to reinstall the first treble peaking circuit (470k/470pf) after the low input, a bright cap - I would use no more than 500pf and to use also the second treble peaking circuit of 470k/470pf between the 2nd and 3rd stage.

If you want a stronger effect out of the presence pot then increase the negative feedback by using a smaller series resistance. If you have a 47k try a 27k. If you have 100k try 47k.

Good luck!
Stephan

[doctorx]

You are all steely eyed missile men, or mad scientists. I don't even know what tube bias is...are some tubes deserving of scorn or disrespect? :(

Tube bias explained. (http://www.aikenamps.com/WhatIsBiasing.htm)