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presence vs. feedback


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8/8/1998 1:30 AM
mike
presence vs. feedback
my question regards the amp in the previous post, marshall 101...after months of fine tuning (well, actually 4 yrs.) i finally have the amp close to what i'm happiest with. but i've come across a strange conclusion...i've modded a feedback vari switch for different values; 27k, 47k, 100k. with 27k the amp sounds great, and with each higher value not only does the gain increase, but the volume increases, and with that the damn treble does too (just as before). is this normal clipping? also, i've read that eric johnson disconnects the presence control to eliminate unwanted highs. what does this refer to? disconnecting the entire feedback section? the cap? ground? would changing the .1 to a higher cap make a difference w/presence already at "0" anyhow? thanks, mike
 
8/8/1998 9:28 AM
Richie

Mike; It looks like you would not mind to experiment,so try changing the .0022 to a .022 cap [on the plate] change the 33k resistor to a 56k and the little 500pf to a 250pf this will change the lead to a bass amp marshall,when you change the feedback resistor with this mod switch you installed,I think if you listen, you will find that you will not have as much volume range,in other words if you have the 27k on at the time you will be able to turn the volume knob further before it starts to distort,if you switch to the 47k, it will distort earlier,and as you change further[56k 68k 100k] you will barley have to turn the volume before you get distortion,not much room for any clean sound.Also if you took off the 500pf across the 470k resistor you may want too put it back after changing to the bass amp mod.I think you said you removed the .005 on the volume control,you may want to put it back,or change the value for a gain boost.You should not disconnect the presence control,If I remember right your amp won't work, if its just disconected,check this pots value,also for a mid boost change the 25k pot to a 50k pot[Big difference in sound] Check all your resistors to see if they have changed value,this will effect the amp alot if they are not in the range they are supposed to be.Your feedback wire should be connected to the 8 ohm leg of the output if your useing EL34s. I hope this will give you some help. [Richie]
 
8/8/1998 1:03 PM
Steve III

Richie:  
 
    Do you have any similiar suggestions for modifying a 5F6A bassman feedback loop, driver/PI and presence control?  
 
Steve Ahola
 
8/8/1998 1:43 PM
Richie

Steve; What would you be tring to do, make the bassman more like a marshall? Most of the fender is the same except for the cap sizes on the preamp plates,[.02 bassman,.022 marshall]and the bassman having the 270k and the marshall having the 470k with the 470pf cap across the bright channel,you could change the feedback resistor to 47k on the bassman [from the org. 27k]for a little more poop! And put a cap across the volume control,you can play around with this to get one you like, .004, or what ever,some use these on a swith for a boost.The mid mod would be the same,[if you like it] put a 50k in place of the 25k.The same for the tone too, both look like they have the same 56k with a 250pf cap,[more bass] if you want less change it to a 33k with a 500pf cap. The driver circut should have low tolerence resistors[2%] on the 1 megs and 470 ohm. And make sure the .1uf cap is a good one.If you do change this, put a 7025 in place of the 12AY7. Try this for a start.[Richie] one other thing is check the resistors on the input jacks make sure they are 68k and 1meg to ground!
 
8/8/1998 4:17 PM
Steve A.

Richie:  
 
    I'm rewiring a 72 SF Bassman head with one channel being like a 5F6A and the other channel being more like a Marshall. I haven't done very much experimentation with the long-tailed driver and presence controls- I've just copied the circuits from one schematic or another without really understanding how each of those resistors and caps affects the tone and response of the amp. So your remarks on the Marshall PI have been very helpful...  
 
Steve Ahola
 
8/9/1998 12:50 AM
mike

thanks for the ideas richie. i think i've tried most of them at some point. right now i have the 56k on the tone circiut, which i found while a/b-ing to reduce gain big time. i did remove the 500pf across 470k, just as the older schems. but i still have the .005 on the volume pot for crisper lower volumes, which sound good. as far as the .0022, i kind of like having two different sounding channels. the nasty high frequency i'm hearing comes in just as well on input 2, hence removing the bright channel might help but won't fix...feedback wire is on the 16ohm tap, as in some schems. other schems have it on 4, and 8. would this make a difference? i have the two speaker jacks hardwired at 8 & 16 to bypass the "selector" piece of crap. the 50k pot sounds like a good idea, actually. the mid pot in general is more like a high cut so i usually have to reduce it to two or so.
 
8/9/1998 7:08 AM
Richie

Mike; I went back to the 101 post to try and see what else you might try. Is this the origanal[output] transformer? Your amp should not be like this.You should read the resistence on the transformer,if your transformer is not original, it may need the leads switched on the plates of the power tubes. If you don't have a scope,find the wire coming off pin 8 of the second tube,before it goes to the 56k.take it loose, and hook it to the hot side of a 14 in. jack then a wire to ground off the jack to the chassis.This will be a preamp,[without tone controls]hook a cord from this jack to the input of another amp,plug into your marshall,using it as a preamp, and keep the other amp at low volume,listen to see if you hear the high noise,if you do, the problem is somewhere from the input jack to to pin 8 of the second tube,if you don't, it is in the tone or driver or power amp section.Some will disagree but I would match the impedence with the cab you are using,marshall are very tender in this area.Have you changed the electrolitics?[big and small] you could also have a bad pot. ]Richie]
 

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