Author Topic: feedback loop (Read 4011 times)

the_bleeding · « **on:** March 12, 2007, 08:39:16 PM »

Hey guys, i have a marshall 6100LM, and i've noticed that it doesnt sound much better when i crank the master volume, and it also doesnt really give me any poweramp breakup. Righth now it has mesa 5881's in the powersection...

Could this little issue be cause by a feedback loop? I have schematics if you need them to check for one, because i dont really know what to look for.
If it is, i'll hook it to a switch so i can disable it...

Thanks so much.

shaman · « **Reply #1 on:** March 13, 2007, 02:33:50 AM »

hmmm..how long have you had amp??has it just started this?are you getting any strange noise/60 cycle hum/uncontrollable feedback?is it getting louder ,but not breaking up?I am not too familiar with 6100, but it seems to be a real cruchy amp from my memory..

the_bleeding · « **Reply #2 on:** March 13, 2007, 06:32:05 PM »

Quote from: shaman

hmmm..how long have you had amp??has it just started this?are you getting any strange noise/60 cycle hum/uncontrollable feedback?is it getting louder ,but not breaking up?I am not too familiar with 6100, but it seems to be a real cruchy amp from my memory..

ive had it for abotu 3 months, i forgot to say its the 6100LM (lead mod, more gain in the 3rd channel).

I'm not getting any noise or feedback, its just not getting me powerdistortion, or sounding any fatter as i turn it up. I have loads of predistortion, but i'm really after the combination of both pre and power to get me that extra grit. Its just usually when i play with tube amps, they sound way better as you crank the volume, and this puppy doesnt really.

shaman · « **Reply #3 on:** March 13, 2007, 09:30:03 PM »

hmmm-this is what I know about the feedback loop-take a mesa boogie road king-I believe the modern setting on it basically opens the feedback loop and allows for more volume/headroom-i am aware that many older fender amps use this mod as a fix for big volume-the output gets gated or blocked at certain voltage-just seems like a more modern,recently built amp would have a setting to deal with this, but I am not an amp builder-when you get to biasing options and mods,it gets tricky-on my boogies, for example, I crank the master and decrease my channel level...if I am at 6 on master volume, I better be under 6 on channel-i usually keep my channel vol around 2-i get more power amp saturation...this is only desiable at gig levels...not bedroom...this gets into the realm of phase inversion..negative feedback...etc...uber tech!!

the_bleeding · « **Reply #4 on:** March 14, 2007, 02:25:52 AM »

feedback loops are in fenders. They essentially block the poweramp from breaking up so you can get louder cleans. I'm wondering if i have one because i LIKE my poweramp breaking up.

I just want to know if i have one, and if i do, what i have to do to disable it.

shaman · « **Reply #5 on:** March 14, 2007, 02:12:35 PM »

I believe it would have a feedback loop-not sure, but i am aware of amp techs playing around with phase inverters ,etc....seems like it could have some side effects on what i would consider a modern amp, but i am no tech-have you ever checked out plexipalace.com? I think you could get some help there..very tech-ie/DIY stuff-i was hoping it was only a power tube issue-best of luck-if you do a mod, just always be aware there are usually side effects-the feedback loop is usually there for a reason!
keep me posted

shaman · « **Reply #6 on:** March 14, 2007, 03:44:00 PM »

ok...my boogie mark iv,according to my more knowledgeable friend, uses a pull pot on the presence-pulling it add a .001 cap to circuit/which adds more negative feedback,resulting in sweeter sound-great for liquid lead tone.basically filtering the high end-called the depth mod and it is used with many boogies-on a dual rec, there is a modern?ssetting that bypasses the feedback loop,resulting in a less tamed beast-seems like this would be a basic mod you could make-on my amp, I usually play with it pulled(therefore using the loop)-I play it on full power(as opposed to the bedroom setting of tweed power-basically acts as an attenuator) to get more power amp sat....big difference-i like both...have you played the 6100 at full power much>??might help..

the_bleeding · « **Reply #7 on:** March 19, 2007, 05:05:47 AM »

i run it at 100 watts, all 4 powertubes workin, pentode mode. I have a bunch of switches to change it to half power (2 powertubes) and triode... but half power kills some distortion, and triode MURDERS the low end and distortion... so, its stayin at full power.
My master volume stays around 9:30 (i know, it should be higher)... but i dont trust my cabinet to handle it right now. Right now its sitting ontop of a line6 spider 4x12 with 2x8 ohm inputs... and the lowest impedance my amp has is 8 ohms, so i can only plug it into 1 side of it.
Though, i have run my master knob up to around 2 oclock, and the tone kinda started thinning out... but i could blame that on the cabinet, and my channel volumes were set pretty low to compensate.... yeah...

HTH AMPS · « **Reply #8 on:** March 19, 2007, 11:38:36 AM »

what you're describing sounds typical of that amp from what I can remember of mine. there is so much preamp gain that you won't notice much more breakup with it cranked.

Also, it's a LOUD 100w amp, you're not gonna get the output stage starting to breakup until its running past at least half volume.

As for the feedback loop, you could disconnect it and you'll get a looser bass end and more breakup. Marshalls typically have a purple wire going to the impedance selector switch - this is the feedback fed back into the phase invertor.

:twisted:

shaman · « **Reply #9 on:** March 19, 2007, 08:37:24 PM »

good points-if you havent yet, go to forum on plexipalace-real knowledgeable amp builders floating around-weird-triode and pentode act opposite on my amp-good point about your cabs-believe you can put together a y cable (2 male/1 female) that would allow you to run both inputs /might have to do a little re wiring,but I have a friend who does it and he hasnt fried anything in 20 years

the_bleeding · « **Reply #10 on:** March 20, 2007, 05:38:45 AM »

Quote from: HEAVIER THAN HELL

Also, it's a LOUD 100w amp, you're not gonna get the output stage starting to breakup until its running past at least half volume.

As for the feedback loop, you could disconnect it and you'll get a looser bass end and more breakup. Marshalls typically have a purple wire going to the impedance selector switch - this is the feedback fed back into the phase invertor.

:twisted:

i have run it past half... still sounded pretty thin.

And from what you just told me, i have a feeling that i have a bunch of buttons attached to it... like a built in buffering system to get the distortion dirtier and the cleans cleaner... i'll play with that and see what i can do :)