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I just got in my 2204, and I'm a newbie. I found this 220k resistor on the 1 meg pre gain pot (connected to the two outside terminals). I don't see this on the schematic, and the previous owner added an effects loop, so maybe this is a mod that's supposed to do something, add more gain? I don't know if it was bad before I took it out, or I may have ruined it, because I checked it with a VOM, and it has no resistance, or conductivity. Can someone tell me the purpose of it? I'm wondering if I should just leave it out, or put a new one in. Thanks.
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putting a parallel resistor across the "legs" of a pot makes it a smaller value
1/1M + 1/220k = 1/180k
its a 180k pot!
it also messes with the taper, better to use the right value... no one makes a 180k pot though
An FX loop? I'd put it back to stock... -
Tedmich, thanks for the technical info. I was wondering what effect it would have? Would it make more gain? ...or, make the amp seem to have more gain at a lower setting, but not any more at 10?
Also, I found a diode across R6, so I'm wondering what that might be for, as well?
I plan to keep the loop there, because the ugly holes will still be there, where the jacks are now. I don't use it as an effects loop, but I have used this amp and a cab as a halfstack, and used the "preamp out" to go to another power amp and it's own cabinet, and just used the controls on this amp.Comment
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The 1M pot puts the signal to ground (throws away gain) and turning it up keeps more signal from ground (raises gain) so a 180k pot would only allow ~20% as much signal to stay; a quarter turn of the original 1M pot = the 180k full on.Originally posted by breeze View Post
the schem I am looking at has no numbered resistors:
http://www.drtube.com/schematics/marshall/2204u.gif
wheres R6?
Can't think of why they did this unless the FX loop and other changes did BAAAD thing to the amp (motorboating oscillation, squealing) and they needed to calm things down a ton. You can, of course, remove all the added crap internally and keep the jacks in place. Then begin your own mods but it is a VERY GOOD IDEA to start from the original, which worked great.... can't polish a turd yada yada...Comment
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Originally posted by breeze View Post
the R6/10k if the famous cathode bypass cap (where a lot of amp tone resides) having a diode across here is definitely non standard practice, but is discussed at the Valve Wizard WEB site (the place I got some of my best tube schooling)
best read it soon, as he is selling it in book form currently:
http://www.freewebs.com/valvewizard1...Gain_Stage.pdf
This identifies the last modder of your unit to be a bit of an "original thinker" IMHO. I would most definitely go back to square one, as your amp in on square [square root(-1)]Comment
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Thanks for that link. Very helpful.Comment
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oh wrong number...I don't know my Marshalls....diodes at the cathode bypass are common in other Marshall series amps...Comment
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