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previous: TMiller Make sure input is wired like a tru... -- 4/22/1998 8:26 PM View Thread

Re: Marshall master volume preamp

4/27/1998 6:28 PM
Michael Tousek
Re: Marshall master volume preamp
Sorry for my long delay in replying -- I've been stretched pretty thin between work and wrapping up this semester.  
I'm glad you mentioned the inputs because this is something I had overlooked as a possible problem. In fact, the inputs were still wired as I had them in the amp's previous incarnation as a McIntyre Bluesmaker. I should use the term "miswired", though, because the way I had them, one of the 68K resistors was in parallel with the 1meg to ground when I was plugged into the first input. This was weakening the signal applied to the first stage.  
The next problem (which is the one you were seeing) is that a 2204 uses a single input to the high gain "channel" and uses only a series 68K and a 1meg to ground, whereas a plexi uses dual parallel inputs which puts both 68k's in parallel and results in a series resistance of 34K. You were anticipating a too-strong signal to my amp's 2204 side due to my use of the plexi's parallel input scheme. This would have been right on the money if I had had the inputs wired correctly.  
What I actually had, though, due to my miswiring, was the equivalent of a low sensitivity plexi input feeding both the 2204 and the plexi preamps. This was resulting in *decreased* gain to the two circuits.  
I have since corrected/revised my input scheme so that when the plexi channel is selected, a 68k resistor is added in parallel to the input's 68k resistor. This results in the correct series resistance of 34K. When you pull the channel-select switch, the extra 68k is removed so that the series resistance is simply 68k, thus keeping the 2204 circuit happy.  
The net result of this fix is that the 2204 circuit has more gain (I always thought it lacked some kick before but chalked it up to single-coil pickups -- now there's plenty of gain), and the plexi circuit is brighter and has more gain, too. Unfortunately, none of this has changed the fundemental character of the 2204's distortion. It still seems to fumble around on low notes kind of like a fuzz box.  
That was an excellent suggestion, though -- I had completely ignored the inputs in my troubleshooting.  
Any and all additional input is still greatly appreciated.  
Michael Tousek