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previous: SProuty >...the output stage getting overlo... -- 12/15/1997 6:16 AM View Thread

Re: Preamp Gain Reduction

12/17/1997 2:23 AM
Steve A.
Re: Preamp Gain Reduction
>>>So, to your point, which resistors and up or down in value?<<<  
    For a grid load resistor, a lower value (in ohms) will shunt more of the signal from the previous stage to ground without effecting the tone too much. For a 12AX7 tube, pins 2 and 7 are the grids of the 2 stages.  
    For a particular gain stage, the ratio between the plate load resistor (at pins 1 & 6) and the cathode resistor (at pins 3 & 8) will determine the gain. For example, replacing a 2K cathode resistor with a 1.8K resistor will lower the gain. Replacing a 100K plate load resistor with an 84K resistor will also lower the gain (contrary to what would be expected intuitively). The ratio between the plate load and cathode resistors will effect the bias current for the preamp tube stage, so you don't want to change the values too much. Dan Torres has a chart on this on page 20 of his book on tube amps.  
     Changing the values of the cathode bypass cap (at pins 3 & 8) will effect both the gain and tonal response of the stage. For example, the Torres "Triple Killer Switch" (great name- eh?) uses an on-off-on mini-toggle switch to select between 2 different bypass caps and none at all for the initial preamp gain stage. With a stock 22-25uF bypass cap, you get a Fender BF sound with a lot of bass; with a 0.47uF cap, you get a midrangy sound like a Marshall and slightly less gain. With no cap, the gain is much lower and it is supposed to be like some tweed Fenders. In the Torres Reverb King amp, he switches the 22uF bypass cap on for the gain boost switch.  
    If you could e-mail a scanned schematic for your amp I could be more specific in my suggestions of what to try. (Does the Vibro King use point-to-point wiring or a printed circuit board?)  
Steve Ahola

SProuty Steve,Thanks for the recommenda... -- 12/17/1997 2:57 PM