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Preamp Gain Reduction


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12/12/1997 10:34 AM
SProuty
Preamp Gain Reduction
Could any one tell me which preamp tube should be replaced, with a lower gain type, to reduce the overal gain the output section "sees"? Is it the phase inverter tube or the first actual preamp tube. I think I've read recommendations for both.  
 
Thanks,  
Stephen
 
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12/12/1997 11:06 PM
Paul Markwalter
The first one the signal sees. Leave the phase inverter alone.
 
12/15/1997 2:16 AM
Steve A.

Stephen:  
 
    If you like the way the preamp sounds, but wish to lower the output level, I'd try putting in a lower gain driver tube. Replacing a 12AT7 in a Fender with a 12AU7 is supposed to create a really nice jazzy, bluesy sort of sound (the output section won't distort much and if you try to drive it to distortion with an OD stomp box, it won't sound very good).  
 
    What are you trying to accomplish here? Same sound, but at a lower output level? Put in a high quality 500k dual pot at the input of the output tubes for a nice master volume control that takes itself out of the circuit when set to 10. Or is the output stage getting overloaded by the preamp signal and you wish to tone it down a bit? In that case, maybe you could fiddle with the values of some of the resistors to reduce the signal a bit.  
 
Steve Ahola
 
12/15/1997 6:16 AM
SProuty

>...the output stage getting overloaded by the preamp signal and you wish to tone it down a bit? In that case, maybe you could fiddle with the values of some of the resistors to reduce the signal a bit.<  
 
Exactly, I think what is happening is that I'm really hammering the input section and I'm trying to reduce it some what. (I'm still on my kick to reduce the gain on my Vibro-King so that I have some more clean headroom.) I replaced the non-functioning speaker and this (obviously) made a huge improvement in the sound. I also placed a lower gain tube in the first stage as Paul recommended, but I'm still not quite happy. I basically want a very loud Vibrolux/Vibroverb sound: loud AND clean up to a point, then distortion. I should also mention that I adjusted the bias, by ear, to a point that the amp became definitely cleaner sounding, but I noticed that at that setting the output tubes started to exhibit a bit of a blue-ish haze in certain locations so I backed off a bit because I wasn't sure if this was a concern.  
So, to your point, which resistors and up or down in value?  
By the way I ve also ordered some Svetlana 6L6GC's to see if they help any.  
 
Thanks,  
Stephen
 
12/17/1997 2:23 AM
Steve A.

Stephen:  
 
>>>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
 
12/17/1997 2:57 PM
SProuty

Steve,  
Thanks for the recommendations, I'll see if I can find a way to get the Schematic scanned and send it to you. And yes the amp is wired point-to-point.  
 
Stephen
 
12/18/1997 1:33 AM
Steve A.

Stephen:  
 
    I forgot to mention another trick to reduce gain from the Torres sheet for fine-tuning all of his mods: add a 470 to 10K resistor in series with the cathode bypass capacitor. That will lower the gain without effecting the tone too much (or so he says- I haven't tried that particular trick myself.) It's probably best not to mess with the plate load and cathode resistors unless none of the other tricks work because if you change the bias too much, it can shorten tube life. Of course, if you had one stage that sounded particularly nasty, changing the bias might be what is needed to sweeten it up.  
 
    What kind of tubes are you using, anyway? I think a Sovtek 12AX7WBT+ works great for the first preamp tube, but its a little too hot for the subsequent stages so I use their 12AX7WB for the second preamp tube. For your clean channel, a 5751 will give you a clearer sound than the WBT+, but there isn't as much gain (the WBT+ adds a warm funkiness to the clean channel so I switched from the 5751). The Golden Dragon 12AX7's and 12AT7's make great driver tubes- clear and strong, without masking the sounds of the preamp stages. I suggest that you try these tubes out first, and then fine-tune the circuits a bit changing a few resistors here and there until you get the sound you are looking for.  
 
Steve Ahola
 

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