I've been thinking about modifying my new Valco (circa 1965ish), and wanted to bounce a couple ideas around here first. See attached schematic for details, but the basic layout of this amp is this: two 12ax7 driven input channels, feeding a quad of 6L6 power tubes. The weird part is there are two output transformers which fed the original 2 by 12 cabinet speakers in a stereo arrangement. This has been modified already. The two output transfomers now feed a mono output jack and a traditional mono 2x12 cabinet. The amp produces about 75 watts of power, so it is very loud and has tons of Headroom. Mod ideas: Since there are 4 inputs feeding two channels (which are almost identical), there seems to be an opportunity to modify three of the existing inputs - so that the amp has four different personalities available. My first impulse is to mimic the 5e3 circuit (Tweed Fender Deluxe Reverb), and simply add 1 Meg resistors to one input each for channels 1 and 2. That should give me a bright channel and a normal one, each with a high and low input. Same as the 5e3. The only difference between the two channels that I can see, is that one has a small 500 pf capacitor that the other does not(off the cathode of V3a). It appears to be a brightness capacitor - but I could be wrong about that. At any rate, the two channels sound identical, so I don't think it is doing much. Perhaps I should try larger or smaller value cap in it's place. Or perhaps add one in different position? - the 5e3 uses one right in the tone control. But what I really would like is to add additional gain to one of the two preamp channels. I'd like to make one channel crunch earlier and harder than the other. I noticed in the schematic that the V3b half of one12ax7 is not used. Would it be possible to use V3b to add another preamp stage on the tail end of the channel 1 preamp circuit - before it dives into the rest of the pre amp circuit? Just before V1b? I think I can handle the easy option of copying the 5e3 input mods, but designing the new gain stage is beyond me. What do you think of this idea? Is it possible to make one channel extra crunchy? Would using V3b cause my house to burn down? The 9151A schematic is at the bottom as a pdf..