Machine Gun Amps
Notes on the silverface
Twin Reverb

The main differences in the circuit of this amp and the blackface is the bias supply, and the phase inverter. If you look at the layouts, it's easiest to find the differences (sorry for the quality of these images, I really can't get it any better with the source I'm using). The bias supply in the silverface amp is a balance. The resistors between the pot and the grids of the power tubes are 68k instead of 220k on the blackface. The plate load resistors on the phase inverter are 47k instead of 82 and 100k on the blackface. The grids of the phase inverter have 330k's instead of the 1 megs, and the cathode resistor is a 270 instead of 470 ohm. The coupling cap to the phase inverter is a .01 in silverface, and a .001 in blackface. If you want your silverface Twin Reverb to sound better, change this stuff back to blackface! This also applies to most silverface amps as well. These changes got done on pretty much all amps from the Deluxe Reverb on up sometime in the late '60's to '70's.

You'll also find that not all amps match this silverface schematic exactly. Fender went through a few incarnations before it came to this one. Then there were some changes they did afterward too. This is a good starting point to look at though.

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