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Interesting stuff for Fatboy 3 channel heads:

There are a whole bunch of 12AX7's and a 12AU7 in these amps. Here is a list of each tube position and what they do. It is fun to experiment with different brands of tubes and tube types.

#1 - 12AX7: Clean channel and #1 tube for channels 2 and 3.
#2 - 12AU7: 2nd stage for channel 2. You can also try a higher gain tube here for more gain in channel 2, like a 12AX7, 12AY7, etc.
#3 - 12AX7: 3rd stage for channel 2.
#4 - 12AX7: 2nd stage for channel 3.
#5 - 12AX7: 3rd stage for channel 3.
#6 - 12AX7: F/X loop.
#7 - 12AX7: Phase inverter.


I've been using a bias probe to bias my amp. I've been told the plate voltage on Fatboys is 480v. My 2 heads measured 490v. I calculated that on mine to run EL34's at 70% plate dissipation, 35.7ma is the number.

EL34 = 25w max dissipation
25 divided by 490 (plate voltage) = .051
.051 x 70% (plate dissipation) = .0357 or 35.7ma

If you are too lazy to do the math, WeberVST has a bias calculator:

The Fatboy bias control has a wide range, so you can use other tubes as well.


Mouser has electrolytic filter capacitors in case yours have seen better days. Make sure you get the correct values/voltages and the correct physical size (measure the ones in the amp) and the 10mm pin width. Currently at, you can get the caps made by Cornell Dubilier that will fit in the right vales/voltages/sizes.

The main ones are:

4 - 100uf/400v
1 - 68uf/400v
1 - 6800uf/25v

Other parts:

I was told that Norb got a lot of the parts such as caps, resistors, pots, etc from Mouser, and a quick check reveals this to be true. They still even have the control knobs if anyone out there is missing some.

Allied still sells the relays:
W172dip-147 (qty 3)
W171dip-7 (qty 4)
W171dip-12 (qty 2)
About the only hard to find parts are a couple of the IC chips in the channel switching section.

Hand wiring the power tube sockets for Texas made Fatboys:

One of my TX Fatboys had bad power tube sockets, and I could not find suitable PCB replacements with mounting bases. I decided to chuck the PCB setup and go with the hand wired sockets like the Norb Funk-built Fatboys. I took great care to wire it exactly like the Norb-built amps. Most of the parts required are easy to find, with the exception of the 2 bolt-on insulated turret lugs for the grid resistors, I had to scrounge in some electronics surplus stores for those, but I did find identical ones. I used 16 awg buss wire. The result is far better sockets and contact integrity than the stock PCB sockets. \


Here is a clean channel boost mod from Fatboy user 6AM:
Clean boost : There's a resistor that goes from the gain knob on the clean channel to the bright switch. Replace the resistor with a .22uF cap (or really any value cap). It will boost the signal and add low en