The F-106Cs are rated 20dBW of output using fixed bias on four 6GT5s per amp with a unique input and driver arrangement. Anyone who is familiar with the Grommes amplifiers will instantly recognise this configuration as many of their amplifiers used it. The amps worked fine when I received them but I decided to replace all of the output tubes with NOS RCAs to ensure dependable service in the future. Here is the schematic. These amps are identical to the Grommes-Precision G-101A and similar to the power amp section in the Precision S100. They are very well designed but I have some suggestions if you own some or plan to build one or some of your own. First, Frazier and Grommes recommend that the output tubes should idle at 40 to 45mA. This corresponds to 0.8 to 0.9 volts across each 20 ohm resistor in the cathode of the 6GT5s. This is nearly 20 watts dissipation per tube. This is pushing the tubes to their limits. I recommend 35mA on each tube. This is much safer and the tubes will last much longer. This corresponds to 0.7 volts at the bias test points. Second, there is little to limit current flow in the output circuit and if a failure or runaway condition occurs it can ruin a lot of expensive equipment quickly. The 5 watt 20 ohm resistors could be replaced with 1 watt or 2 watt resistors which would act as a fuse under abnormal conditions. Alternatively, one could fuse each tube with a 150 mA fuse. Third, the solid state rectifiers are very low impedance and give the amp a lot of kick. Unfortunately, they are also very hard on the tubes by applying 440 volts to their plates before the cathodes are warm. A circuit or switch needs to be installed to prevent this from happening. It is preferable to ramp the B+ to full voltage after the heaters have been on for 30 seconds or more. A standby switch would be the simplest way to accomplish this. Last, fix a resistor to the bias pot such that if the wiper goes open the tubes will be pulled down to a safe bias. Safety and longevity are important enough to warrant doing these few modifications in this case. These amps weigh 35 pounds each.