I gave it more thought and doodled up two different schematics in response to the other thread. Just a cut and paste, OK dis some more figurin'. The first, second and Phase Inverter stages of the BF amp and the 5E3 run of of 250V so no having to change the supply voltage to them. The outputs and screen voltages do change as well as the bias voltage. If you were so inclined you could have some solid state diodes in series with the 5Y3 and in BF mode two switch poles shunt the current around the 5Y3 giving you a boost in voltage of 50-60V putting the amp in BF territory. The BF amp uses a 6.6k output transformer while the 5E3 uses a 8k. I would stick with the 8k. With the voltage on the output tubes higher you need more voltage on the grid so an additional cathode resistor in series with the normal cathode resistor is needed. One pole of a switch can be used to short this resistor out when in 5E3 mode. In BF mode the other contact can connect another bias cap, 470uF, to ground which stiffens up the bias and makes it more like fixed bias. Another pole is used to switch the NFB on for BF and off for 5E3. The additional voltage will increase the voltage on the preamp sections making them a little more clean. I am not including a schematic for this version, don't want to freak people out. Anybody capable of making the amp with the voltage boost could draw up a schematic from the description. Now moving on to the simpler version. Well two versions. There are always options and compromises. The first is the power supply does not change so the BF version has the same power as the 5E3, not a big difference but it is there. In these two versions the stiffening capacitor on the outputs cathode is increased. The difference is the first schematic you need a four pole switch to change the preamp version from 5E3 to BF but in BF you still get a Tweed channel. So in both modes you can change the NFB and bias stiffness, might find it useful. The next version, one four pole switch. There is a catch, when changing to BF mode the other channel is disconnected. The negative feedback and bias capacitor gets switched at the same time as the input configuration. You could easily enough have the Tweed channel hooked up also in BF mode, rather than where it is now connect it to the second stage input. The thing to remember here is that it will interact with the BF channel. So there you have it, I think I exhausted the realistic options. The layout and the schematics should be able to give an electronically competent person enough to put together an amp with the features/complexity they want. Not for the first time builder, you have to consider wire routing when going off the beaten path. I am going to try the second last schematic, I like what it has to offer the most and I have a chassis that can accommodate it.