This will not work in quite the way you expect. Both R12 and your 50k feedback adjust pot have much bigger resistances than the 5k presence pot. This means that changes to the feedback will also cause changes to the frequency response,a s though you had turned the presence pot; your adjustable negative feedback will turn into (unintentional) adjustable frequency response.
Here is the revised schematic 2.1
Some other suggested mods:
pending Mod-8 Safety load on OT
With all that switching on the output of the transformer, you really need to put a 1000 ohm 3 watt resistor right on the output of the transformer. That way if a contact in a switch oxidizes in future & doesn't make contact, the transformer doesn't short a turn from excessive voltage.
(from indianajo)
pending Mod-9 Second cathode bypass
I would add a 25uF switchable cathode bypass to V1B for a 5E3 position.
(from RobRob)
I have to read up on both of these mods. I like the idea of the safety load. In a similar vein I also like the idea of a few more fuses to protect more expensive components.
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