Negative Feedback, Reverb in 4-10" Bassman

[skeezbo]

I have three questions, but maybe they are just three ways to get to the same answer. I know I should just build it myself and see, but I don't have a suitable donor amp to experiment on.  Someone here has been through this before, I'll bet. 
1. Why did Fender the move the feedback loop from under the tone stack (5F6) to the phase inverter (5F6a)?
2. If I wanted to add reverb to the iconic 5F6a circuit,the obvious solution seems to be to move the negative feedback back over to the bottom of the tone stack 5F6 style and use the now open side of the inverter for a KOC-style side-chain reverb loop.  How would that change in the entry point of the negative feedback affect the performance of the amp?
3. If the negative feedback needs half the inverter in order to create the iconic tone, could the entry point of the 5F6 negative feedback (under the tone stack) be used as an entry point for a reverb circuit? 

Thanks!
Skeezbo
 

[HotBluePlates]

You could reinvent the wheel, but Hoffman already posted it in layout form here.

I don't know specifically why Fender moved the feedback injection point. However, aside from the 5F6 Bassman and 5F8 Twin, I can't think of any circuit that applies the feedback to the bottom of the tone stack.

Even if there are others out there, the more phase shifts you have inside the loop (each coupling cap, each gain stage, the output tubes and the OT), the more difficult it is to keep the feedback negative and avoid oscillation due to the feedback becoming positive at some frequency.

[skeezbo]

Thanks, HBP. Hoffman's design looks like it would work on any of the four input tweed Fenders from the late 50s. I hadn't thought about the possibility of having the reverb return so early in the chain. Great stuff!

Skeezbo