How would this boost work???

[rake]

I always thought losing a cathode bypass cap would negatively effect the signal.
This is from the Bedrock Royale Reverb schematic. (Second gain stage)

[stratomaster]

That will work as a boost.  I prefer to short across a higher value resistor (33k-100k, 1/4w) instead of just closing the switch. Less of a pop that way. 

[kagliostro]

More readable

[Merlin]

I always thought losing a cathode bypass cap would negatively effect the signal.

Losing the bypass cap causes a gain reduction. Therefore adding the cap back causes a gain boost.

[SEL49]

You can also make the gain adjustable.

https://sluckeyamps.com/smoky/smoky.pdf

[rake]

Losing the bypass cap causes a gain reduction. Therefore adding the cap back causes a gain boost.

So, "normal operation" would be with the boost switch open and boost is when the switch is closed?

[Merlin]

So, "normal operation" would be with the boost switch open and boost is when the switch is closed?

Yes

[rake]

Thanks Merlin. For some reason my brain was in bass ackwards mode and couldn't see it the other way around! LOL! 

[shooter]

Losing the bypass cap causes a gain reduction.

seem to recall it also "narrows" bandwidth, lower frequencies "fall off" ?

[HotBluePlates]

Losing the bypass cap causes a gain reduction. ...

seem to recall it also "narrows" bandwidth, lower frequencies "fall off" ?

Not really, because there's no frequency-dependent element to the local negative feedback.

Adding a "small" cap would boost gain for highs, while adding a "big" cap would boost gain for lower frequencies, too.

[acheld]

Go to https://ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/ and you can easily see how size affects frequency response and signal strength.