Tubes behaving weird in my S-C au-34

[PFVictor]

Hi, I got a Stromberg-Carlson au-34 25-watt amplifier from my grandfather's basement. This is my first tube amp, so bear with me. I haven't gotten the new tubes for it in the mail yet, but its 5u4gb was acting strangely. Upon turning the amp on, a blue glow moves up and down the inside of the tube and disappears, only lasting about one second. Also, the 6l6g's glows will fade slightly and come back. I hear small popping noises coming from the speakers that are subtle, and become quieter but more frequent as I play my guitar through it. One of the big 20-10-10-10 caps has white goop coming out of it that has faded the schematic on the inside of the metal cover. I tested the tubes for microphonics. The second stage amplification tube, a 6sf5, makes an EXTREMELY loud *snap* that sounds like a rimshot more that a rubber band, like people tell me microphonics should sound. The way that I use this amp currently is that I put another jack on it hooked up to the circuit right after the signal exits the preamp tube, to use the amp as a preamp for my Dukane 35-watt solid-state PA. (Once those tubes arrive, I'll use the tube amp in its entirety.) So, what I want to ask is that if the 5u4gb or the 6l6's supposed to glow like that, where the popping noises are coming from, and if the cap is affecting this mess? (I'm replacing that cap as well) Thanks.

[Resonator Guy]

A cap that looks that bad will certainly affect everything else after it. I would carefully replace all the electrolytics before putting any new tubes in it.