Hi Jeff; V1 ... the first pre-amp tube ... one half (A) is fed from the instrument jacks, the other half (B) from the mic / other set of jacks. Otherwise, the tube halves are independent of each other. If replacing V1 with a known-good 12AX7 doesn't help, chances there's a bad plate or cathode resistor on the B half. Hiss is characteristic of a bad resistor ... hum is usually either a capacitor or grounding problem depending on how it sounds.
Jeff said:
This guy has the same power tubes that my Ga20t has but the Monkey Ward has way more umph.
The 15" speaker is moving roughly 25% more air than the 12" Gibson speaker ... all things being even, it will seem like it has power ... or presence. In
Gibson Amplifiers by W. Marx, there's a lot of discussion about what Ted McCarty and Seth Lover were trying to do with the tone of Gibson amps. Their goals weren't necessarily the same as MW/Dano's; the MW is newer than the Gibson and was marketed to a different buyer. My experience with Gibsons v. Silvertones (Airlines under a different brand name ... many Silvertones and Airlines share identical schematics) is that the Gibsons are more mid-rangey, compressed, less gainy / 'twangy' ... not as hot or forward.
Interesting twist to wind up with your Bro's amp!