Splawn Nitro white noise coming out of standby then....

[Redline]

....it’s playable with the gain below 5. Turn the gain past 5 and it squeals. Even without a guitar plugged in. The tubes are brand new and I did a quick test on my tube tester just to make sure I didn’t get a dud. I probed around and everything checks out. I’m thinking that I have a leaky electrolytic somewhere but IDK. Help please!

[pdf64]

Any component can fail at any time, especially tubes, whose manufacture / sale has been an unregulated wild west for decades.
Tube testers are generally pointless, they may reveal a tube that is a complete fail but pass a tube that is noisy / microphonic / low gain.
You likely have a bad tube.
It’s a good idea to buy from a vendor who tests and burns their tubes in, checks for noise, microphonics and gain, and provides a warranty.

[Redline]

They came fron Dougs

[Jazz P Bass]

Sounds like positive feedback.

Any chance that the OT primary leads got swapped?

[alexradium]

Plug the guitar directly in the fx return jack,if it doesn't squeal its the preamp,probably a microphonic tube,a bad lead dress or a bad electrolytic cap.

[dstrat]

having just put new tubes in the preamp likely more gain from the new tubes are "too much gain" they do a lot of lower gain 12ax7s in lots of those custom high gainers. might try going back to older ones in places like gain stages.