Strange tone Marshall DSL40C

I’m relatively new to working on tube amps. I did my research and just replaced the tubes in my Marshall DSL40c. When I tried playing through the amp, there was a distinct ring after I’d play a staccato note. It’s not feedback, just a short ringing tone almost like bad reverb. Did I potentially make a mistake while replacing the tubes or is there another issue with the amp?
This distortion you are hearing could be oscillation caused by circuit negative feedback interacting with a limited bandwidth output transformer (OPT). Tube amps are very sensitive to the amount of negative feedback applied. Less is better! And a better OPT (wider bandwidth) would help! Guitar amps are notorious for having poor quality OPT's. Cheap and distortion-prone! Then again that's what electric guitar players want- DISTORTION!
I confess that I have  Fender Twin Reverb and a Mesa Boogie! Plus a Laney 100 watt tube head and 412 cabinet!
The problem is caused by a microphonic tube. Tap on the tubes while the amp is running and it should not be hard to find out which is causing it.

If the amp makes a sound no matter which tube you tap, try just rubbing the top of the tube gently- that will make it easier to find.