Hot Pickup+Tube Combo=Nasty Squealing. Please help!

[somebrains]

Easiest is to unscrew the 4 screws on top, pull the speaker + power cables, unscrew grill, and slide out the front.
Kinda gives you enough space to work instead of taking out the aluminum bar in back and dealing with all that.
You have to lift the chassis a little when reinstalling the screws from the top but they catch easy.

It's a lot like working on an SWR in that going out of the front is simpler than going out the back.
Somehow these amps catch weird in the combos when you rear load.

 

[Riff Ranger]

I managed to figure out how to get new preamp tubes in without electrocuting myself, and this seems to have cured this combo of fits of uncontrollable squealing. I had gotten used to babying my attack and taking it easy with the EQ, but I pushed it harder with the new tubes (if I had a separate cab to plug into, than I could have isolated the bad one, but oh well; I'll find a way to see if either of the old ones would still work) than I had in weeks with both the T-bird and the hot-rodded Electra and everything just kept sounding good. Glad the fix was relatively cheap and easy, glad the pickup will work after all, and glad to have gotten inside the amp and learned a bunch. Thanks for your wisdom and interest, everybody. Now, to find out how to drive this thing without burning through tubes or blowing speakers, bwahaha

 

[somebrains]

I kinda took you thru the tone stack and the bump the EQ throws in when it's kicked on.
Now find what the tone stack does with your basses within a reasonable volume.
The EV15L has a breakup curve that becomes predictable as you push into it.
Learn to use it, it's a lot like playing with old Fender tube bass amps.

Get super acquainted with it, then take the Buster out cab browsing and note the changes with various different cabs and drivers.

It'll be a nice journey for you.