A tube amplifier chassis contains lethal high voltage, even when unplugged! If you have little or no experience or feel unconfident working with high voltages then have an amp technician check your amp. See https://robrobinette.com/Tube_Amp_Safety.htm for more information.
Peavey 5150
With the band McClane I used a Peavey 5150 (a signature MkI). When one of the tubes was failing I checked what the original tube configuration of a 5150 should be, mine consisted of two KT88's and two 6L6GC's. Turned out my 5150 was fitted with an integrated quads mod and a hot bias mod. Even though the mods were properly done I wanted the amp to get its original personality and character back again so I removed the mods and popped four new JJ's 6L6GC's in again.
Integrated quads mod
This mod consists of using two different types of tubes in the power amp stage of a 5150, in my case two KT88's on both sides and two 6L6GC's in the middle. Removed all the power tubes and replaced them with a matched quartet of JJ's 6L6GC's.
Hot bias mod
This mod allows you to bias the power tubes hotter than the factory default. But from what I understood the power tubes being biased colder is one of the characteristics that help define the sound of a 51501). Also if the amp is biased this way it's easier to replace the power tubes as the amp doesn't have to be biased again for the new tubes which isn't even possible in a stock 5150 MkI as it has a fixed bias.
After opening up my 5150 first thing that struck me was the build quality and the quality of the used components. It all looks very tidy and sturdy, this thing is built like a tank. First I pulled out the worn out tubes and unscrewed the tube sockets. Then I took out the power board.
As you can see this mod is pretty invasive, several traces were cut, a 1 Ohm resistor was added and a bias measurement construction with four 10 Ohm resistors was added. I took that all out and restored the board to its original condition.
I desoldered the potentiometer and resistor and replaced the cut 15K resistor with a new one.
Putting the tubes back in
Once the four 6L6GC's got in that I ordered from Tonefactory I popped those in, hooked up a small 1×12 cab and powered up the amp. No blown fuses, blue smoke or weird sounds so this restoration was a success. Had a rehearsal with McClane that very same night and the revised 5150 stood up way better against the Mesa Boogie Dual Rectifier of the other guitarist, the blend of those two now felt like it should, just awesome.
