Here goes methode A)
A1) Turn on your amp like normal enough to start to play.
A2) Pull the power chord out of the wall socket without touching anything else.
A3) Just let the sound/hiss/hum die away naturally - it will at one point sound horrible.
Now the amp is drained - still be careful when you open the head up.
Methode B)
First do methode A - then to play it safe do B:
B1) With the amp opened up find the rectifier cap can
B2) Loacte the cap connectors - probably underneath
B3) Take good long screwdriver with an insulated handle (plastic/rubber)
B4) Let the screwdriver metalshaft connect to one of the connectors on the Cap.
Let the screwdriver tip hit the chassies or the "grounding bar" somewhere.
Caveat: I don't know if Doc:s amps uses a grounding-bar/strip. Chassies is OK
B5) Apply the same procedure on all connectors from the Cap.
Now you are safe.
IF the amp WAS NOT drained in step A - you will see a spark and hear a crackle when you empty the Cap can - with the screwdriver.
Regarding BIAS-adjustment - during this procedure the amp MUST BE ON -
so there is no point really trying to drain beforehand.
Use methode A - if you want to feel safer.
The Route66 has a bias-pot somewhere inside - so you must open the head up for bias-adjustment.
Maybe methode A draining - is a good idea - hadn't really given that wrinkle much thought.
Thanks for mentioning it.
Cheers: Dixiechicken