Terms as used in this site
 
Amp – the whole combo; cabinet, speaker, chassis, the works
Cabinet – black vinyl over wood; the box that packages the whole thing
Chassis – the thing which does the actual amplifying; metal box which is screwed into the top of the cabinet, featuring knobs, switches, valves, and loads of internal wiring. The dangerous bit.
Reverb Unit – dirty-gold-coloured metal block screwed into the bottom of the cabinet
Valve – tube to you North Americans
Tube – valve to us Brits

Safety / Damage Disclaimer
Valve (tube) amps develop LETHAL VOLTAGES while running, and store them in charged components EVEN WHILE SWITCHED OFF AND DISCONNECTED FROM MAINS SUPPLY. These voltages are MUCH HIGHER than mains, and higher than anything you’ll find inside a transistorized amp. If this scares you, good. Inside a chassis, don’t use your fingers to touch anything which isn’t insulated or earthed (grounded). Don’t stick more than one hand in at a time, and keep the other hand well away. Use fine-nose pliers to manipulate components. If you’re not sure what you’re doing, get local help.

I have to disclaim any responsibility for injury, damage, loss of value or loss of gig due to inoperative equipment. The external web pages I’ve given as links all say roughly the same thing, and  their content is of course not my responsibility.

To work on the electronics you’ll have to slide the metal chassis out of the wooden cabinet. If this procedure seems obvious to you, fine, but it took me a little while to work out, and I’m assuming there are some other folks out there as slow as I am. Here’s how it’s done;

1
Check it’s not plugged into the wall. I don’t mean to insult your intelligence but we’ve all done some crazy things before now. Stand the amp on a table or workbench.

2
At the back of the amp, remove the upper rear panel (wood covered with Tolex (black vinyl) which protects the valves.) It’s a snug fit, so you may have to knock it out once the 4 woodscrews are removed.

3
Remove the speaker grille; it’s held on with Velcro at the four corners. The trick here is to PUSH it off from the rear rather than lever it off from the front, which would be more likely to damage the grille cloth at the edge. Using good lighting, look into the rear of the amp, just below the chassis. There's a small gap where you can see the back of the grille. At one end of this gap (the 'input' end is easiest), NOT in the middle, push in a flat-blade screwdriver and the grille will come away  (ie you're pushing at a corner of the grille, not halfway along its top edge.) The ripping noise is just the Velcro pads separating. Probably.

If you want to remove the speaker (not normally necessary) see here. To do this, you'll still have to remove the chassis as described below.

4
Unplug the speaker jack and the reverb cables from the amp. Don’t bother to unplug the cables at the other end (i.e. at the reverb unit). (When you come to reassemble, the red reverb cable is for the input, which goes to the right-hand connector on the reverb unit as viewed from the rear of the amp.)

5
Turn the amp on its side and remove the 4 long screws from the top of the amp (the 2 metal trim strips will come away too).

6
Turn the amp upside down. The chassis is now only held by 2 woodscrews passing through the rear edge of the chassis and into the wood of the amp top, so remove them.

Intermission
You’re just about to slide the entire chassis rearwards. Either push some of the mains cable through the cable-tie which is screwed to the inside of the cabinet side, or unscrew the cable tie altogether. Check that the edges of the black vinyl Tolex covering won’t catch the sides of the chassis as it passes by. And most importantly, be aware you’re just about to expose BARE WIRES with DANGEROUS VOLTAGES, even though you’ve unplugged the amp from the wall. (Please tell me it IS unplugged from the wall…)

To explain about the dangers of electrocution here; THERE IS NO ‘TOP’ TO THE CHASSIS. If you had the amp the right way up you’d be able to see down into the naked wiring of the chassis as you slid the chassis out. Since you’ve got the amp upside down, it’s the table-top which will be looking up at those wires. The point is this; as you persuade the chassis out of the cabinet, DON’T let your fingertips curl up and into the chassis where you can’t see them; they could stray onto something with a big fat charge on them.

7
So… push the front panel, and jiggle the chassis up and down a little so it clears the thin sheet of metal which is stuck to the inside of the cabinet top… and ease the chassis rearwards until it’s clear of the cabinet and sitting on the table-top. Make a note of the transformer date codes (606-xxx) and email them to me with your amp's serial number! Thanks! Keeping your fingers out of the chassis, turn it over and find a way of supporting it so that the weight isn’t taken by the valves and other sticky-out components. You can now see all the innards. If you’re going to be soldering or probing around with volt-meter probes, make sure the chassis is firmly supported and unlikely to wobble at a critical moment.

8
Now you can see all the works, you can safely discharge the high voltages before you start work. In the corner near the power valves, near the back panel, you can see the underside of the filter capacitor assembly (on the outside of the chassis, that's the big silver metal tube standing up at the end of the row of valves/tubes). On the inside of the chassis, the tags for this thing are arranged in a circle. Two of them are joined by a resistor. Take a resistor bigger than a few k and HOLDING IT WITH INSULATED PLIERS, CAREFULLY use it to bridge from one of these 2 tags to the metalworkof the chassis. Remove it after a few seconds and then CAREFULLY measure the voltage from one of those 2 tags to the chassis. The voltage should be zero or fairly small now. Obviously, if you power the amp up while the chassis is out of the cabinet, those lethal voltages will return.

Digression; While You're in In There
... there are a couple of modifications you could do. They're simple, they're cheap, they boost the reliability and they don't affect the tone. You'd be crazy not to. Not as crazy as people who go through life without checking out the claims of Jesus of Nazareth, but crazy nonetheless.

Operating the Amp while Dismantled...
... could kill you if you don't know what you're doing. Never, ever, work on a live, open amp while holding a connected electric guitar. If you need to check something by playing - connect the guitar, turn on the amp, set the controls as desired, stand away, play the guitar, put it down, go back to the amp, make adjustments, walk back to the guitar, etc etc. And to avoid expensive damage to the amp, reconnect the speaker and make sure air can flow around the valves/tubes before switching on.

Reassembly…
… is pretty much the reverse of the above. 

The Reverb Unit is held into the base or the side of the cabinet by 4 woodscrews and if you must remove it, you'll have it out with a few twists of a cross-head screwdriver. One point which wasn't obvious to me; the metal casing has no base - once you start to lift it away from the cabinet, those fragile springy things inside are exposed. So gently does it.