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When taking measurements there are some things to consider. Some components cannot be tested in circuit because they are parallel with other components via the circuit path. That means many resistance reading may seem wrong. Also, there is usually some voltage lingering in the circuits. Even with power supplies designed to drain the voltage after power off there can still be some lingering and it may mess with your reading because your meter uses voltage to do it's job. So unless you can tell what can and cannot be tested accurately the results of in circuit readings can't give you any right or wrong answers.
Since you've already had the amp powered up and it didn't explode I would venture to measure some voltages. Be sure the meter is set to the correct parameters if this is necessary. The amp would be live when you do this and you need to work safely. Do not hold the chassis, your guitar, the metal jack plug or freekin anything in the hand that's not holding the red probe. You may need to hold the black probe in that free hand to touch it to ground, but if you can you're better off securing it to a ground point with a clip lead.
Start by making sure the amp is plugged into a load and all knobs are at zero. Then power up the amp and take it out of standby.
The first thing you'll want to check for is DC voltage on the chassis. Touch the black probe to something that is grounded that ISN'T your amp. Now touch the red probe to your amp chassis. If you don't get zero the amp is dangerous and must be unplugged immediately. Hopefully you get zero. Then we can move forward.
Ok, actually, if you're using a digital meter you probably won't get zero. You'll get some tiny default amount. Touch the meter probes together. That is your default reading. You can ignore it because it's probably tiny or trouble to subtract it from all your readings. Your choice. Moving forward.
Next you'll want to check for DC voltage on jacks or pots. Black probe grounded to your amp chassis and red probe to the + terminal of jacks. Note that some jacks have switches built into them that function when you plug in and out, this should be readily visible. Plug a cable into these jacks and take your reading from the tip of the plug. Measure the center lug of all potentiometers. There should be no voltage on any jack or pot except perhaps the "presence" control. If all this checks out we can move forward.
Now we will check DC volts on pin 5 of each output tube. These should read some amount of negative voltage. Probably in the range of -35V to -50V. If it's not there you're cooking your power tubes. Turn the amp off. If it's there we move forward.
The power supply has a sequence. The main filter is the first large capacitor off the rectifier. Measure DC voltage at the + lead of that capacitor. Proceed with the other filter capacitors in sequence. You should have some 4XX volts on the main filter and a little less at each consecutive filter. Moving forward...
Are you taking notes? You're going to have to tell us this stuff!!!
Now measure the DC volts on every other tube pin. We call the first preamp tube V1, the second V2 and the one nearest the power tubes is V3. The power tubes will be V4 and V5. If your amp is using a rectifier tube that would be V6. Make your notes like this V1-3-1.8V That would translate V1, pin 3, 1.8 volts.
We'll be here when you get back
Since you've already had the amp powered up and it didn't explode I would venture to measure some voltages. Be sure the meter is set to the correct parameters if this is necessary. The amp would be live when you do this and you need to work safely. Do not hold the chassis, your guitar, the metal jack plug or freekin anything in the hand that's not holding the red probe. You may need to hold the black probe in that free hand to touch it to ground, but if you can you're better off securing it to a ground point with a clip lead.
Start by making sure the amp is plugged into a load and all knobs are at zero. Then power up the amp and take it out of standby.
The first thing you'll want to check for is DC voltage on the chassis. Touch the black probe to something that is grounded that ISN'T your amp. Now touch the red probe to your amp chassis. If you don't get zero the amp is dangerous and must be unplugged immediately. Hopefully you get zero. Then we can move forward.
Ok, actually, if you're using a digital meter you probably won't get zero. You'll get some tiny default amount. Touch the meter probes together. That is your default reading. You can ignore it because it's probably tiny or trouble to subtract it from all your readings. Your choice. Moving forward.
Next you'll want to check for DC voltage on jacks or pots. Black probe grounded to your amp chassis and red probe to the + terminal of jacks. Note that some jacks have switches built into them that function when you plug in and out, this should be readily visible. Plug a cable into these jacks and take your reading from the tip of the plug. Measure the center lug of all potentiometers. There should be no voltage on any jack or pot except perhaps the "presence" control. If all this checks out we can move forward.
Now we will check DC volts on pin 5 of each output tube. These should read some amount of negative voltage. Probably in the range of -35V to -50V. If it's not there you're cooking your power tubes. Turn the amp off. If it's there we move forward.
The power supply has a sequence. The main filter is the first large capacitor off the rectifier. Measure DC voltage at the + lead of that capacitor. Proceed with the other filter capacitors in sequence. You should have some 4XX volts on the main filter and a little less at each consecutive filter. Moving forward...
Are you taking notes? You're going to have to tell us this stuff!!!
Now measure the DC volts on every other tube pin. We call the first preamp tube V1, the second V2 and the one nearest the power tubes is V3. The power tubes will be V4 and V5. If your amp is using a rectifier tube that would be V6. Make your notes like this V1-3-1.8V That would translate V1, pin 3, 1.8 volts.
We'll be here when you get back
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