Hum
Hum is the most common problem which is usually caused by AC line noise leaking into the filament wiring or input stages and getting amplified. Here are comprehensive troubleshooting guide step by step.
(a)Reliable Grounding
The resistance from each parts ground connection to the chassis  should be less than 1ohm, typically 0.5 ohms.
Make sure the Mains Ground at the chassis is very tight.
(b)Volume Test
Turning the volume control(s) to find where the hum is. If the hum changes levels when you turn the volume control, then the source of the hum is somethings before the volume control of amp, checking anything around first preamp tube. Conversely, if the volume control does not affect the hum, the cause is somewhere after the volume control.
(c)Faulty tube
Tubes sometimes develop internal hum. Do some tube swapping to locate the problem. Use the volume control test.
(d)Severely unmatched output tubes in a push pull amplifier
Matched push-pull amplifier(Class AB amp) can cancel the ripple in the output transformer. If the output tubes are not matched because of different bias current of power tubes, the cancellation can be upset, the hum will be loud.
(e)Faulty power supply filter caps
One bad power supply filter capacitors would be possible cause hum of amplifier also. If you suspected power filter capacitors, please check them like following :
(1). Unplugged amplifier power cable and open the chassis .
(2). Check the capacitor(s) by eye for any signs of bulging, leaking, dents and other mechanical damage. If you found any of these, replace the capacitor. Same time, pay attention to the condition of series dropping resistors connected to the capacitors if they were damaged by heat.
(3). Discharge the filter capacitors. All of the power filter capacitors store fatal voltage and charge, even if amplifier power cable was unplugged.So you need discharge them before you test these capacitors.
(4)Use one lead with clip at two ends. One clip connected to the ground of chassis. Another clip connected with one end of 10K 1/2W resistor. Touch the another end of the resistor with feet of the each power filter capacitors at least 30 seconds.
(5)After discharge, measure the resistance between the (+) foot and the foot(-) of each capacitor by multimeter .
(6)The resistance should be over 15K ohms (Q). If you get less than that on any capacitor, unsolder that capacitor and re-measure just the capacitor. Less than 15KQ indicates a dead or dying capacitor; If the resistance is now much higher with the cap unsoldered, there is a low resistance load pulling current, not a faulty capacitor.
(7)If you did not find problems in these aspects. Try to temporarily solder a new capacitor, which is at least as high a capacitance and voltage across the suspected capacitor or section. If amplifier could work normal again, the problem is in capacitor still.
(f)Faulty bias supply in fixed bias amplifiers
A bias supply with excessive ripple injects hum directly into the grids of the output tubes. Check that the bias supply diode is not shorted or leaky, and then bridge the bias capacitor with another one of equal value to see if the hum goes away.
(g)Unbalanced or not-ground-referenced filament winding
The filament power must be referenced to the DC in the tubes in some way, otherwise you may get a lot of hum. The filaments are usually a center tapped 6.3VAC winding, with the center tap(CT) grounded for the necessary reference. If the winding is not grounded and balanced around ground, it will cause hum. Measure the voltage from each side of the 6.3V to ground; it should be pretty much exactly half the AC voltage at either end. If it is unbalanced to ground, tweak the pot or change the resistors to get it to be.
Note: If you have grounded center tap style supply that is not centered on ground, this indicates a faulty power transformer.
TIP: If your heater wires did not have a center tap to connect to ground, then put a 100 Q anti-hum resistor to ground from each side of the heater wires to the common ground point.
This will add a ground reference to the heater voltages and help to reduce hum. Other methods are low value pot (200-500 Q) across the whole 6.3V with the wiper grounded.
(h)Defective input jack
If the input jack is not making good contact to the guitar cord shield, it will hum. Likewise, if the jack has a broken or poorly soldered ground wire, or not-very-good connection to the grounded chassis, it will cause hum. If messing with the jack changes the hum, suspect this.
TIP: If hum or noise exists when the input plug is removed, try re-soldering the connections to the Input jacks.
(i)Poor AC grounding
In amps with two wire cords, defects of the 'ground reverse' switch and/or capacitor can cause hum. A leaky power transformer can also cause this.
(j)Induced hum
Placement of the amplifier near other equipment can sometimes cause it to pick up radiated hum from other equipment. Suspect this if the hum changes loudness or tone when you move or turn the amp. There is usually nothing you can do about this except move the amp to where the hum is less.
(k)Poor internal wire routing
If the signal leads inside the amp are routed too near the AC power wires, transformer, or alongside the high-current filament supply wires, they can hum. Sometimes using shielded cable for signal runs inside the cabinet can help. It is hazardous to do, but you can open the amp up and use a wooden chopstick (NOT A PENCIL) to move the wires around inside to see if the hum changes. This is hard to do well and conclusively, since the amp may well hum more just because it is open. BE VERY CAREFUL NOT TO SHORT THINGS INSIDE THE AMP.
(l)Poor AC Chassis Ground at Power Transformer
A common problem is the main ground point to the chassis. The green wire ground to the chassis, the Nine reverse' cap, the CT on the filament windings, the CT on the high voltage windings, and other things associated with power or RF shield grounding are often tied to lugs held under one of the power transformer mounting bolts. If this bolt becomes loose, or if there is corrosion or dirt under the lugs, you can get an assortment of hum problems.
(m)Defective internal grounding
There are potentially lots of places that must be tied to ground in the internal wiring. This varies a lot from amp to amp. If one is broken loose or has a poor solder joint or poor mechanical connection, it can show up as hum. Note that modified amplifiers are particularly susceptible to this problem, as the grounding scheme that the manufacturer came up with may well have been modified, sometimes unintentionally. With the amp unplugged, open and the filter capacitors drained, carefully examine the wires for signs of breakage.

Hiss
Some noise or hiss is normal. These amps are supplied with Carbon Composition resistors similar to the original. This style of resistor has inherent noise. If this amount of hiss is bother some, you will need to replace the resistors in the signal chain with Metal Film resistors.

Squealing/Feedback
Squealing usually occurs when there is coupling between the input and output stages. The positive feedback causes the amp to become an oscillator. Vary the volume and tone controls to see if it affects the oscillation. That will tell you if the coupling is occurring before or after the control. Sometimes the problem can be solved by minor changes to the wiring (moving output wires away from input wires, shortening excessively long wires, etc.).
Use shielded wire on the input jack to help a hum or squealing problem.Ensure the shielded wire goes on top of the board, not underneath it.
when you turn up the Volume & Master Volume together, you may have a squeal. Try the following to eliminate it.
1. Attach the grid stopper resistor of the phase inverter (V3) R10 and R12 (1M) directly to pin 2 of the tube .
2. Use shielded wire from the Volume pot wiper to the resistor.
3. Connect wire shield to the socket, or chassis at V3(phase inverter); or attach to the potentiometer.
4.Reduce grid stopper resistor of the phase inverter R10,R12 1M value (to 68K min.) until the squeal goes away; 68K will give you a nicer impedance, but if you are going for gain, just bypass that resistor with shielded wire and enjoy.
5. If all that doesn't work reverse the output transformer leads on pin 7 of power amp tubes(V4, V5).If this step was necessary, you might be able to return to grid stopper resistor of phase inverter value.
One of the primary leads is in phase with one of the secondary taps. In a high gain amp, this phase relationship needs to be maintained.
Note: If you are looking for gain, you could omit the grid stopper or bypass it with a cap. If not, you should try to leave it in there. This point in the design happens to have the most effect on gain.
In extreme cases, you may need to ground the output jacks to the chassis.

Radio Interference
If you are picking up radio stations on your amp:
1. Try a .01 uF or 47 pf capacitor on very short leads between the 'ground' side of the input jack and chassis.
2. Make sure the chassis is fully enclosed electrically. Install a piece of thin Aluminum sheet metal or HVAC Aluminum tape sandwiched between the chassis & cabinet and make sure it makes contact with the chassis.
3. Make sure the 68K grid blocking resistors are located at/on the V1 tube socket.
4. Use shielded wire between the input jack and the 68K grid blocking resistor.
5. Place ferrite beads over the shielded input cable.
6. Try grounding the shield of the shielded input cable to the chassis instead of the preamp ground.
Other useful measures to take in extreme circumstances:
1. Use a filtered IEC connector for your mains power connection.
2. Put a loop across the V1 Plate and Cathode pins 1&3
Scratchy Sounds on Potentiometer(s)
If you are hearing scratch sounds on a pot when you rotate it, measure the DC voltage from the terminals to ground. A leaky coupling capacitor or tone stack can cause this to happen.
Amp Buzz or Rattle When Installed in Cabinet
If you get a buzz in an amp when it's installed in a cabinet, it could be due to any one or a combination of the following things. Start with the easy things and work your way through the tests.
First, is it a metallic buzz? Is it a tube (ringing) buzz? Is it a softer buzz (wooden/plastic sound?)
Try using an external speaker, isolated from the amp to see if it goes away. This should tell you it's related to the cabinet mechanics or not.