Excessive Hiss

• noisy tubes
• Noisy plate resistors. The carbon composition resistors used on the plates of preamp tubes often go very noisy, especially in older Fenders. If swapping tubes does not fix the trouble, locate the circuit that seems noisiest by tube swapping, and then replace the plate resistors in that circuit with equal-value metal film or metal oxide resistors. You can use carbon comp if you can find them, but the problem is likely to recur.
• Noisy resistors in the B+ decoupling string, often around 10K in value
• Unusual: An ultrasonic oscillation can cause an intense insect-like hissing that sounds very much unlike normal hiss.
• Rare: leaky coupling capacitors or faulty controls
• Rare: a slight ongoing arc on the output tube socket(s)
• Rare: a bad solder joint somewhere in the signal chain
• Rare: internal arcing or noise in almost any part in the preamp section

A good way to divide and conquer is to turn the volume control(s). If the hiss changes levels as you do this, then the source of the hiss is something that affects the stages of the amp before the volume control. A faulty, hissing preamp tube will be turned up this way very quickly. Conversely, if the volume control does not affect the hiss, the cause is somewhere after the volume control. In general, the volume level of the hiss is an indicator of where the hiss is occuring - the louder the hiss, the more likely its source is near the input of the amp where the gain applied to it will be the greatest

The procedure of locating by removing one tube at at time working from the phase inverter/driver back towards the input until removing a tube no longer stops the hiss should then localize the problem to one tube's worth of circuitry.