Hi,
ok, lets summarize: ALL voltages from the schematic are correct, reverb unit is between guitar and amp, guitar to input, output to amp.
The hum changes in response to the MIX potentiometer. So you can trace it back to the dry or to the reverb-path of the circuit.
If only in the wet-path (Mix turned UP):
first guess: is the reverb tank connected in the right direction?
second guess: did you properly clean off the insulating yellowish varnish on the cables of the reverb drivers output-transformer before soldering? Bad connection would result in anything between no-reverb to strange noise.
If only in the dry path: maybe doublecheck the cathode-follower. It is the only thing between in and out in the dry path. Vin should be more or less exactly Vout. You can use the two channels of an oscillograph to compare them.
Another questions: do you have an oscillograph and a sine-generator (computer with soundcard will do) to trace your signals? Then you could step by step check your signal path (200mVpp 1000Hz sine is fine as an input).
Maybe you could provide some high resolution photos of your circuit. A lot of things have been identified in the past via photo-inspection.
To provide some information about yourself (skill-level, experience in tube amps, tech and test equipment) in the beginning could help others to help you.
Cheers,
Andreas
