Fender 68 Custom Deluxe Reverb

[bsco]

Here's a good one...This amp had a slight hum coming from the speaker....If you played a guitar through the amp, you couldn't hear it but when you stopped the hum was very noticeable..but was low in volume....sounded like power supply hum...and did not change when controls were turned up and down...So before jumping in and going for faulty caps etc.... I sat back and thought......hummmmm.......ac type hum.....it can be caused by a number of things...can also be caused by a tube which is the easiest and fastest check to make( I heard of heater to cathode leakage in tubes before).......so I removed all the tubes except the two output tubes and the rectifier tube...powered it up and guess what ...no hum at all....(now, if the hum had to remain then I would have eliminated all the circuitry before the power amp/power supply sections) ...so I re-installed each tube one at a time and rechecked.....I was able to locate a faulty tube that was causing the hum.....It was V4 on the schematic...and just to make sure I re-installed the bad tube and sure enough...the hum came back....removed it and installed the good spare I had and no hum....so the tube is leaky for some reason....and I didn't even have to pull the chassis or fire up the soldering iron or break out any test gear.....I hope this helps somebody else out at some point...and this can apply to any type of tube amplifier not just this make and model.....
Cheers

[Leo_Gnardo]

Your method gets my 100% approval! Sometimes even new tubes inject hum into the signal as well - annoying fact of life - when that happens I try a couple one after the other & if no joy then have to chase down & stop the hum in other ways.

[bsco]

Thanks Leo...I have finally come to realize that a good visual inspection and overall evaluation is the best way to tackle a repair...before I used to pull a chassis and dive in there...now I just sit back and think about the problem first and if needed check out this forum just to get some extra info on problems similar to the ones I am having...then I go from there......electronics is a wild field to be involved in..
Cheers