Princeton Reverb: Loud humm independent of volume. Garbled sound.

[anteriorl5]

Where to start? Hum started after about 15 minutes during practice.

[sluckey]

Change tubes. Then bridge a test filter cap across each filter cap.

[anteriorl5]

Tubes all good. Filters good, but all the bypass caps are original as is the bias cap. Sounds has to be coming from the power section I'm assuming. Removing preamp tubes has no effect.

[Latole]

How do you test filter caps to say they are good ?

Hum may come also from bias caps or from one defective power tube. 

Hum is there with no preamp tubes ; You have now only Power tubes with B+ voltage and bias voltage. Look more a power tube issue.

[sluckey]

...bridge a test filter cap across each filter cap.

[tubeswell]

... all the bypass caps are original as is the bias cap...

A bad bias cap can make an amp hum

[anteriorl5]

Okay so I bridged the filter caps and one WAS open. Hum mostly went away. Ordering a new can cap and going to replace all the bypass caps and the bias cap as well. It was a fairly new can cap so sort of surprising...thanks for the help!