Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: anteriorl5 on September 24, 2020, 10:19:37 am

Title: Princeton Reverb: Loud humm independent of volume. Garbled sound.
Post by: anteriorl5 on September 24, 2020, 10:19:37 am

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

Title: Re: Princeton Reverb: Loud humm independent of volume. Garbled sound.
Post by: sluckey on September 24, 2020, 10:24:22 am

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

Title: Re: Princeton Reverb: Loud humm independent of volume. Garbled sound.
Post by: anteriorl5 on September 24, 2020, 01:41:50 pm

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.

Title: Re: Princeton Reverb: Loud humm independent of volume. Garbled sound.
Post by: Latole on September 24, 2020, 02:24:56 pm

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.

Title: Re: Princeton Reverb: Loud humm independent of volume. Garbled sound.
Post by: sluckey on September 24, 2020, 02:39:46 pm

Quote from: sluckey on September 24, 2020, 10:24:22 am

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

Title: Re: Princeton Reverb: Loud humm independent of volume. Garbled sound.
Post by: tubeswell on September 24, 2020, 02:57:45 pm

Quote from: anteriorl5 on September 24, 2020, 01:41:50 pm

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

A bad bias cap can make an amp hum

Title: Re: Princeton Reverb: Loud humm independent of volume. Garbled sound.
Post by: anteriorl5 on September 24, 2020, 04:16:01 pm

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!