[Doug Gifford]

symptoms:
-began suddenly this morning
-rich 60 hz hum at all times when on
-unaffected by where it is or what outlet it's plugged into
-unaffected by instruments or footswitch being plugged in (amp works but with hum)
-constant: unaffected by any of the knobs
-traded out rectifier tube: no change
-when I turn on the amp (standby switch up) the hum swells then fades slightly
-when I let the amp warm up fully and switch standby from down to up I get the same swell and fade but the swell is louder

Any ideas?

 

[Chito]

Usually that's when I bring it to @nonreverb
Sorry for not being helpful.

 

[Doug Gifford]

Usually that's when I bring it to @nonreverb
Sorry for not being helpful.

I may have to call in the pros but just in case it's something common and simple…

 

[Doug Gifford]

Here's a recording of the standby getting switched off and the start of the hum: http://jazzagejazz.ca/resources/audio/amp hum.MOV

 

[Jerome]

Try a new set of output tubes , if you have some .

 

[nonreverb]

Depending on what year it was made, there was a run of Fender everything amp-wise that got poor quality filter caps. I can't tell you how many I've replaced over the years but it's a shit-load. I'd start there first. Look for goo coming out of one(or more) of the filter caps in the dog house.

 

[nonreverb]

Try a new set of output tubes , if you have some .

A good point. Check to see if one power tube is not lighting up.

 

[Doug Gifford]

Depending on what year it was made, there was a run of Fender everything amp-wise that got poor quality filter caps. I can't tell you how many I've replaced over the years but it's a shit-load. I'd start there first. Look for goo coming out of one(or more) of the filter caps in the dog house.

Well, it wasn't the output tubes. I assume I'll need to take the amp out of the cab. What is the "dog house"?

 

[nonreverb]

Well, it wasn't the output tubes. I assume I'll need to take the amp out of the cab. What is the "dog house"?

The metal box attached to the underside of the chassis where the tubes are mounted. 4 screws and lift the lid. Inspect all the caps for dried gunk where the positive leads come out. Sometimes they fail without any material oozing out but most of the time, it does.

 

[DaddyDog]

Here's a recording of the standby getting switched off and the start of the hum: http://jazzagejazz.ca/resources/audio/amp hum.MOV

Wow that's nasty. Is the volume turned right off?

A no-risk, no-cost test is to switch off, let cool, remove the V1 tube. Power up... still noisy?

Keep doing this through rest of the pre-amp tubes. This helps narrow down if it's those tubes or not.

I don't think doing the same with power tubes will help. Better to swap in 1 tube (don't worry about matching them at this point). Just be sure it's a 6V6, assuming you have one handy.

 

[Doug Gifford]

The metal box attached to the underside of the chassis where the tubes are mounted. 4 screws and lift the lid. Inspect all the caps for dried gunk where the positive leads come out. Sometimes they fail without any material oozing out but most of the time, it does.

Had a look. I assume the black and silver is probably a filter cap and maybe the orange ones? They all seem clean

 

[Doug Gifford]

Wow that's nasty. Is the volume turned right off?

A no-risk, no-cost test is to switch off, let cool, remove the V1 tube. Power up... still noisy?

Keep doing this through rest of the pre-amp tubes. This helps narrow down if it's those tubes or not.

I don't think doing the same with power tubes will help. Better to swap in 1 tube (don't worry about matching them at this point). Just be sure it's a 6V6, assuming you have one handy.

The sound reminds me a bit of turning on an old old television.

I powered up with the power tubes out. Still makes the same sound, which surprises me -- I'd have expected silence.

on to the pre-amp tubes

 

[Doug Gifford]

Removed and replaced all the pre-amp tubes (one at a time) to no effect. sigh

 

[nonreverb]

Had a look. I assume the black and silver is probably a filter cap and maybe the orange ones? They all seem clean View attachment 459625

The other side of the chassis.

 

[Paul Running]

I powered up with the power tubes out. Still makes the same sound, which surprises me -- I'd have expected silence.

The excess power supply ripple is modulating the output transformer, passed on to the secondary and outputted by the speaker.

 

[Latole]

Had a look. I assume the black and silver is probably a filter cap and maybe the orange ones? They all seem clean View attachment 459625

It's the bias filter cap ; look good , check for broken solder under the board
Very very unmatched power tubes may be noisy

 

[Latole]

Under the "dog house" :
DON'T PUT YOUR FINGER THERE ; lethal voltages

Put stanby switch in "play " mode to discharge capacitors if you want to look under the board.
Wait few hours if you don't have a voltmeter.
Not sure; see a tech.

Look at both end of the filter caps to see if any buble.
Buble ; replace filter cap.

Look under the board for some broke or burn solder or copper wiring

IC ; Illinois Capacitor; low budget caps.
I put F& T caps

 

[Doug Gifford]

Under the "dog house" :
DON'T PUT YOUR FINGER THERE ; lethal voltages

Put stanby switch in "play " mode to discharge capacitors if you want to look under the board.
Wait few hours if you don't have a voltmeter.
Not sure; see a tech.

Look at both end of the filter caps to see if any buble.
Buble ; replace filter cap.

Look under the board for some broke or burn solder or copper wiring

IC ; Illinois Capacitor; low budget caps.
I put F& T caps
View attachment 459735

Looks okay to me, but my eyes don't have much experience…

 

[nonreverb]

Middle one is definitely bad and so is the 47uF large one. Replace them all.
The large one is the primary filter cap. It's the one causing most if not all the hum.

 

[Latole]

Middle one is definitely bad and so is the 47uF large one. Replace them all.
The large one is the primary filter cap. It's the one causing most if not all the hum.

Right and many solders close to the side need to be renew.
I'll replace all caps for new and better ones.