‘68 Custom Vibrolux Transformer Hum

[Lowspeid]

I recently bought a used ‘68 Custom Vibrolux Reverb, and now that it’s home I hear a loud hiss at idle, but there’s also loud hum from the power transformer (This the most recent reissue with Silverface aesthetics, the “bassman” Custom channel, and a vintage channel). It’s PCB, which I don’t mind, but I’m concerned about the transformer humming. I have several tube amps, some vintage Fender, but have never heard transformer hum on any of them.
Any Ideas what it might be and how to fix it? I know the lead dress coming off the transformer pretty much sucks (they ziptie them all together), so is that the issue?

 

[Michael Smith]

Is the hum coming thru the speaker, or directly from the transformer? I have quite a few vintage Fender amps and have had the occasional crackles and hiss associated with bad solder joints or failing tubes or toasty resisters, but not a transformer that made any audible noise. Is there still hum with all of the pots set to "0"?

 

[archetype]

I recently bought a used ‘68 Custom Vibrolux Reverb, and now that it’s home I hear a loud hiss at idle, but there’s also loud hum from the power transformer (This the most recent reissue with Silverface aesthetics, the “bassman” Custom channel, and a vintage channel). It’s PCB, which I don’t mind, but I’m concerned about the transformer humming. I have several tube amps, some vintage Fender, but have never heard transformer hum on any of them.
Any Ideas what it might be and how to fix it? I know the lead dress coming off the transformer pretty much sucks (they ziptie them all together), so is that the issue?

Search results for query: hum transformer

www.tdpri.com

 

[schmee]

Man, it's a rash of humming amps lately!
If they'd hum in harmony it wouldn't be so bad...

 

[Michael Smith]

Before proceeding, make sure the fuse is of the proper value. How does the amp sound with a guitar? My 1969 Super Reverb has a pretty loud hum on the normal channel with nothing plugged into either normal channel inputs that gets louder when the volume is raised. I assume this is a grounding issue with one or both of the input jacks. Not a problem really, because the hum disappears when the volume is all the way off and nothing plugged in to the channel, which is how I keep it set when playing the vibrato channel.

 

[dan40]

Some power transformers are prone to humming (ringing is the technical term) while others are perfectly silent. As long as the PT is physically too hot to touch which would indicate a problem, the humming noise is ok though it may be annoying in a recording environment. I have had one or two amps over the years that exhibited this same phenomenon.

 

[Lowspeid]

Is the hum coming thru the speaker, or directly from the transformer? I have quite a few vintage Fender amps and have had the occasional crackles and hiss associated with bad solder joints or failing tubes or toasty resisters, but not a transformer that made any audible noise. Is there still hum with all of the pots set to "0"?

Both. I can hear the transformer humming, very clearly when I’m behind the amp. I also hear a loud hiss that gets louder when the volume (either channel) or the reverb get turned up.

 

[Lowspeid]

Before proceeding, make sure the fuse is of the proper value. How does the amp sound with a guitar? My 1969 Super Reverb has a pretty loud hum on the normal channel with nothing plugged into either normal channel inputs that gets louder when the volume is raised. I assume this is a grounding issue with one or both of the input jacks. Not a problem really, because the hum disappears when the volume is all the way off and nothing plugged in to the channel, which is how I keep it set when playing the vibrato channel.

Checked the fuse first thing. It’s good and correct for the amp. Guitar sounds great through the amp on either channel, but as the volume goes up so does the hiss/white noise. I’ve gone through each tube swapping a known good tube. The hiss went away when I pulled V3. Transformer hum is ever-present.

 

[Michael Smith]

Checked the fuse first thing. It’s good and correct for the amp. Guitar sounds great through the amp on either channel, but as the volume goes up so does the hiss/white noise. I’ve gone through each tube swapping a known good tube. The hiss went away when I pulled V3. Transformer hum is ever-present.

Well, if pulling V3 causes the hiss/white noise to go away, then it is something in the reverb circuit. With V3 installed, disconnect the reverb cables and raise the volume and the reverb level. If there is still no hiss/white noise, then the problem is likely the cables or the reverb tank. When looking in from the rear of the amp, are the rca jacks on the reverb tank facing away from you? They should be. You may have to take out the screws holding the bag to the floor of the cabinet (if they still use that method on the reissue amps) and partially slip the tank out of the bag to determine the orientation of the rca jacks.

 

[Wally]

Can you feel a vibration in the PT that is in sync with the hum? You can safely touch the body of the PT…while keeping the other hand away from any contact with the chassis. good link in post #2 by @archetype.

 

[SoK66]

Among all the other noises common to the '68 Custom line, my '68 CDR has a humming OT. I've exorcised virtually all the other racket (buzz, hum, hiss) but this remains and probably would require an OT replacement to cure.

 

[Lowspeid]

Among all the other noises common to the '68 Custom line, my '68 CDR has a humming OT. I've exorcised virtually all the other racket (buzz, hum, hiss) but this remains and probably would require an OT replacement to cure.

Kinda what I was afraid of.

 

[Michael Smith]

Have you made sure the transformer(s) and all other hardware are tightly fastened?

 

[dan40]

In some cases, a noisy PT can be caused by a DC offset on the incoming AC line. Loose laminations and/or a poor potting job can also cause the noise. The AC line voltage is alternating 60 times a second within the transformer so if the laminations are slightly loose or the potting job was poorly done, the vibrations with the transformer can become audible. Here are a couple links discussing this issue.

What can cause a power transformer to buzz or hum?

Ok, let's say you have a power transformer in a tube integrated amplifier and it hums / buzzes. Not out of the speakers; just the transformer itself. You can hear it within 2 feet. It's old. Not torroidial. Now here is the big question: If the transformer is bolted tight on the chassis, and...

audiokarma.org

transformer hum normal with tube amp?

Hi, Got my new PPX3-6SN7. Loving the sound , even while the tubes burn in. Glorious. While signal path is abolutely dead quiet, the transformer hums just a little bit. Mikhail says that's normal, and I believe him. But how much is normal? I didn't notice it for the first 24hrs, and...

www.head-fi.org

 

[Jasonpatrick]

I see A LOT of these “ my amp hums posts. All my amps have a little hum at zero volume when I’m sitting ear level a foot away, some more then others but I just figured that was normal haha

 

[Lowspeid]

Well, if pulling V3 causes the hiss/white noise to go away, then it is something in the reverb circuit. With V3 installed, disconnect the reverb cables and raise the volume and the reverb level. If there is still no hiss/white noise, then the problem is likely the cables or the reverb tank. When looking in from the rear of the amp, are the rca jacks on the reverb tank facing away from you? They should be. You may have to take out the screws holding the bag to the floor of the cabinet (if they still use that method on the reissue amps) and partially slip the tank out of the bag to determine the orientation of the rca jacks.

I was mistaken. It’s when I pull V4 that the hiss goes away.

 

[Michael Smith]

I was mistaken. It’s when I pull V4 that the hiss goes away.

Hmmm I will have to some more research on that.

 

[dan40]

I was mistaken. It’s when I pull V4 that the hiss goes away.

The hiss is being generated somewhere in the preamp so pulling the PI tube will kill all signal before it reaches the power amp. Hiss can be created by resistors (especially carbon comp) or a bad preamp tube. Some of these Fender Custom series are also famous for having a hiss at idle due to the circuit configuration. I'm not very familiar with the Custom line of amps so maybe one of the members can point you in the right direction.

 

[Wally]

I was mistaken. It’s when I pull V4 that the hiss goes away.

V4 in this amp is the third gain stage, which in this amp exists in both channels due to the application of the effects to both channels.

 

[SoK66]

The hiss is being generated somewhere in the preamp so pulling the PI tube will kill all signal before it reaches the power amp. Hiss can be created by resistors (especially carbon comp) or a bad preamp tube. Some of these Fender Custom series are also famous for having a hiss at idle due to the circuit configuration. I'm not very familiar with the Custom line of amps so maybe one of the members can point you in the right direction.

Hiss in these amps is a combination of the negative feedback resistance being doubled (reduced), the bridging of the two channels to put reverb & trem on the "Custom" channel and the circuit layout. On my '68 CDR I returned the NFB resistance to AB763 spec, removed the jumper that bridges the channels, essentially making the "Custom" channel same as the "Normal" channel on a garden variety DR, and cut the trace to the V2 input from the volume pot and ran shielded cable from the pot to V2. To put the reverb back on the Custom channel I may do Rob Rob's mod, but I have to do some study.

This amp was the noisiest thing right out of the box. The grounding scheme is a bit nuts with loops everywhere. However you can make it work. I made quite a few bulletproofing changes, complete recap with quality capacitors (F&Ts), eliminated the 220/100 filter, uprated the filter & screen resistors to 3w, raised the screen resistors up off the board, moved the heater circuit off the PCB and mounted directly to the power tubes, Mounted the ACT near the PT, etc. I tried moving the power section grounds off the board to points near the PT, but because Fender has the screen cap (also reverb & trem) grounded with the power supply, rather than the preamp ala AB763, it created a ground loop hum in the reverb. The only fix was to either cut the ground trace on the main PCB or revert back to stock. Still a work in process, but quiet as a mouse now.