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Fender Twin Reverb Hum/Hiss Issue

Philip Martin

Member
Messages
39
Hey all,

I have a very loud hum and hiss coming from my fender twin reverb that will make the noise even when the amp has no cables connected to it. I inspected all of the tubes and physically they seem to be fine. I conducted the tap test and found a faulty tube with the last 12AX7. I changed this tube but still am experiencing this hum and hiss. Do you all have any experience with this? Any suggestions or hint in the right direction would be much appreciated.
 

HotBluePlates

Member
Messages
10,015
The noise stays the same with all knobs between 0-10; noise is also just as present with no guitar cables plugged in
I was hoping the noise was lower with the Volumes off, indicating there was a problem with the input jacks' switched contacts. Those normally short hot-to-ground when no cable is inserted, killing noise from an "open input." If those switched contacts were sprung, you'd definitely get a loud buzzing noise.
______________________________

The next step is to "pull preamp tubes."

Start with the phase inverter, the 12AT7 closest to the 6L6s ("V6"). Pull that tube out of the socket & turn the amp on. Assuming the noise is stopped, reinstall that tube.

Skip the 12AX7 to the right of the phase inverter ("V5"), and go 1 more 12AX7 to the right ("V4"). Pull this tube from the socket & turn the amp on. Has the noise stopped?

If yes, reinstall V4 and move 1 tube to the right to the 12AT7 ("V3"). Pull it from the socket & turn the amp on.

You're going to rinse & repeat this process until you find the 2 tubes next to each other where removing one kills the noise while removing the other doesn't.

- Since the noise is common to both channels & regardless of Volume setting, probably the pair of 12AX7s furthest-right ("V1 & V2") won't have any impact.​
- V3 probably won't have any impact, since this 12AT7 is the Reverb Driver (so would only impact the Reverb).​
- V4 (12AX7 to the left of the reverb-12AT7) is probably the first tube-stage that might kill the noise (where V1 & V2 do not when removed). This is where the reverb & dry signals are mixed in the Vibrato channel, and is after this channel's Volume control.​
- V5 is for the tremolo only, so almost certainly has no impact.​
- Signal for both channels has to pass through V6 to drive the output tubes.​
 

Jahn

Listens to Johnny Marr, plays like John Denver
Silver Supporting Member
Messages
28,491
A blurb that may help:


Is your amp getting noisy? In vintage Fenders, the 100K plate & slope resistors in the preamp tend to get noisy over time (hiss, snap, crackle, pop)!! That intermittent staticy/crackly sound is often caused by one of three things:

1) Spent or noisy preamp tubes

2) Dirty or loose tube sockets

3) Noisy 100K plate resistors

These 100K plate resistors regulate the high voltage feeding the plates of your preamp tubes. They often become crackly as the decades roll by. Replacing these resistors is a big help in reducing the noise floor of your Vintage Fender Amplifier.
 

Philip Martin

Member
Messages
39
I was hoping the noise was lower with the Volumes off, indicating there was a problem with the input jacks' switched contacts. Those normally short hot-to-ground when no cable is inserted, killing noise from an "open input." If those switched contacts were sprung, you'd definitely get a loud buzzing noise.
______________________________

The next step is to "pull preamp tubes."

Start with the phase inverter, the 12AT7 closest to the 6L6s ("V6"). Pull that tube out of the socket & turn the amp on. Assuming the noise is stopped, reinstall that tube.

Skip the 12AX7 to the right of the phase inverter ("V5"), and go 1 more 12AX7 to the right ("V4"). Pull this tube from the socket & turn the amp on. Has the noise stopped?

If yes, reinstall V4 and move 1 tube to the right to the 12AT7 ("V3"). Pull it from the socket & turn the amp on.

You're going to rinse & repeat this process until you find the 2 tubes next to each other where removing one kills the noise while removing the other doesn't.

- Since the noise is common to both channels & regardless of Volume setting, probably the pair of 12AX7s furthest-right ("V1 & V2") won't have any impact.​
- V3 probably won't have any impact, since this 12AT7 is the Reverb Driver (so would only impact the Reverb).​
- V4 (12AX7 to the left of the reverb-12AT7) is probably the first tube-stage that might kill the noise (where V1 & V2 do not when removed). This is where the reverb & dry signals are mixed in the Vibrato channel, and is after this channel's Volume control.​
- V5 is for the tremolo only, so almost certainly has no impact.​
- Signal for both channels has to pass through V6 to drive the output tubes.​

Hey there, I appreciate your advice, def a noob any trying to work on any of this amp stuff my self. When the noise is on the hum all of a sudden has started to get louder with volume gain. Also when I remove the v6 the one closest to the 6L:^s the noise stops completely? what's next step now?
 

HotBluePlates

Member
Messages
10,015
... the hum all of a sudden has started to get louder with volume gain. ... Also when I remove the v6 ... the noise stops completely? ...
V6 Out = Noise Stops = Noise in the Preamp
Noise Changes with Volume Setting = Noise is (at least partly) in the 1st gain stage.

We're going to divide & conquer: Pull V1 (furthest right) from the socket & leave it out for now. Also pull V2 out (the tube next to V1). Is all noise gone or do you still have some hum?
 

Philip Martin

Member
Messages
39
V6 Out = Noise Stops = Noise in the Preamp
Noise Changes with Volume Setting = Noise is (at least partly) in the 1st gain stage.

We're going to divide & conquer: Pull V1 (furthest right) from the socket & leave it out for now. Also pull V2 out (the tube next to V1). Is all noise gone or do you still have some hum?

So when I pulled the v6 out ( one closest to the 6L6s) all sound cut. So no hum and also no ability to play when I plugged in the guitar. When I pulled out the V1 and the V2 (the two farthest from the 6L6s) there is still a super small hum but the hum no longer changes when you change the volume. Also with v1 and v2 out I can not plug in the guitar and have it amplified i.e no sound.

Also the reverb has a massive hum. Almost deafening when turned on at all and gets louder the more reverb you "apply." Thank you for your time and help so far. What would be the next step?
 

HotBluePlates

Member
Messages
10,015
... When I pulled out the V1 and the V2 (the two farthest from the 6L6s) there is still a super small hum ...

Also the reverb has a massive hum. Almost deafening when turned on at all and gets louder the more reverb ...
Okay so you've found the bulk of the hum is in the reverb circuit (there could be more than 1 humming thing).

What kind of pedal are you using to turn the reverb on/off (original Fender pedal, or something else)?
 

zenas

Member
Messages
8,500
I bought brand new in 2012 and I think the Power Tubes may have been changed once. No other maintenance/service done to it.
Ok only 8 years old. Wouldn't expect any maintenance/service work needs to be done then. If you'd have said 1972 we'd be talking electrolytic capacitors.
Try what Hot Blue Plates said. If that does nothing, try disconnecting the reverb send and return cables next. A bad connection there will make some weird things happen.
 

Philip Martin

Member
Messages
39
Okay so you've found the bulk of the hum is in the reverb circuit (there could be more than 1 humming thing).

What kind of pedal are you using to turn the reverb on/off (original Fender pedal, or something else)?
No pedal to turn the amplifier reverb on or off. Just manually turning the reverb knob on the vibrato channel
 

HotBluePlates

Member
Messages
10,015
No pedal to turn the amplifier reverb on or off. Just manually turning the reverb knob on the vibrato channel
Then you'll probably need a tech to sort out the hum source & how to resolve it. My last "user serviceable" bit was if you had the wrong kind of footswitch in use (non-shielded), it would be an antenna picking up the noise and injecting it at the Reverb Recovery stage.
 

Andygee

Gold Supporting Member
Messages
408
A blurb that may help:


Is your amp getting noisy? In vintage Fenders, the 100K plate & slope resistors in the preamp tend to get noisy over time (hiss, snap, crackle, pop)!! That intermittent staticy/crackly sound is often caused by one of three things:

1) Spent or noisy preamp tubes

2) Dirty or loose tube sockets

3) Noisy 100K plate resistors

These 100K plate resistors regulate the high voltage feeding the plates of your preamp tubes. They often become crackly as the decades roll by. Replacing these resistors is a big help in reducing the noise floor of your Vintage Fender Amplifier.
What type/wattage for those plate resistors?
 




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