Fender 60's, 70's Tremolo Thumping

[Mars Amp Repair]

Hello folks,
I;ve already posted this under '70's Fender Super Tremolo Thumping Fix' but now realize this issue spans a number of the models. I now have a 60's Pro-Reverb that had the same 'Thumping/Snapping' issue with the reverb.
SO, I decided to investigate moving the grid p2 on V5 as I did on the Super. Turns out in this amp, just lifting the wire that goes from the tube to a hole in the rivet board eliminated the Thump & the Snap completely & as a result improved the interruption in the wave form at the speaker when inputting an oscillator to view on the scope.
So, there it is.
Glen

[SoulFetish]

Hello folks,
I;ve already posted this under '70's Fender Super Tremolo Thumping Fix' but now realize this issue spans a number of the models. I now have a 60's Pro-Reverb that had the same 'Thumping/Snapping' issue with the reverb.
SO, I decided to investigate moving the grid p2 on V5 as I did on the Super. Turns out in this amp, just lifting the wire that goes from the tube to a hole in the rivet board eliminated the Thump & the Snap completely & as a result improved the interruption in the wave form at the speaker when inputting an oscillator to view on the scope.
So, there it is.
Glen

Dude, I have fought this symptom in a couple of amps for hours. I used to be able to sometimes fix it by re-routing the wire going from the RCA back panel to the board.
However, I think I've figured out what is causing that audible thumping, but I'm convinced I know how to fix it reliably.
Firstly,from time to time, a bad tube can cause this. But usually, I think that its caused by AC ground currents in the LFO stage coupling through to the audio. The most fender amps, and all the ones that I've seen with this have the cathode resistors in the mixing stage and the oscillator stage grounded to the same eyelet on the board.
The problem is that in order to switch the vibrato, Fender uses the chassis to ground the oscillator when in use.
I've been able to fix the audible thump by either grounding the cathode bypass capacitor of the oscillator to the back of the chassis at the RCA jack, and sometimes both the resistor and cap.

[Mars Amp Repair]

Dude, I have fought this symptom in a couple of amps for hours. I used to be able to sometimes fix it by re-routing the wire going from the RCA back panel to the board.
However, I think I've figured out what is causing that audible thumping, but I'm convinced I know how to fix it reliably.
Firstly,from time to time, a bad tube can cause this. But usually, I think that its caused by AC ground currents in the LFO stage coupling through to the audio. The most fender amps, and all the ones that I've seen with this have the cathode resistors in the mixing stage and the oscillator stage grounded to the same eyelet on the board.
The problem is that in order to switch the vibrato, Fender uses the chassis to ground the oscillator when in use.
I've been able to fix the audible thump by either grounding the cathode bypass capacitor of the oscillator to the back of the chassis at the RCA jack, and sometimes both the resistor and cap.

Hey man, I hear you on the common grounds. If that ground is a solid one, and if I recall it ends up right at the chassis then it should be easy to realize that.
So, next time give this a whirl. It's worked on both of these different model vintage fenders, so I imagine it's a viable source for many of them.
Thanx for the info...glen