Super Reverb Vibrato Fading in/out

[kcguitar2003]

My SF SR's vibrato has been fading in/out ever since I got it a few months ago. I already replaced the LDR with no luck. I have to play pretty hard and at a decent volume to get the vibrato effect going again. If I don't constantly play, the effect will diminish within seconds until theres no effect at all. The depth control doesn't really make a difference either. Any suggestions?

[Wakarusa]

It's likely that the tremolo oscillator is failing. Possible causes include (all referencing the oscillator circuit):
- weak driver tube
- bad cathode bypass cap
- bad feedback loop cap
- bad plate resistor

In a nutshell, it sounds like oscillator can't sustain oscillations. This is either because the circuit can't develop enough gain (tube, cathode cap, plate load) or because the feedback loop doesn't give a full 180 degrees of phase shift (feedback loop caps and resistors).