AB763 Vibrato problem

[marcfrom]

Hi Guys,

The vibrato of my Vibroverb take a long time to be active (30 -40 seconds) when the amp is cold (first start of the day). After that it works properly.

Does anyone have an idea?

Thanks.
Marc

 

[KFW]

Try replacing the vibrato tube - my schematic says it's a 12AX7. A single tube stage has just enough gain to make up the losses in the phase shift network (39:1 for the ideal phase shift network, even more loss with variable frequency), and if the gain is low, the oscillator may not start, even though it runs once started.

Also, check the cap in the vibrato oscillator's cathode - if it's lost capacitance, (dried up or failed), the oscillator will have less gain.

This part of the amp doesn't need a low-noise 12AX7, it needs a high-gain one. But a 12AX7 in that circuit should have plenty of gain to make the oscillator start if there's nothing wrong.