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Ampeg Vibrato

10/5/1999 3:37 AM
Ampeg Vibrato
I've got an Ampeg B12-XT and I'd like to know if anyone knows a way to slow down the vibrato speed. The intensity is fine, but even all the way down the vibrato is still quite fast. Any ideas? As always, thanks.
10/5/1999 3:31 PM
Here's the schematic
Go to:  
and take a look at the schematic.  
REAL TECH ALERT: please jump in if I get this wrong.  
Looks like V7a is the oscillator (or LFO - low frequency oscillator). Notice that the grid and plate are connected through an RC network. This feedback circuit is in a constant state of phase flux, since the plate signal and grid signal are out of phase by virtue of the tube's design. The AC coming off V7a's plate modulates the (cathode) bias on V8a, which has the *effect* of 'turning the tube on and off'. (*very* rough explanation)  
Sounds like your problem could possibly be a bad cap or drifted resistor in the phase-shift RC network between the plate and grid of V7a.  
I think changing any of the cap or resistor component values will alter the speed. In terms of user-controllable parameters, typically this is done by messing with the resistance loading, as you can see from how the speed control is set up. I think its pretty common to see a larger cap value for the 1st cap off the plate, and I believe that this contributes to dialing in the speed range. Each one of those caps charges and discharges as the phase shifts back and forth between plate & grid; the size of the cap in combination with the size of the resistive load determine the rate of charge/discharge, and therefore the rate of phase shift, or "speed".  
Again, real techs, please jump in if I am completely out of my mind, or even just mildly disturbed.  
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