Tube Amps / Music Electronics
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|10/5/1999 3:37 AM|
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: http://users.aol.com/portaflex/schems/b-12xt.gif
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.
Again, real techs, please jump in if I am completely out of my mind, or even just mildly disturbed.
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