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previous: Enzo In looking at your schemo, my first... -- 4/29/2003 3:33 AM View Thread

Re: Wacko little amplifier

5/4/2003 8:44 AM
Tim Gagan
Re: Wacko little amplifier
Playing with some of Enzo's suggestions this is what I've found:  
The voltage on the cathode of V1B does change as the intensity pot gets turned, going as high as 14.19vdc. Taking the DC off the grid by putting a cap between the intensity pot and the grid does bring the cathode voltage up, but only to 30vdc. There's no audible difference in the vibrato's intensity, though.  
Enzo mentioned that both the speed and the intensity pots are working off the same node in the cap train, so I set out to move the speed pot to a different node. However, I realized that even though my drawing makes it look like the speed pot is on the same node as the intensity pot, the speed pot actually works with the 470K resistors in the other two nodes. The 470K resistor in the intensity pot node is always going to ground when the vibrato is on.  
What I've ended up with is the speed pot working off the 470K of the second node, with the third 470K grounded. Here's the updated drawing:  
The speed pot now does slow the vibrato down a bit, but it still affects the intensity as well.  
I did check my drawing against the circuit, and it looks right. Woody Crane pointed out to me that there are some Magnatone vibratos that appear somewhat similar to this one, but the Magnatones use varistors instead of the tube diodes. He wasn't sure whether the varistors serve the same purpose as the tube diodes.

Enzo Hey, close is better than nothing. ... -- 5/6/2003 5:47 AM