What is the purpose of the centertap terminal? The pic is from a '72 Super Reverb AA270 that was mod to Blackface specs. The pot was originally intended as a bias balance pot. Now it's set up as a bias adjust, correct?
AA270 layout and schematic...:
Potentiometers with a Centertap
Moderators: pompeiisneaks, Colossal
Potentiometers with a Centertap
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Greg
Re: Potentiometers with a Centertap
That should still work as a bias balance pot; adjusting to either extreme differentially changes the voltage divider ratio on either leg. Using the tapped pot simplifies the circuit, which would otherwise require a dual-ganged pot to have the same functionality. I did notice that this circuit doesn't require a safety resistor to connect on of the ends to the wiper. Additionally, it would avoid the typical failure pattern that occurs if the wiper loses contact where Rg1 dramatically increases since bias would be across the safety resistor, which I generally think of as a large value. Rather, if the wiper fails open, bias voltage would dramatically increase, as in more conventional bias supply designs, but the impedance to the bias supply remains well within the maximum datasheet value for Rg1. I wonder if this was the goal or if it was just the result of making the design work with parts that happened to be comparatively when bought in bulk and the added benefit of a safer failure mode was ancillary.
Re: Potentiometers with a Centertap
The center tapped pot allows the balance circuit to use fewer components (important when components are wired point to point) and it keeps the bias almost unchanged on one side while increasing bias current on the other side.
A circuit could be devised to balance bias currents in a push pull output stage, but you would also need another pot to adjust overall bias Voltage.
A circuit could be devised to balance bias currents in a push pull output stage, but you would also need another pot to adjust overall bias Voltage.
Re: Potentiometers with a Centertap
The balance pot is intended to adjust for minimum output hum without changing average bias.
This is best achieved with a center-tapped pot.
This is best achieved with a center-tapped pot.
Re: Potentiometers with a Centertap
Other than bias circuits, tapped pots are used in some Fender eq circuits. I believe these are 70% taps.
Mark Bartel uses CT pots for the eq in his amp line.
I wish these were more available these days. It’s a PITA trying to roll your own.
Mark Bartel uses CT pots for the eq in his amp line.
I wish these were more available these days. It’s a PITA trying to roll your own.