Low Power Tweed Twin Build: Sounds great but low bias

Fender Amp Discussion

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JoeTele
Posts: 127
Joined: Sun Oct 27, 2013 2:20 pm
Location: Milwaukee, WI

Re: Low Power Tweed Twin Build: Sounds great but low bias

Post by JoeTele »

Great info all around. My only question: the smaller value bias pot I understand completely, but why the larger than 10k fixed resistor? Or is that just to raise the minimum amount of current (16 millivolts currently [no pun intended!]) while the lower pot value lowers the maximum?

Thanks!

Joe
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martin manning
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Re: Low Power Tweed Twin Build: Sounds great but low bias

Post by martin manning »

The total resistance (pot plus resistor) sets the minimum bias level and the pot sets the range of adjustment. This is a form of voltage divider, the same network as most volume controls. Another way to wire it is shown below in a snip from the Fender '57 Twin schematic. Note the bias comes from the pot's wiper instead of using it as a variable resistor (as in Weber's version). Some will warn of a pot failure causing loss of bias, but the risk is very low; it was done this way in many Fender amps.

You could set the desired bias with the circuit you have, and then measure the total resistance required from the diode to ground by measuring across the pot and resistor (disconnect the ground first to get an accurate reading). Then, you can figure the resistor value for either type of circuit using a 10k pot set at mid-range.
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