Bias pit and resistor FAQ

[Snake5150]

I’m sure I’m not the first to ask. But can someone give a brief rundown of the relationship between bias pots and there tail resistor.

I have a jtm45 planning on using a 25kl pot with a 68k resistor. Would this produce enough range.

I have also read that a 10k is suffient ?

Obviously layman’s answers only as I’m not as gifted with the technical stuff.

[stratomaster]

Search "resistive voltage divider". You will find everything you need to know.

[AlNewman]

The tail resistor between the pot and ground is basically used as a safety net so you always have negative bias at your grids and won't fry your tubes (or OT, or resistors, etc.) by overadjustment.
What you choose as the tail resistor depends on tubes and B+ voltage.  I believe the pot on a marshall circuit is wired as a variable resistor, so you can parallel resistors with the pot to dial in the amount of adjustment you want.
Without all the math, it's possible to figure out using trial and error, if you have an idea of what you want your center bias to be.

[shooter]

it's possible to figure out using trial and error, if you have an idea of what you want your center bias to be.

best done with NO PA tubes in and measuring bias voltage at the tube side of the power tubes socket.