#124 Resistor question

[Mondoslug]

First I grabbed this pict off of this site...from somebody's build so I'm hoping it's okay to use here(never assume?)

The resistor circled in green, what exactly is the function of this and what are some of the higher values you've seen used for this spot. Newbie!
TIA!
[IMG:640:467]http://i32.photobucket.com/albums/d38/m ... estion.jpg[/img]

[heisthl]

It's a "tail" resistor covered in numerous posts re: presence and Global negative feedback, but to simplify its one of 2 resistors that form a voltage divider to regulate the amount of output transformer signal to feedback into the PI on the out of phase input of the PI tube. Smaller tail = less PI frequency cancellation = more overall amplifier volume especially in the bass frequencies because the highs are bled off to ground(the 1uf cap in your picture does this) and never make it to the inverting input anyway. Bear in mind the Dumble style amps need the presence circuit to sound like Dumbles and most of the schematics will show the typical values used with Bassman or Showman type iron. The divider values will change slightly with other OT types and also will change if the PI is the "Bluesmaster" style.

[odourboy]

That's the 'other half' of the global negative feedback circuit. The ratio of that resistor to the feedback resistor off the OT speaker tap define how much negative feedback is going to the phase invertor (presence settings aside).

edit to say... oops, too slow. Heisthl gave a more thorough reply to boot!

[Mondoslug]

Thanks for the taking the time to reply...excellent explanation I'm thinking. You guys are good!