Bias resistors in my marshall ?

[jamme61]

My question is about a JMP 2204 marshall head, with 6550 tubes. When I look at the circuit board, the bias resistors are marked on the board, as being 82K but, what is installed is 150K? I know both values are good for the 6550 tubes but, would really love to know what these resistors do. Warning, I'm a novice and only know enough to not kill myself. Thanks for any time on this, just curious.

[roberto]

150k should be the right value:
http://www.drtube.com/schematics/marshall/2203u.gif

would really love to know what these resistors do

They are the output load for the PI, the input ground resistor for the power tubes, and at the same time they feed the right bias voltage to power tubes.

[M Fowler]

The board should have had 220k silk screened but for 6550 tubes your safe to go down as low as 150K when using 6550 tubes.

Mark

[Darkbluemurder]

In a fixed bias amp like the 2204, these resistors have two functions:

(1) pass the negative voltage to the power tube grids for biasing purposes - essential for the power tubes to survive - and
(2) act as grid leak resistors for the power tubes. Together with the PI output coupling caps they form a high pass filter, i.e. the lower the resistors the less signal and less bass get to the power tubes.

From a technical perspective 150k is not a correct value for 6550 tubes because the maximum g1 resistance (grid leak resistors plus grid stopper resistors) would be exceeded (see the 6550 data sheet for more details). I know that many amp designers exceed these values merely because Leo Fender always did but remember how the amps were supposed to be operated - CLEANLY. In a 2204 you may get away with the 150k resistors as it does not entirely depend on power tube overdrive to get its signature sound.

Cheers Stephan

[jamme61]

In a fixed bias amp like the 2204, these resistors have two functions:

(1) pass the negative voltage to the power tube grids for biasing purposes - essential for the power tubes to survive - and
(2) act as grid leak resistors for the power tubes. Together with the PI output coupling caps they form a high pass filter, i.e. the lower the resistors the less signal and less bass get to the power tubes.

From a technical perspective 150k is not a correct value for 6550 tubes because the maximum g1 resistance (grid leak resistors plus grid stopper resistors) would be exceeded (see the 6550 data sheet for more details). I know that many amp designers exceed these values merely because Leo Fender always did but remember how the amps were supposed to be operated - CLEANLY. In a 2204 you may get away with the 150k resistors as it does not entirely depend on power tube overdrive to get its signature sound.

Cheers Stephan

This is great stuff! thanks guys. I'm getting it, there's more stuff going thru those resistors then just the signal, great. I have changed the bias resistors to 82K and like what I hear. Its cleaner and still a lot of bass, less fizz & mush, to my ear. thanks so much to all you guys for helping me undersatnd this better. one last thing, I'm running NOS GE 6550's at 56ma. and the amp is reading 400 volts on the plates. From what I've read I'm good on the bias, right?