Blues deville bias control

[joeyb]

Hi i am brand new to this site,i need help on how to wire in a bias control in my hotrod blues deville,

[Enzo]

Hot Rod DeVille or Blues DeVille.


On old style BLues DeVille, make R87 variable. SUggest replacing the 27k resistor with something like a 15k resistor with 10k pot in series.

[joeyb]

bias control

thanks enzo i think my amp is a 1996 do those same mods work on it

[joeyb]

thanks alot enzo, i just looked at the schematic and it does have 27k for r87. i see now by adding the 15k in series with the 10k pot brings it back to 25k,will that give me enough adjustment both ways from center of the pot adjustment

[joeyb]

one more thing,where is the test point ? can i run a wire to a place i can get to it,like insulated test point

[Enzo]

First, is the amp not living up to expectations in any way? I mean as regards bias? The thousands of these amps out there without the adjustment have gotten along fine over the years.

99.999% of the time, when someone wants to make bias adjustable, it is to make the tubes run hotter. Not many want to run them colder than stock. SO there is little point in looking to increase the bias voltage. SO I don;t look at the existing voltage as the center point, I look at it is the end point. The change I suggested then lets you start from here, and dial the voltage down to get the tubes hotter. the values I suggested are not critical, they just seemed reasonable. There is no need to dial the voltage down to 10v or something, so the 15k limits how low you can go.

Unlike the later ones, there is no current sensing resistor here. You could insert a 1 ohm resistor betwen the power tube sockets' pins 8 and ground. There may or may not be a convenient trace to cut on the board. Otherwise you will need to get a bias probe.

[joeyb]

Thank you for the info. I may never make this mod but if i change output tubes i thought you had to adjust the bias. Fender says replace tubes with the same color. I have thought about putting some j/j tubes,so i was unsure of the bias settings

[Enzo]

People chant "bias bias bias" like a mantra. it is not really all that critical. if your new tubes sound good, and the plates inside don;t start getting red hot, then it is fine.

[R.G.]

People chant "bias bias bias" like a mantra. it is not really all that critical. if your new tubes sound good, and the plates inside don;t start getting red hot, then it is fine.

As my daddy used to say, "It's like shoveling sand against the tide."

[Enzo]

Maybe my obit will read, "he tried to fight the good fight... the dummy."

[Robert M. Martinelli]

As my daddy used to say, "It's like shoveling sand against the tide."

Maybe my obit will read, "he tried to fight the good fight... the dummy."

...Here in Italy we use to say: "It's like trying to empty the sea with a bucket" ( to which I would add "full of holes" )

Cheers

Bob