Bias test point first gen Fender Blues deluxe

[guile]

I have a Fender Blues Deluxe (1993, 1st generation). These have a fixed bias via resistor R87. The bias is reportedly set 'cold'. I want to install a potentiometer instead of R87, but where on the pcb can I test ma current?

Another solution would be a different resistor value, the current one is 27k.

Would love to hear suggestions, thanks in advance.

[xtian]

The Hot Rod Deluxe (not sure about your Blues Deluxe, but think it's the same) has a test point on the edge of the power tubes' PCB for power tube cathode current.

[martin manning]

I don't think the Blues Deluxe has a bias current test point; the schematic shows the cathodes going directly to ground. It appears that there are some test points and OT CT access, making it easy to use the OT primary resistance to get at bias current.

[sluckey]

I have a Fender Blues Deluxe (1993, 1st generation). These have a fixed bias via resistor R87. The bias is reportedly set 'cold'. I want to install a potentiometer instead of R87, but where on the pcb can I test ma current?

Easiest non-invasive way to monitor mA would be at CP1, CP2, CP3. With amp off measure the resistance between CP1 and CP3, and also between CP2 and CP3. (Write it down somewhere because you don't have to do this again.) Next measure the voltage between CP1 and CP3, and also between CP2 and CP3. Do the math. Divide the voltage by the resistance to get the mA.

[guile]

Thanks a lot!
I will get to it asap.

[FUCHSAUDIO]

FWIW: I usually unclip the output transformer CT wire from the main board, and use clip leads and a 1-ohm resistor. Same with Blues Juniors.
If the bias is where it should be (or close), I don't mess with a trimmer. If we're doing an overhaul (cap job and tweaks etc), I'll add a ten turn pot with a Dremel and wire gauge drill bits.