Mesa Express 5:50 Bias mod questions

[Turret]

Hi,

After having success adding the bias mod to a Fillmore, I’ve decided to do the same modification on my Express 5:50+. The bias supply is similar in topology, but the component values are different.
Could you offer some guidance on an appropriate value range for R1?

Thank you.

[martin manning]

Stock is 22k1//33k2 = 13k27
For reference I'd guess a reasonable range might be half to twice the stock value, 6k6 to 26k5. A 6k6 and a 10k pot might be good choices. Using a 25k pot, you might make the fixed resistor a 4k7 to get a range of 4k7 to 29k7.

[Turret]

Stock is 22k1//33k2 = 13k27
For reference I'd guess a reasonable range might be half to twice the stock value, 6k6 to 26k5. A 6k6 and a 10k pot might be good choices. Using a 25k pot, you might make the fixed resistor a 4k7 to get a range of 4k7 to 29k7.

Thank you. I have both the 10k and 25k (model Fender bias trim with 25k linear waffle). Which one you recommend to use?

[martin manning]

I like the Fender-style 10k since they can be made accessible from outside the chassis, and here the adjustment will be easier with the 10k since the range is smaller.

[Turret]

Thank you, Martin.

Legend

I went with 10k pot with 4k7 + 2k7 in series. With a pair of STR440 grey rank I get the ranges in 43% - 88% on 50w settings. On the 25w it runs a way cooler. Really happy with the amp, and grateful for the advice.

[martin manning]

Good to hear, thanks for the follow-up!