Tonepad Boss CE-2 trim pot value?

[slashandburn]

I'm building the Tonepad CE-2 and just realised I'm all out of 10k Trimpots.     I have plenty 5k and 20k.   The 20k seems the logical option but looking at the schematic I'm wondering what the Trimpot is doing there in the first place.  Is it even necessary? If it is, Would the 5k be a better option?

Schematic here: (power supply section, bottom left of image)
https://www.electrosmash.com/images/tech/ce-2/boss-ce-2-schematic-parts.png

Google is suggesting that it's job is to bias the BBD chip. I can see Vb plays a role around pins 7 and 8 but looking at the power supply, my limited knowledge tells me that the Trimpot shouldn't be necessary to achieve 4.5v on Vb, so long at the resistors either side of the trimmer are closely matched. If it is necessary for fine tuning, a value like 10k seems completely overkill.

I'm conscious of my knowledge being very limited and hoping someone can explain (like I'm 5) what it's doing there and why it's such a (relatively) high value.

[Max999]

The output of the 10K trimpot is connected to all the points marked with "4.5V". The 4.5V is biasing the opamps at mid supply levels and also biases the BBD.
So the question now is "why the trimpot". My take is ( please others correct me if I am wrong!) that it is to fine tune the bias voltages because the resistive divider before the trimpot is pulled off center because of the current draw.

If this is the case your 5K pot would be a better option than the 10K, because it provides a stiffer bias supply.

[PRR]

> I'm wondering what the Trimpot is doing there in the first place.  Is it even necessary?

The BBD is dead at either rail and has an optimum near halfway between rails. Near but NOT halfway exactly! It is different on every chip. Depending on the doping and baking it got in the vat. Enough to matter.

I was gonna say "20K" but OK, try the 5K. It will almost-sure be fine, enough range. If in the fine-biasing you discover you have to cramp the trim to an extreme to get maximum output with least distortion, either change the 4.7Ks to 3.3Ks or go to the 20K trimmer.

[slashandburn]

Excellent info!  Thanks. 

So this Trimpot should be set by ear, rather than by measuring exactly half voltage?   I'm glad I asked!

[Max999]

As we ( both) learned from PRR, the optimum of the BDD bias is not exactly halfway but varies per chip. I did my ce-2 by ear and it was actually very easy to set it right enough, there was only a small section of pot rotation in which the chorus actually worked.