Sovereign/Monarch J201 biasing?

[mywmyw]

I built a Monarch with some no name J201s from Smallbear and it works except there is a a strange splatty/ducking effect on the note attack when I play with drive knob set anywhere but absolutely maxed out. Could this be a result of the 201s needing to be biased?

 

[music6000]

You could try removing R101-10k & putting a 25K trimmer or temporary pot with leads to see if you can dial it in better.
That gives you around 10K in either direction from where you are now.

 

[mywmyw]

ok i may give that a try. im curious why this pedal doesnt have the bias trim pots like the pineapple and other j201 emulation type pedals

 

[Mcknib]

ok i may give that a try. im curious why this pedal doesnt have the bias trim pots like the pineapple and other j201 emulation type pedals

Put simply the upper fet kinda maintains the power requirements for the lower one which drives the load or does the amplification it's a mu amp arrangement

Have a look at cascode amplifiers

Page 21 of Mr Wamplers book albeit he does use trimmers in the example circuit

Google Drive: Sign-in

Access Google Drive with a Google account (for personal use) or Google Workspace account (for business use).

drive.google.com

Geofex:

Foolin' with FETs

www.geofex.com

 

[mywmyw]

Thanks for the link.

I measured the drains of the j201s, here is what I got :

Q1 - 5v
Q2 - 9.24v
Q3 - 6.74v
Q4 - 5.11v
Q5 - 9.24

is it just me or are all of these too high?

 

[music6000]

Try swapping Q2 with Q4 & see what Voltage your getting, They might be Duds!

 

[Chuck D. Bones]

Q1 & Q2 are fine.

Q3 is too high, but since the Q4 measurement makes no sense (Q4 drain connects to Vcc), recheck both of them.

Q5 definitely has a problem, its drain voltage is way too high. Verify R19 and R20 are 2.2K and 27K respectively and check your solder joints.