M800 EQ section does very little

[sticky1138]

I used SMD J201s from PedalPCB, all biased with a DMM to to 4.5V. The tone is set by ear. It sounds great and just like a JCM, but each of the four EQ pots do very little. The difference between max and min on each is no more than `15-20%. The drive is also unusably muddy past noon, if that means anything. I experimented with the tone pot for a while, but these problems persist in the sweet spot.

 

[Harry Klippton]

The eq section barely does anything on a jcm 800 too

 

[blackhatboojum]

The eq section barely does anything on a jcm 800 too

This^^^. I will preface by saying I don’t own the original pal 800 nor have I built the M800. My understanding of these pedals is that they’re supposed to almost perfectly emulate a JCM 800. If that’s the case then yeah… there’s not going to be much range with the eq. I own a jcm800 clone and the eq doesn’t do much. Especially when you start cranking up the preamp gain.

 

[sticky1138]

Interesting. I briefly owned a JCM 900 head a while back and if I kind of remember the same thing. I read someone's build report here saying they liked the EQ response from the M800, which threw me off because mine seems to be mostly for looks.

 

[blackhatboojum]

You could have something wonky going but, I’m of no help with this one. Hopefully, someone with this circuit will chime in and give you some definitive answers.

 

[Bricksnbeatles]

The configuration and values for the M800 tone stack perfectly match up to the configuration and listed values for a Marshall TMB tone stack as shown in the Duncan Tone Stack calculator.
After you verify that you haven’t made any errors, you want to play around with the values in the tone stack calculator to see what components you may want to swap out for different values. As it is, I find the Marshall tone stack to be my least favorite popular amp tone stack, but with some tweaking it could probably be made into something a little more useable.

TSC in the web

 

[Bricksnbeatles]

After playing around with the TSC a bit, I think if I was building this myself, I’d replace C6 with a 100pF cap, C8 with something around 47nF, and R10 with a 50K cap.
Strongly increases the mid scoop you get when the bass and treble and maxed out with the kids below 5, increases the flatness of the 1-10-0 setting, and gives you a nice upper mid scoop without too much bassiness when you’ve got the bass and mids all the way down with treble between 1-2. You still get a good mid hump with all controls turned down, and the range of the controls is increased as a whole.

 

[Danbieranowski]

Built one a few weeks back, and yes, the EQ section does very little. lol

 

[Sturdag Lagernathy]

Wait, the eq is supposed to do something? I thought you just put 'em all on 10 and rocked out?

 

[djmiyta]

built this recently using smd j201s '( at fist I used thru hole and they did'nt work ) now I have signal and the tone section I can hear the pots doing something but it acts only as a booster checking my gain pot value it would only go to 210 k ohms and as I'm still turning clockwise it goes down to 60 ohms I'm thinking this would a problem? and probably 1st place to start checking then audio probe the fets all biased up nicely then I tried by ear still no distortion

 

[sticky1138]

built this recently using smd j201s '( at fist I used thru hole and they did'nt work ) now I have signal and the tone section I can hear the pots doing something but it acts only as a booster checking my gain pot value it would only go to 210 k ohms and as I'm still turning clockwise it goes down to 60 ohms I'm thinking this would a problem? and probably 1st place to start checking then audio probe the fets all biased up nicely then I tried by ear still no distortion

Use a DMM touching the ground and the drain on each J201 and bias each to 4.5V as a starting point before setting by ear.

 

[djmiyta]

I did the biasing with a DMM to 4.5v and only get clean boosted signal no distortion then after biased by ear and can get no distortion.
2 thoughts as to why 1 is a long shot and that is I ordered a 100 mmbf j201 from mouser and a 100 smd to thru hole adapter boards from oshpark. Possibly a bad j201? I have doubts thats it. 2nd is I soldered the j201's to the boards and although I was very quick maybe I fried 1 or 2 when soldering? guess Ill have to pull them and find out.
It bummed me cause after reading how several others were using thru hole fets when they swapped them for SMD j201s it made a huge difference. I had high hopes for experiencing the same thing. will have to try in other circuits