# M800 Overdrive



## music6000

Awesome JCM800 Emulation !!!
All the Trimmers were adjusted to 4.5v & it was meh, 4.75, 5v, 5.25v & 5.5v, Louder with no Gain!
*Old School*, Strummed a Chord & tuned each Trimmer by ear.
Critical Trimmer is the Tone trimmer, This effects overall EQ & gives it the Power of The JCM800 sizzle!
The *Tone *Trimmer is an Internal Potentiometer, It doesn't adjust the J201(*Q4*) below it, that has Fixed Voltage.

*UPDATE - November 2019 : The J201's have been replaced with Siliconix J201's & are set to 4.5v.
It now has more Headroom & Sustain!!!*

The JCM 800 label represents Jim's Number Plate, rumoured to be where the Amp got its name.








Here is a Good demo :


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## zgrav

Nice looking build!  Glad that adjusting the trimmers by ear unlocked the tone you were looking for.


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## mad5066

That's awesome, you convinced me to build one now and I still have 5 in que


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## geekmacdaddy

Nice build. Like the demo too. Just might have to build it.


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## music6000

*Update *: The internal Tone trimmer is just that, It adjusts the Low Mid to High End frequencies.
 The J201 below it is separate & has fixed Voltage.


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## Barry

Schweet


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## DrumBuster

Hey! Having issues with mine, followed the wiring but I’m having ground issues. The switch I wired is wired differently than yours, why so?


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## JetFixxxer

I'm having issues with mine as well. Found the replacement PF5102 and J112 might be the culprit. I'm just waiting for my real 201 to show.


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## Chuck D. Bones

music6000 said:


> *Old School*, Strummed a Chord & tuned each Trimmer by ear.



You didn't happen to write down the drain voltages after you dialed in the timmers, did you?  Might be a good starting point for the next guy...


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## DrumBuster

my j201s all work correctly, they all read well, I’m guessing the issues is the switch, got rid of the grounding. Clean sound on bypass but when I toggle it on it doesn’t work.


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## music6000

DrumBuster said:


> Hey! Having issues with mine, followed the wiring but I’m having ground issues. The switch I wired is wired differently than yours, why so?


This is what I have used for years, Pedal PCB Footswitch wiring is correct & PCB matches the Footswitch breakout Board.


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## DrumBuster

music6000 said:


> This is what I have used for years, Pedal PCB Footswitch wiring is correct & PCB matches the Footswitch breakout Board.


The one in the build instructions looks different to yours, the last row Is jumped, safe the middle one which is jumped with the input prong.


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## Chuck D. Bones

There is more than one way to wire a 3PDT stompswitch and get the same results.  What Music6000 did is equivalent to the wiring diagram in the Build Docs.


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## music6000

*M800 Latest Update : *I revisited the pedal yesterday as I had recently acquired some *Genuine NOS Siliconix J201 *with the Dot.
I removed the Fairchild J201's & replaced with the Siliconix J201's.
I set them all to 4.5v except Q4 which has fixed voltage.
It worked instantly & Sounds better than before at *4.5V *with even better sustain & more Headroom.
The Fairchild J201's had to be adjusted by ear as they were out of spec at 4.5v, remnants of what is left for the picking & the poor Receiver at the other end!
I have spoken to a few members that have been less than happy with their Build.

Chuck  D. Bones has listed the specs that need to be measured to get maximum performance from the J201.

PedalPCB now offers the more stable SMD J201 & Adaptor Board pre soldered or as separate purchases.

This Pedal sounds Great but has to be with Genuine J201's that are in Spec with IDSS & VGS.


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## JetFixxxer

music6000 said:


> This Pedal sounds Great but has to be with Genuine J201's that are in Spec with IDSS & VGS.



I agree.  I couldn't get it work with the substitutes.  I wasn't expecting any better results from genuine J201, but I was completely wrong.


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## music6000

JetFixxxer said:


> I agree.  I couldn't get it work with the substitutes.  I wasn't expecting any better results from genuine J201, but I was completely wrong.


Did you use the Siiliconix J201's I suggested for your Final build?


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## JetFixxxer

music6000 said:


> Did you use the Siiliconix J201's I suggested for your Final build?


Yes I did.   Thanks!


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## cooder

So what would be the ideal values (Vp / Idss) for the jfets in this? (paging Sir Chuck....)


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## Chuck D. Bones

It's subjective, but lower Vp will give more gain.  With trimpots that large, any J201 that's in-spec should work.  I'd use the lowest Vp parts for Q1, Q3 & Q6 because they have the smallest source resistors.


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## cooder

I knew you'd be right on it, thanks.
So (asking for a friend...  ) what do I measure to find out if my smd J201 that I banged already in there are a good choice and how would I (well my friend...) improve the biasing on that circuit?


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## Chuck D. Bones

Not to be flippant or anything, but give it a listen and see if even needs "improving."  Any idea where the trimmers are set?


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## cooder

I'll measure later on to confirm but from memory I set em all to 4.5 volt. I shall try also to tweak and listen a bit more, so far I'm not overwhelmed with it. Will post my results back here.


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## music6000

cooder said:


> I'll measure later on to confirm but from memory I set em all to 4.5 volt. I shall try also to tweak and listen a bit more, so far I'm not overwhelmed with it. Will post my results back here.


The Magic is also where you set the internal Tone Trimmer, it has a *big* impact on the pedal!


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## cooder

Thanks I shall try to conjure up some more magic.


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## Chuck D. Bones

cooder said:


> I'll measure later on to confirm but from memory I set em all to 4.5 volt. I shall try also to tweak and listen a bit more, so far I'm not overwhelmed with it. Will post my results back here.


I should have been more clear.  I meant to say "at what % of rotation or resistance did you set the trimmers?"  4.5V is a starting point, but I'd expect Q2 and Q4 to be set higher.  I really need to finish writing that JFET biasing article!


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## Chuck D. Bones

music6000 said:


> The Magic is also where you set the internal Tone Trimmer, it has a *big* impact on the pedal!


The tone trimmer interacts with the Q6 trimmer, so every time you adjust one, you should see if the other one needs tweaking.


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## sticky1138

Just finished this build. I set the all the trimmers to 4.5V and the tone pot by ear, which ended up sounding best around 9.3V. It does that Marshall stack sound, but it's muffled, the EQ is not that responsive and it's very noisy with the gain past noon.

Not sure if it's the J201s I'm using or improper biasing, but it's pretty underwhelming so far.


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## zgrav

sticky1138 said:


> Just finished this build. I set the all the trimmers to 4.5V and the tone pot by ear, which ended up sounding best around 9.3V. It does that Marshall stack sound, but it's muffled, the EQ is not that responsive and it's very noisy with the gain past noon.
> 
> Not sure if it's the J201s I'm using or improper biasing, but it's pretty underwhelming so far.


try the settings for the trimmers listed in the first post to see if that brings the pedal to life.  note also that changing the j201s could make a big difference.


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## sticky1138

Just sat down with a looper pedal for 20 minutes and tested a bunch of combinations by ear. I was able to clear up the noise and got a lot more high-end clarity, but the distortion is nasty and gates quite a bit regardless of setting. The EQ is still ineffective, too. Probably going to swap the J201s when I can get my hands on some.


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## peccary

sticky1138 said:


> Just sat down with a looper pedal for 20 minutes and tested a bunch of combinations by ear. I was able to clear up the noise and got a lot more high-end clarity, but the distortion is nasty and gates quite a bit regardless of setting. The EQ is still ineffective, too. Probably going to swap the J201s when I can get my hands on some.



The SMD J201's are reliable and easy to find, and you can use these and some pin headers to install them thru-hole: https://www.pedalpcb.com/product/sot23adapter/ 

Might save you some time and headache down the line.


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## music6000

sticky1138 said:


> Just finished this build. I set the all the trimmers to 4.5V and the tone pot by ear, which ended up sounding best around 9.3V. It does that Marshall stack sound, but it's muffled, the EQ is not that responsive and it's very noisy with the gain past noon.
> 
> Not sure if it's the J201s I'm using or improper biasing, but it's pretty underwhelming so far.


What you are describing is out of spec J201's like my first set!
Purchase a set of these pre soldered  from PedalPCB and save yourself some heartache & you will get the pedal to fire up correctly.









						MMBFJ201 JFET (Pre-Soldered) - PedalPCB.com
					

JFET




					www.pedalpcb.com


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## sticky1138

music6000 said:


> What you are describing is out of spec J201's like my first set!
> Purchase a set of these pre soldered  from PedalPCB and save yourself some heartache & you will get the pedal to fire up correctly.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> MMBFJ201 JFET (Pre-Soldered) - PedalPCB.com
> 
> 
> JFET
> 
> 
> 
> 
> www.pedalpcb.com


Thanks, I'm definitely doing that once they come back in stock.


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