# M800 PAL 800 JCM Emulator run at 18V?



## cooder (May 5, 2020)

Wondering if I can run the M800 / PAL 800 JCM Emulator with a charge pump at 18 V for increased headroom and punch as well as on 9V for 'normal' sound? 
Silly idea, yay or nay?


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## MightySmallElectrons (May 5, 2020)

Looks like the J201’s will run fine. I’m guessing that as long as your electrolytic is rated for over 25v you would be good.
But I wouldn’t trust my judgement and I’m curious to hear from someone who knows what they’re talking about.


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## Nostradoomus (May 5, 2020)

Should be fine, might need to tweak the trimmers so they’re at 9v-ish.


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## cooder (May 5, 2020)

It seems an overall similar topology like the Deofol / Okko Diablo which has the switchable charge pump 9 to 18 V, so i guess I give that a go and will report back what I find. I'll check the drain voltages when I do that. Cheers.


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## Chuck D. Bones (May 9, 2020)

Actually, the topology is different.  The Okko Diablo does not have trimmers because the drain voltage is set by the upper JFET.  The trimmer setting in the M800 will have to be a compromise between 9V and 18V operation.  Not necessarily a bad thing because you probably want it to sound different at 9V vs. 18V. Just trim by ear at both voltages.  BTW, 1/2 Vcc is not a magic number, it's just a starting point in all of the JFET pedals.


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## cooder (May 10, 2020)

Chuck D. Bones said:


> Actually, the topology is different.  The Okko Diablo does not have trimmers because the drain voltage is set by the upper JFET.  The trimmer setting in the M800 will have to be a compromise between 9V and 18V operation.  Not necessarily a bad thing because you probably want it to sound different at 9V vs. 18V. Just trim by ear at both voltages.  BTW, 1/2 Vcc is not a magic number, it's just a starting point in all of the JFET pedals.


Yeah cheers I noticed the absence of trimmers on Okko Diablo. Thanks for explaining!


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## music6000 (May 10, 2020)

I just tried it on my Build & it sounds Huge @18v, I found it more controllable at 9v meaning at so called ''Bedroom'' levels!
My trimmers are all set at 4.5v @ 9v Power supply.
Yes it works but you need to tame that Volume knob @ 18v's !
I always use 35v or higher on my Builds.
People looking for that ''Marshall'' sound should seriously look at building this pedal
It has gone a little ''Under the Radar'' as it is built in Small Quantity's.
Purchase the PedalPCB Adaptor Board & save yourself some heartache with Fake J201's:








						MMBFJ201 JFET (Pre-Soldered) - PedalPCB.com
					

JFET




					www.pedalpcb.com


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## cooder (May 10, 2020)

music6000 said:


> I just tried it on my Build & it sounds Huge @18v, I found it more controllable at 9v meaning at so called ''Bedroom'' levels!
> My trimmers are all set at 4.5v @ 9v Power supply.
> Yes it works but you need to tame that Volume knob @ 18v's !
> I always use 35v or higher on my Builds.
> ...


Awesome, thanks for that!
What do you mean by "_I always use 35v or higher on my Builds._"? 
Would you run this from an even higher voltage supply to do that?


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## Moltenmetalburn (May 13, 2020)

I have run mine at 33-36V with an external charge pump.  Sounds ok. Muxh louder.  Caps are all 50V or higher. I previously tweaked trimmers by ear from the 4.5/half supply starting point at 9V. 

hmm had another look at this. Been tweaking for a while at different voltages.

Even at 18V not all the the trimmers can limit the voltage enough to reach the halfway bias point.

seems like a bad idea unless the trimmer values can simply be adjusted higher  to compensate.

Hopefully others will chime in again. Definitely takes more than just higher voltage caps.


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## Elijah-Baley (May 13, 2020)

Hello.
I still never built none JFET emulator amp, but I guess I read about the Dr. Boogie that at higher voltage like 36v can help bigger trimmer value: 250k, 500k maybe?


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## Moltenmetalburn (May 13, 2020)

Elijah-Baley said:


> Hello.
> I still never built none JFET emulator amp, but I guess I read about the Dr. Boogie that at higher voltage like 36v can help bigger trimmer value: 250k, 500k maybe?


That is what I am hoping that a larger trimmer value is all that is needed. 

The misbias at 18- 36 did sound a bit worse than 9.


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## Chuck D. Bones (May 13, 2020)

I would expect that higher voltage operation would require proportionately larger trimmers to find the sweet spot.


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## cooder (May 13, 2020)

And how about we crank up the voltage to 250 V and make it the real tube amp....? LOL...
Interesting discussion though. I wonder if at some point of increasing voltage it is a matter of diminishing returns, aka not hearing much difference...

And I'd love to see more schematics of circuits available here as I'm on a binge trying to learn and see what's happening on circuits. 
For example how the 9 to 27 V switch is implemented on Duocast.

Paging Dr. BuGG...


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## Chuck D. Bones (May 13, 2020)

cooder said:


> For example how the 9 to 27 V switch is implemented on Duocast.



When you see it, you will shit bricks.

Deceptively simple.  All you need is an SPST switch between +9V and pins 1 & 8 of the charge pump.  Disconect the charge pump and the 9V flows straight thru the diodes with no boost.  

Alternatively, you can put an SPDT switch between the power rail and the charge pump to select either +9V or +27V.  But why leave the charge pump running when it's not being used?


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## dankmetalsounds (Jul 21, 2020)

Newbie question - Instead of using a change pump, could you run 18v directly into the pedal to get the desired effect?


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## Chuck D. Bones (Jul 21, 2020)

Yes, absolutely.  All the aforementioned caveats apply...


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