PAL Fx JCM 800 Emulator  [schematic]

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
User avatar
plush
Cap Cooler
Information
Posts: 639
Joined: 08 Dec 2015, 09:29
Location: Moscow, Evil Russia
Has thanked: 36 times
Been thanked: 169 times

Post by plush »

Pedalist666 wrote:Thank you for the reply.

I am brand new to pedal technology and i read on youtube that mosfets are the way to emulate tube sound using solid state electronics. i guess that info is wrong.
It's not solely mosfets, but their implementation in circuit design.
In current design you can't simply throw in bunch of mosfets and hope that they will make this circuit sound good.

Some circuits implement DMOS or MIS FETs and emulate tubes quite well. They don't use one transistor per one tube stage, but bunch of them to emulate inherent feedback and other properties of a vacuum tube.

Google Fetron or AMT's approach to triod emulation (their Warmstone is build around it) it's not that comprehensive, but you'll get the idea (cons - it's all in russian but google translate will help).

User avatar
POTL
Resistor Ronker
Information
Posts: 335
Joined: 26 Sep 2016, 03:11
Has thanked: 58 times
Been thanked: 67 times

Post by POTL »

You will need to make some changes to the circuit.
MOSFets in schemes of a similar style.
1) Zvex Box Of Rock, Distortron, etc
Image
2) Catalinbread RAH.
Image
But you should understand that MOSFets sounds completely different than JFets.

I found their sound less transparent, they are more middle, more noisy and dirty. I don’t like their high gain sound.
They sound interesting in fuzz patterns and are good for clear sound (although I don't remember if they were noisy).
Also, they can be very pleased with the sound at first, and then disappoint because of their characteristic overtones.

User avatar
Ichabod_Crane
Resistor Ronker
Information
Posts: 354
Joined: 26 Apr 2016, 15:11
Has thanked: 82 times
Been thanked: 36 times

Post by Ichabod_Crane »

What about the voicing switch in newer Pal800 pedal?

How does the "Gain Voicing switch" work?
This three-position switch will allow you to get different tone responds (Voicing) in the gain section of the pedal, the change between positions will be more radical at low gain settings and smoother as you turn up the "Gain" knob. At maximum "Gain" knob position this switch will not affect the gain voicing respond.
The middle position is the most flat respond setting of the three positions, ideal to use at low gain settings (bellow 11 o'clock position of the "Gain" knob), the lower position will give you more middle and bass response and the upper position more brightness and less bass response.
Each position will affect the output volume level of the effect depending of the "Gain" knob position, in this case, you will need to adjust also the "Volume" knob to your taste when you use the "Gain Voicing switch".

Listen in some demo the effect that that voicing switch do I'd say it could change the bypass cap (680nF) in the first stage.
- No cap (or small cap) for the lower gain setting voice.
- Stock 680nF for the middle setting voice.
- A bigger cap, like 1uF (or even more) for the more high gain setting voice.

But I dont' get how it could be no effect at max gain pot setting.

Edit:
Or, watching an official video, the high gain setting voice could be the stock 680nF. So the other two setting should beone with smaller cap and the other one with an even smaller cap or no cap at all.

Edit 2 (sorry):
Watching this: I see just three transistor (transistors?) a none bias trimmer. What the...? :scratch:

User avatar
tinkercreek
Information
Posts: 13
Joined: 12 Jun 2010, 16:28
Been thanked: 2 times

Post by tinkercreek »

Hello all

I built this circuit from the Dirtbox Layouts site. I kept working on this but could never get it biased so that it didn't have a farty note decay. I even built the JCM Emulator from Sabro's site to see if it acted any differently - basically the same circuit with an added "master volume" between Q4 and Q5. Same issue.

I looked back here and re-read back through a few times. On the last pass I saw something that I had passed over previously. bmxguitarsbmx had posted drain voltages from an actual unit. I biased both to these voltages and the farty decay is 99% gone. Here are the voltages :

Q1 - 3.742V
Q2 - 3.361V
Q3 - 3.362V
Q4 - 9.45V ( tied directly to 9V supply )
Q5 - 4.03V
Q6 - 3.115V

Not sure why this circuit is so unforgiving for other values closer to 4.5 - 5V. I've built many of these J201 cascading gain stage circuits and they usually sound fine if you get them close to the 4.5V value.

Now, the qualification. It really only sounds really good at or near full gain. When I lower the gain to lower settings, the farty decay comes back - same on the Sabro JCM 800 build. I've watched the demos and they often play the pedal at lower gain settings and there is no decay weirdness. I'm flummoxed at this point. Been tinkering with this for a couple of months. Maybe someone here has an idea for resolution? I've been building for about 10 years but I'm still not super well versed in the depths of circuitry. Still trying to learn though.

And I tried multiple sets of J201s from different sources and they all exhibit the same behavior.

With all that said, I'm glad I built it. Even without the capability to turn down the gain on the pedal, it sounds really good at the top gain setting - and you can lower the volume on your guitar and it cleans up pretty well. Thick and full distortion - I did swap the 680n for a 1uf for a bit more gain - made a pretty big difference. I would recommend that change.

Thanks

Tim/tinkercreek

User avatar
tinkercreek
Information
Posts: 13
Joined: 12 Jun 2010, 16:28
Been thanked: 2 times

Post by tinkercreek »

And thank you bmxguitarsbmx for the posted voltages. I was ready to give up on the pedal until I used those voltages. And thanks to all for the expertise here.

User avatar
tacianocanassa
Information
Posts: 8
Joined: 25 Apr 2018, 20:03
Been thanked: 13 times

Post by tacianocanassa »

tinkercreek wrote: 26 Aug 2021, 14:46 Hello all

I built this circuit from the Dirtbox Layouts site. I kept working on this but could never get it biased so that it didn't have a farty note decay. I even built the JCM Emulator from Sabro's site to see if it acted any differently - basically the same circuit with an added "master volume" between Q4 and Q5. Same issue.

I looked back here and re-read back through a few times. On the last pass I saw something that I had passed over previously. bmxguitarsbmx had posted drain voltages from an actual unit. I biased both to these voltages and the farty decay is 99% gone. Here are the voltages :

Q1 - 3.742V
Q2 - 3.361V
Q3 - 3.362V
Q4 - 9.45V ( tied directly to 9V supply )
Q5 - 4.03V
Q6 - 3.115V

Not sure why this circuit is so unforgiving for other values closer to 4.5 - 5V. I've built many of these J201 cascading gain stage circuits and they usually sound fine if you get them close to the 4.5V value.

Now, the qualification. It really only sounds really good at or near full gain. When I lower the gain to lower settings, the farty decay comes back - same on the Sabro JCM 800 build. I've watched the demos and they often play the pedal at lower gain settings and there is no decay weirdness. I'm flummoxed at this point. Been tinkering with this for a couple of months. Maybe someone here has an idea for resolution? I've been building for about 10 years but I'm still not super well versed in the depths of circuitry. Still trying to learn though.

And I tried multiple sets of J201s from different sources and they all exhibit the same behavior.

With all that said, I'm glad I built it. Even without the capability to turn down the gain on the pedal, it sounds really good at the top gain setting - and you can lower the volume on your guitar and it cleans up pretty well. Thick and full distortion - I did swap the 680n for a 1uf for a bit more gain - made a pretty big difference. I would recommend that change.

Thanks

Tim/tinkercreek
I Just Build it with smd J201 types (with adapters for trought hole), set the bias at half of the incoming power supply voltage (if it's 9V then 4.5v) and it works both with high GAIN and lower Gain settings without any gated sound. I just used my own PCB desing.
WhatsApp Image 2021-12-12 at 09.31.38.jpeg
PCB PAL 800
PCB PAL 800
WhatsApp Image 2021-12-13 at 20.36.00.jpeg

User avatar
tinkercreek
Information
Posts: 13
Joined: 12 Jun 2010, 16:28
Been thanked: 2 times

Post by tinkercreek »

Thanks tacianocanassa

I will order some smd J201s and adapters and give it another go. Appreciate the follow up post.

User avatar
tinkercreek
Information
Posts: 13
Joined: 12 Jun 2010, 16:28
Been thanked: 2 times

Post by tinkercreek »

tacianocanassa

I tried my build with the smd j201s with adaptors. I get the same issue I had before. So I've either built 2 similar circuits with the same mistakes or something's not working on the layouts that I have used. Can you possibly show me the schematic that you made your pcb from? I would like to check it against the layouts that I have used.

May try buying a pcb if I can't discern what is going wrong on mine.

Thanks

User avatar
Gebrey
Information
Posts: 37
Joined: 09 Mar 2023, 23:09
Has thanked: 9 times
Been thanked: 6 times

Post by Gebrey »

Zombodroid_06062023011700.jpg
Schematic + compact layout
1686087924438.jpg

User avatar
Gebrey
Information
Posts: 37
Joined: 09 Mar 2023, 23:09
Has thanked: 9 times
Been thanked: 6 times

Post by Gebrey »

Ichabod_Crane wrote: 20 Mar 2021, 14:37 What about the voicing switch in newer Pal800 pedal?

How does the "Gain Voicing switch" work?
This three-position switch will allow you to get different tone responds (Voicing) in the gain section of the pedal, the change between positions will be more radical at low gain settings and smoother as you turn up the "Gain" knob. At maximum "Gain" knob position this switch will not affect the gain voicing respond.
The middle position is the most flat respond setting of the three positions, ideal to use at low gain settings (bellow 11 o'clock position of the "Gain" knob), the lower position will give you more middle and bass response and the upper position more brightness and less bass response.
Each position will affect the output volume level of the effect depending of the "Gain" knob position, in this case, you will need to adjust also the "Volume" knob to your taste when you use the "Gain Voicing switch".

Listen in some demo the effect that that voicing switch do I'd say it could change the bypass cap (680nF) in the first stage.
- No cap (or small cap) for the lower gain setting voice.
- Stock 680nF for the middle setting voice.
- A bigger cap, like 1uF (or even more) for the more high gain setting voice.

But I dont' get how it could be no effect at max gain pot setting.

Edit:
Or, watching an official video, the high gain setting voice could be the stock 680nF. So the other two setting should beone with smaller cap and the other one with an even smaller cap or no cap at all.

Edit 2 (sorry):
Watching this: I see just three transistor (transistors?) a none bias trimmer. What the...? :scratch:
These are 2x trannsistor coupled under a plastic case. So 6 trannies as we expect.

About the voicing switch, maybe could be a bypass cap on the pin 1 & 2 of the gain pot. Since these caps are used to let pass some freq, when you turn gain to 100% the signal pass totally (in fact this gain pot is a volume pot) making the cap useless (no difference at max gain)

And last, the trimmers are not present because they use discrete resistors. They use a trimmer to check the voltage and they use tables of voltage/resistor for opportune choice.

User avatar
Ichabod_Crane
Resistor Ronker
Information
Posts: 354
Joined: 26 Apr 2016, 15:11
Has thanked: 82 times
Been thanked: 36 times

Post by Ichabod_Crane »

Thank you for your reply. Later I learnt what you explained. I don't know however I didn't know it when I asked. :D

After that I am very curious about the newer version, the V3, the one with I/II footswitch (like a high and low input amp style). And the #34 toggle switch, that should sound is like «a modification that can be made in a 1959SLP or JCM800 amp and that has been the main tone of one of our favorite guitar player "SLASH". »

https://www.pedalpalfx.com/pal800-v3-gold-overdrive

User avatar
Ichabod_Crane
Resistor Ronker
Information
Posts: 354
Joined: 26 Apr 2016, 15:11
Has thanked: 82 times
Been thanked: 36 times

Post by Ichabod_Crane »

Hello.
I built the PAL800 circuit using this verified layout here:
http://dirtboxlayouts.blogspot.com/2020 ... lator.html
Setting the trimmer to get the voltage you can see here viewtopic.php?p=264332#p264332 the distortion seems work fine without gate problem I got with the voltage setted at 4.5v.

Anyway, I got a problem. It sounds alright just with the Mid at lower setting. If I increase the Mid I got bias/gate sound, but just at the medium gain setting. It sounds still right at lower gain and higher gain, but passed a certain points the sounds gated suddenly. These points change turning the Mid and the Gain control.

I noticed the voltage on the gate of Q5 change if I turn Mid and Gain.
Position of the parts is ok, so the values, orientation of the parts and wires, potentiometers measure ok. None jumper or cut track is missing.
Transistors are smd J201 I soldered on PCB, they seems ok. All of them are on the socket pin.
I passed a knife between the stripes, I found any bridge.
The eq is a bit limited, but I knew this. Overall it sounds a bit muddy, but the tone trimmer helps.

Usually I don't have this kind of problem, but this one seems very tricky and I need some help.

Thanks.

User avatar
bmxguitarsbmx
Cap Cooler
Information
Posts: 510
Joined: 25 Mar 2010, 21:15
Has thanked: 700 times
Been thanked: 216 times

Post by bmxguitarsbmx »

That is weird. The tone controls are isolated from the gate of Q5 with capacitor C9, so adjusting the midrange should not effect the voltage at the gate of Q5. is there any DC voltage measurable in the tonestack? Could be a bad C9

User avatar
Ichabod_Crane
Resistor Ronker
Information
Posts: 354
Joined: 26 Apr 2016, 15:11
Has thanked: 82 times
Been thanked: 36 times

Post by Ichabod_Crane »

Thank for your reply. I want to correct a bit the thing after another test. It could be the Gain pot that change the voltages in every transistors. Somehow include the Mid setting, but with the gain at max or very low the mid doesn't change the voltages. But the voltages change when the gain is not lower until is not almost at higher gain. Indeed in the central part of the gain setting it sounds like misbiasing and gated.

I'll do other tests. I have to try to replace the transistors one at the time.

User avatar
andregarcia57
Cap Cooler
Information
Posts: 572
Joined: 16 Nov 2008, 15:42
Has thanked: 72 times
Been thanked: 108 times

Post by andregarcia57 »

PAL800-V3 GOLD Overdrive (2020 Model)
Attachments
beooypottl0ps7tewsmr.jpg

User avatar
Ichabod_Crane
Resistor Ronker
Information
Posts: 354
Joined: 26 Apr 2016, 15:11
Has thanked: 82 times
Been thanked: 36 times

Post by Ichabod_Crane »

I measured the voltage and made some tests.
Voltage with Bass, Treble, Presence at half, Mid at zero, Gain at max. Every drain voltage is very close the original pedal. The sounds seems ok.
With the Mid at zero I can use all the gain range without any changing in the voltages, and it still sounds fine.

Q1 Q4
D: 3.7v D: 9.2v
S: 0.4v S: 3.7v
G, 0.0v G: 3.4v

Q2 Q5
D: 3.6v D: 3.9v
S: 0.5v S: 3.0v
G: 0.0v G: 1.7v

Q3 Q6
D: 3.3v* D: 3.2v
S: 0.2v S: 0.2v
G: 0.0v G: 0.0v

*The trimmer can't allow to get 3.3v like the original one.

But, when the mid is not at zero voltage for Q3, Q4, Q5, Q6 changes if the Gain is not at higher setting or lower setting.
For example, with the Mid at max and Gain at half I get:

Q1 Q4
D: 3.7v D: 9.2v
S: 0.4v S: 4.5v
G, 0.0v G: 4.3v

Q2 Q5
D: 3.6v D: 6.7v
S: 0.5v S: 1.5v
G: 0.0v G: 1.4v

Q3 Q6
D: 4.2v D: 3.0v
S: 0.1v S: 0.2v
G: 0.2v G: 3.0v

What does it mean?

User avatar
blackboarcult
Breadboard Brother
Information
Posts: 72
Joined: 28 Mar 2018, 19:18
my favorite amplifier: one that works.
Location: Norway
Has thanked: 89 times
Been thanked: 70 times

Post by blackboarcult »

I think I had a similar problem with one of my jfet emus a few years ago, but I ended up binning it instead of debugging. I'd try to put a 1uF cap between the jfet follower and the tonestack, and see if it changes anything.

User avatar
Ichabod_Crane
Resistor Ronker
Information
Posts: 354
Joined: 26 Apr 2016, 15:11
Has thanked: 82 times
Been thanked: 36 times

Post by Ichabod_Crane »

Do you mean a 1uF cap between Q4 and the tonestack?

User avatar
blackboarcult
Breadboard Brother
Information
Posts: 72
Joined: 28 Mar 2018, 19:18
my favorite amplifier: one that works.
Location: Norway
Has thanked: 89 times
Been thanked: 70 times

Post by blackboarcult »

Yep!

User avatar
Gebrey
Information
Posts: 37
Joined: 09 Mar 2023, 23:09
Has thanked: 9 times
Been thanked: 6 times

Post by Gebrey »

Ichabod_Crane wrote: 16 Aug 2023, 16:51 I measured the voltage and made some tests.
Voltage with Bass, Treble, Presence at half, Mid at zero, Gain at max. Every drain voltage is very close the original pedal. The sounds seems ok.
With the Mid at zero I can use all the gain range without any changing in the voltages, and it still sounds fine.

Q1 Q4
D: 3.7v D: 9.2v
S: 0.4v S: 3.7v
G, 0.0v G: 3.4v

Q2 Q5
D: 3.6v D: 3.9v
S: 0.5v S: 3.0v
G: 0.0v G: 1.7v

Q3 Q6
D: 3.3v* D: 3.2v
S: 0.2v S: 0.2v
G: 0.0v G: 0.0v

*The trimmer can't allow to get 3.3v like the original one.

But, when the mid is not at zero voltage for Q3, Q4, Q5, Q6 changes if the Gain is not at higher setting or lower setting.
For example, with the Mid at max and Gain at half I get:

Q1 Q4
D: 3.7v D: 9.2v
S: 0.4v S: 4.5v
G, 0.0v G: 4.3v

Q2 Q5
D: 3.6v D: 6.7v
S: 0.5v S: 1.5v
G: 0.0v G: 1.4v

Q3 Q6
D: 4.2v D: 3.0v
S: 0.1v S: 0.2v
G: 0.2v G: 3.0v

What does it mean?
I'm stucked too with this pedal. I think because my jfet are fakes (bjt reprinted?). I can't measure less than 8.9V on Drains @9V psu.

Back to you, I would probably put GAIN to max and maybe MID too and bias the trannies. Let me know if biasing at max setting could improve

Post Reply