Vandal from Hell power supply and trim pots questions Topic is solved

[micmac]

Hi all,

Newbie alarm, first post

I searched the web for some information but didn't find any build reports for the Vandal from Hell yet.

I ordered a kit of the Vandal from Hell kit from Musikding. The build document is here:

https://pcbguitarmania.com/wp-content/u ... g-Docs.pdf

PCBGuitarMan posted a picture of the PCB here on FSB in '21:

viewtopic.php?f=13&t=28945&p=286151&hilit=hell#p286151

In the above thread people wrote that they used their circuit with 12V or 18V power supplies, because the Randall circuit used 24V. But in the build doc of the Vandal from Hell it says 9V.

Also, there's two 100k trim pots mentioned in the build doc. But it doesn't say what to do with them. In the PCB layout they're marked as POT1 (close to Q1) and POT3 (close to Q2).

My two questions:

1. Is 9V the correct power supply voltage for the Vandal from Hell or should it be used with higher voltage for more headroom?
2. Are the trim pots to be used to bias Q1 and Q2 to 4,5V (half of the power supply voltage) on the transistor drains?

[mauman]

Welcome!

My experience with J201 preamps is that you can't just increase the voltage from 9V to 18V without adjusting the drain and source resistors. It won't hurt anything, but it will sound quite different than designed.

Yes, Q1 and Q2 are biased using the pair of 100k trim pots. For a 9V supply, somewhere between 4.5V and 6V at the drain is usually recommended, and you can fine-tune the drain resistor (trim pot) by ear. There's a concept called the Fetzer valve that proposes optimum bias points for various supply voltages and FET types when doing a conversion from tube design to FET design. If you like that idea, details can be found here, there's a calculator at the bottom of the page. https://www.runoffgroove.com/fetzervalve.html

[micmac]

Thank you mauman for all this information! Much appreciated!

The linked fetzervalve paper seems a bit over my head. I shall read it a couple more times

Musikding sent a mail that the shipment is almost on its way. So I think I should have the kit sometime next week. I'll post back once I have the device boxed up.

Thanks again for your help!

Kind regards,
Seb

[mauman]

Looking forward to hearing about it! BTW, using the Fetzer Valve approach for your build would be setting the trim pots for Q1 and Q2 closer to 6V at the drains than to 4.5V, for a 9V supply.

[micmac]

Good morning!

Finished the Vandal. I had some issue with the transistors, possibly of my own making. I won't bore you with the details, but I think having a cheap 25€ transistor tester around for this build is a good idea, even if only to check that the transistors are working.

Main PCB was easy to solder. The only "problem" I had was the spacing between C15 (220u) and R12. You need to pull R12 down a fraction to make space for the cap. It'd have been easier if I had known this up front.

6 pots, 3 switches and a LED into a 125b is a tight fit in my opinion, so it wasn't easy to box it up. But in the end it worked out fine. I didn't drill the holes myself. That was done by Musikding. Good job Musikding!

I bent the pot pins upward a bit so that they stick out of the top side of the PCB a bit, otherwise they'd have been just about flush, because the "Channel" switch is a bit on the high side (higher than the "Sustain" switch).

The PCB Mania 3PDT mini PCB gave me some grief. The holes are pretty close together and some solder ran from one hole to the next so I had to clean it up again. Never had this issue with mini PCBs from other vendors. But in the end it worked out fine.

When I first tested the pedal yesterday I had Q1 and Q2 biased to 4,5V on the drains. It didn't sound so good. It felt unfinished, out of whack. Today I biased both to a bit over 6V like mauman suggested. Like this it sounds great.

I was watching some online videos about the original amp and so my expectation was that the pedal alone is not going to be high gain. When I plugged it into the FX loop return of my amp it sounded a bit like a JCM800 but darker. So I added a 6 band EQ in front of the vandal and boosted a band here and there. But I needed a little bit more boost, so I added a PedalPCB Closed Circuit between the EQ and the Vandal. The CC is a clean boost and I ran it with the compression off. With the gain at 11 or 12 I felt like I had enough boost for the Vandal. It sounds brutal like this. I think the Vandal and I are going to be good friends

Attached some pics. Forgive the missing knobs, they're still in the mail.

Kind regards,
Seb

[mauman]

Congratulations! Your first thread completed, with a working pedal, and it looks good too. Neat job on the wiring, I like running those in/out jack wires far apart, against the sides of the enclosure like that, it helps reduce the opportunity for signal leakage and feedback on gain-y pedals. I know what you mean about boosting the output level, these JFET amp emulator pedals are more about the character of the sound, and less about adding volume.

[micmac]

Hi mauman!

No no, I'm not boosting output, the output is fine. I'm boosting the input of the Vandal.

And thank you!

Kind regards,
Seb

[M.H.Halo]

Hi micmac
Thx for your thread.
Im about to build this vandal as well ..
Not sure what voltage I Will use.
9, 18 or 24V .
I Will use 2n 5484 transistors
Any suggestions?
Im novice what comes to pedal building,
Changed damaged pcb and op- amps on my mxr 6-band eq .
Greetings/ Miika

[micmac]

Hi @M.H.Halo

Mmh, I think having a cheap tester around for the transistors (like 25€) to weed out the duds is helpful. But even if you don't have a tester, my advice would be to socket the transistors if possible. I usually use sockets for transistors. This way I

a) cannot fry them while soldering
b) I can easily take them out again for whatever reason, like trying them in different places

Have fun with your build!

[M.H.Halo]

Cool, thx nan