Vandal - a 2-channel preamp inspired by the Randall RG100ES

rowelej · Post by **rowelej** » 04 Apr 2019, 09:37

Thank you guys for your answers!
Still waiting for 4pdt delivery)

jarrodthebobo · Post by **jarrodthebobo** » 03 Sep 2019, 01:29

I just recently went ahead and tried to build the simpler version of this schematic; just the red channel and I'm having some issues with gain and volume; I've noticed that you've posted that you didn't have good results with 2n5485s; which are the only trannies I had on hand at the time. Is this why?

I assumed that 2n5484s and the 85s were close enough spec wise to be mostly interchangeable... I've gone ahead and ordered some 84s in the mean time and I'll post something else when I get them in, otherwise, has any one else had gain and volume issues with this build?

I've checked over the circuit numerous times and can't really find any issues with it; running it at 24 volts and everything appears to be biased right; no solder bridges either. The sustain switch works, adds tons of gain at the expense of volume, but without the switch engaged the base tone is very clean and only slightly above unity volume. The eq section, gain control, master, and presc all work fine as well. Any thoughts?

Thanks!

marcao_cfh · Post by **marcao_cfh** » 06 Sep 2019, 20:55

2N5485s, maybe? I built the red channel a few days ago using 85s, and had some issues too. Running the circuit at 18V, I got less distortion than I was expecting and volume below unity, even with the switch disengaged. Didn't tried other fets or voltages.

sixthfloor · Post by **sixthfloor** » 07 Sep 2019, 10:48

That's interesting, my 2n5485s distorted quite a lot contrary to both of your experiences. The trouble I had was with the distortion being kind of farty with too harsh highs; as far as I can remember the other transistor types I used led to the same problem. If i remember well, changing the bias voltages didn't solve the problem - the 5484s were the only satisfying transistors there.

jarrodthebobo · Post by **jarrodthebobo** » 09 Sep 2019, 17:10

sixthfloor wrote:That's interesting, my 2n5485s distorted quite a lot contrary to both of your experiences. The trouble I had was with the distortion being kind of farty with too harsh highs; as far as I can remember the other transistor types I used led to the same problem. If i remember well, changing the bias voltages didn't solve the problem - the 5484s were the only satisfying transistors there.

I'm going to be rebuilding the circuit this weekend fresh; with new parts and all. I'll report any updates when they happen

Optical · Post by **Optical** » 12 Sep 2019, 22:38

Has anyone tried the SMD 5484 fets?

jarrodthebobo · Post by **jarrodthebobo** » 18 Sep 2019, 03:56

Completely rebuild the board and still have a lack of gain; the volume appears to be a bit better, but the gain is still very weak.

Have tried all different combinations of jfets to no avail; I'm so confused as to what could be causing this.

I mean, I can't have made the same error twice, could I?

Does anyone have a sound clip without a boost pedal in front of it so I can hear what it sounds like?

Thanks!

sixthfloor · Post by **sixthfloor** » 18 Sep 2019, 08:17

jarrodthebobo wrote:
Does anyone have a sound clip without a boost pedal in front of it so I can hear what it sounds like?

Thanks!

I just recorded a quick (and crappy) one for you:

Strat bridge humbucker, full volume -> Vandal red channel, gain 2 o'clock, EQ to taste -> computer speakers -> smartphone.

EDIT: do you want to share your modified layout and/or schematic, to have a second pair of eyes check it ?

Manfred · Post by **Manfred** » 18 Sep 2019, 08:18

Where have you purchased your FETs?
Have a look at this post.
viewtopic.php?f=17&t=24756&start=40#p276841
That had been the fourth time that I bought Fets by Chinese Sellers,
whereby the imprinted type not match the type as expected.
Once one time even the P-channel JFET was a P-Channel JFET.
As a result of this experience with the Chinese sellers I will never buy semiconductors from.
Left the Id_Vds curve form the data sheet right one measured curve from Chinese JFET,
all others had almost similar curves.

jarrodthebobo · Post by **jarrodthebobo** » 18 Sep 2019, 22:02

sixthfloor wrote:
jarrodthebobo wrote:
Does anyone have a sound clip without a boost pedal in front of it so I can hear what it sounds like?

Thanks!
I just recorded a quick (and crappy) one for you:

Strat bridge humbucker, full volume -> Vandal red channel, gain 2 o'clock, EQ to taste -> computer speakers -> smartphone.

EDIT: do you want to share your modified layout and/or schematic, to have a second pair of eyes check it ?

Thats what mine sounds like for the most part with the sustain switch engaged... Maybe a small bit less gain. Maybe I'm just expecting more from this preamp than it can give, haha!

Here is the layout I used:http://guitar-fx-layouts.42897.x6.nabbl ... ndall2.png

Manfred · Post by **Manfred** » 19 Sep 2019, 02:59

Hi Jarrodthebobo,
you wrote:

Completely rebuild the board and still have a lack of gain; the volume appears to be a bit better, but the gain is still very weak.

... Maybe a small bit less gain

Is the amount of gain enough now?
That is why I asked you, about the supplier of the JFETs above.
Perhaps the JFETs not fulfil its specifications.

Could you please measure the set resistance of the trimmer potentiometer voltages at of the JFET terminals,
it could helps to determine the forward conductance of the JFETS.

sixthfloor · Post by **sixthfloor** » 19 Sep 2019, 07:40

jarrodthebobo wrote: Thats what mine sounds like for the most part with the sustain switch engaged... Maybe a small bit less gain. Maybe I'm just expecting more from this preamp than it can give, haha!

My sustain switch wasn't engaged though.

jarrodthebobo wrote: Here is the layout I used:http://guitar-fx-layouts.42897.x6.nabbl ... ndall2.png

I think there's a slight mistake in the layout: the G8-A8 link should be G8-B8. As it is, the trimpot doesn't regulate the bias on Q1, which might explain your issues.

jarrodthebobo · Post by **jarrodthebobo** » 19 Sep 2019, 15:11

My Q1 biases fine though; got it down to 12.5v using the trimmer.

I'll post resistances and full voltage readouts when I get home... It's odd how clean this thing sounds, almost as if signal is being lost somewhere...

This is the 2nd time building it and having the same issue... So strange!

duckduckgoose · Post by **duckduckgoose** » 03 Oct 2019, 19:37

did you try sixthfloor's suggestion?

edit:

oh actually It's not one wire. It looks like the wire is going from G8 to C8 then there is another wire from b8 to a8.

sixthfloor · Post by **sixthfloor** » 04 Oct 2019, 09:05

duckduckgoose wrote:did you try sixthfloor's suggestion?

edit:

oh actually It's not one wire. It looks like the wire is going from G8 to C8 then there is another wire from b8 to a8.

Yes, I didn't look at the bottom part of the layout. My bad !

duckduckgoose · Post by **duckduckgoose** » 09 Oct 2019, 02:00

no worries. i though the same thing lol.

have you seen this?
https://reverb.com/item/2554060-lpd-eig ... ll-rg100es

looks like they had the same idea.

that was the prototype and now it looks like they are selling a production model
but they don't mention anything about randall now.
https://reverb.com/item/3974100-lawrenc ... llic-black

Fretts · Post by **Fretts** » 30 Mar 2020, 20:54

Regarding the photo of the hand-drawn schematic. Is this exactly as shown in the perfboard layout? Many of the part values are just about incomprehensible to me, like what the diode numbers are and how a 20uF capacitor at Q2 could possibly be right, or what the capacitor value is between Q2 and the 1K resistor. The writing is kind of ambiguous at best, at least to my eye which is accustomed to clear annotations.
Is the perfboard layout the key to correct values?
Thanks,
Fretts

Fretts · Post by **Fretts** » 31 Mar 2020, 02:23

OK never mind - I was able to deduce the component values by referring to the stripboard layout. Wow - some of the handwriting quirks really stumped me for a brief time! Once I translated what was a uF to me into nF, and realized that the 6s and 9s were all actually 4s, it came into focus.
I also had to check the datasheet for the 2n5484 to understand that the Source, Gate and Drain on that transistor is in Drain, Source, Gate order which really twists the layout around when I was trying to correlate the two.
I think I've got it now.

PCBGuitarMan · Post by **PCBGuitarMan** » 02 Jun 2021, 15:49

I really love these types of circuits, which are only available in the DIY world!

This is a great pedal; it really captures Dimebag's tone. I call mine " Vandal from hell"

I finished mine using 2n5458 transistors, and it sounds pretty brutal. I'll like to try how it works with J201 next.
I used a 3pdt toggle on the first batch, but I'll go back with the 4PDT for the channel switch.

I really recommend this build to anyone who's looking for an excellent Metal pre-amp.
I have made some PCBs for anyone who'd like to try this brutal high gain pre-amp/distortion.

https://pcbguitarmania.com/product/vandal-from-hell/

fcoberrios · Post by **fcoberrios** » 16 May 2023, 17:09

Anyone here already built the pedal and is working correctly?