Black Hole : Sunn Model T preamp

This is my last pedal I made : a Sunn Model T preamp in a box!

Coda Effects Black Hole - Sunn Model T preamp

I named it "Black Hole" because of the antimater doom riffs it can generate :)



Coda Effects Black Hole - Sunn Model T Preamp

If you are interested by the pedal, you can subscribe to the waiting list:

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Here is the original prototype. For those who do not know the Sunn Model T, a bit of history...

Sunn Model T preamp

I was introduced to the Sunn o))) band by a friend a few years ago. At first, I was quite amused by their druid look and fan of the Les Paul of the lead guitarist. However, when I listened to them, I was not really found of it.... Until I saw them live!

It was in a festival in June at the begining of the night. We waited for nearly one hour before the show started, first by envelopping the crowd in a thick mist of smoke.

There was a full row of amps on the stage, and we could distinguish the red warm lights of overheating tubes and standby indicators in the dark.

Then a sound.

Sunn o))) concert

And what a sound! I think I have never heard a band play that loud. It make the whole crowd's chest vibrate in rythm with the heavy Nâzgul music played in this mystic mist in the night of June... What an experience! I think the only way to understand this is to see them live, it is a crazy experience that almost looks like a ritual...

Since then, I have becomed quite obsessive with the amp they used to achieved that heavy sound: the Sunn Model T.



History of the Sunn Model T

Originally, the first Sunn amplifier was created in the late 60s by a bass player who thought his amp was not powerful enough for large stages.

The first model T was marketed as an amp that could suit both guitar and bass, a 150 Watt beast with four 6550 tubes, inspired by the Fender Bassman 5F6A amp.

Sunn Model T head


There are two channels, a bright and a normal channel. It is possible to use both for maximum gain and power... Guess which is stoner's favorite setting! :)

It has lots of bass and is super loud! No wonder why stoner rock bands like it so much. It is also very rare and quite expensive nowadays, easily reaching a 3,000$ tag price.

A reissue was released in 2008 by Fender, and is also quite rare. Sunn did also issued solid state amplifiers that are quite hyped today as well like the Sunn beta bass.



My version

So I tried to reproduce this killer sound in a box. To do that, I basically reproduced the preamp circuit of the Sunn Model T and replaced the tubes by JFET transistors.

It is very different from the Earthquacker Devices Accapulco God that is completely different and simply use a LM1458. The circuit has nothing to do with a Sunn model T, even if it sounds very good.

I made the layout myself, and added a few modifications to have more gain. There is a LPB1 boost before the preamp, that you can switch on or off with a DPDT switch, and tweaked a bit the circuit to make it a bit more gainy,

Sunn Model T preamp

The preamp is wired with both channels added for more gain. Turning a dial up makes this channel louder, but it is also possible to cut a channel by putting the pot to zero. So you can choose easily between

Good news is: I made the PCB available if you want to build one for yourself:

 Make one yourself

Through-hole JFET are very hard to come by these days, so I made the layout compatible with SMD JFET. Most of the through hole JFET that are available online are not legit, so I used Fairchield SMD J201 instead. It is however totally possible to use through hole transistors as well.

Sunn Model T preamp JFET inside circuit

Even if I made it gainier, it is still very usable as a clean preamp, or as a light drive. It is possible to use it as an overdrive, the JFET are sounding very well for this, there is a nice warm crunch which is quite different from diodes clipping.

The three band EQ is very nice to switch from a heavy bassy drive to a light crunchy drive with more mids and trebles.

When the LPB1 is activated, you go directly in "beast mode", super heavy, almost fuzzy drive that sounds actually quite like Sleep Dopesmoker album



How does it sound?

Here is a short video I recorded to give you an idea of the sound of this little beast!




Circuit Analysis

This circuit is basically the original preamp circuit plus the three band EQ and a few tweaks to have more gain, pretty much like the Supreaux Deux I made before.

Here is the original schematic:
Sunn Model T original schematic

Basically, I took the top-left part of it and replaced tubes with JFET, with minor tweaks to have more gain, and added a volume stage at the end to have plenty of volume on top.

Here is my schematic:
Sunn Model T preamp JFET schematic

Let's delimit it in several parts:
Sunn Model T preamp JFET schematic

First, the signal goes through the LPB1, which is a clean boost in order to have more gain. There is a switch first to choose whether you want it or not, depending on how much gain you need. For a more detailed analysis of the LPB1 circuit, please check this post.


1. Bright channel

The bright channel reproduce exactly the amp chanel 1, replacing the tube by a J201 JFET.
Sunn model T bright channel schematic
The signal first gets through a 68k resistor, and a 1M resistor is used to increase the input impedance of the circuit. It also avoid "pop" noises when you activate the effect.

The signal is then amplified with a first JFET transistor, that simply replace the tube in the orginal schematic. The trimpot is used to bias it correctly, the 220uF capacitor increases a bit the gain of it.

There is then a coupling capacitor that prevents any parasitic DC current to get through the circuit, and a Bright pot allows you to set the gain of this stage. The lower its value is, the bigger the signal will be, resulting in more gain later on.

The 470pF removes a bit of trebles as well, increasing the value will result in more trebles in the bright channel.


2. Normal Channel

This is basically the same layout with some minor changes:
Sunn model T preamp normal channel schematic
The only change is the presence of a low pass filter with R7 et C4. It removes some trebles to have a bassier sound. Well, it is the "normal" channel after all!

I also removed the resistor that bias the JFET, in order to have more gain on this channel. This way, I can achieve a very bassy, saturated sound that is very close to stoner rock standard "Dopesmoker" by Sleep



3.Gain and volume stage

This part of the circuit is where the magic operates!
Sunn model T preamp gain and volume stage
The JFET Q3 is smashed with the amplified signals of both channels, and start to saturate. It is this transistor which create the gain of the pedal.

Then, another transistor re-amplify the signal before it goes through the tonestack.


4. Three band EQ

It is a classic passive tonestack that you will find in many other pedal. This schematic was made popular by Marshall amps:
Three band EQ schematic
It is basically three low pass RC filters that will cut bass, mids and trebles respectively


There it is! I hope that you like this blog post! You can follow Coda Effects on the Coda Effects facebook page or on Instagram for more news and infos.

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18 Comment
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I have a modified Sound City to have the Sunn Model T circuit! Very nice for bass guitar as well, sounds like a darker Fender Bassman. Awesome pedal you got there!

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Sound City amps are my dream amp #2 (#1 being an old Orange/Matamp...), I love them! Thanks!

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Hello! Have to tried adding a charge pump to this pre-amp? if so, what voltage pump did you find worked best? 17V or 25V maybe?

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Hello! I have never tried it... yet! But you are right, it should be interesting to test it. I can test 18V, but maybe not the 25V because of the 220uF capacitor (small voltage rating in order to have a small footprint)

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OK! If you end up testing the 18V output, let me know how it works out compared to the 9V! Thanks!

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Do you happen to have a drill template for your enclosure layout? Thanks!

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Yep! Have a look on the build doc ! (link in the sales page here : http://www.coda-effects.com/p/black-hole-pcb.html

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Hi!

I'd better add an output buffer to have a steady impedance source.

Nice job btw.

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Exactly. For now, the output impedance is not good enough to use it as a preamp per se. I am currently testing it at my place, and might add it to the v2.0 if conclusive!

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Got mine in the mail a last week, was wondering if there's any concern about soldering near those built-in transistors? Do you have any suggestions for not frying them? This will either be the first or one of the first builds I do, thanks!

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No problem at all: all the soldering that you have to do is on the other side of the board. They are also more heat resistant that they seem!

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Thanks, I bought some Panasonic fc capacitors but the 50v 220uf is way to big for the pcb, I see some 25v on ebay that should fit, is it ok to mix and match voltages? The rest of the electrolytic caps i have are 50v...

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Eya! Thnx 4 the lush pedals. Whats the ldr on the board/kit list all about? Light dependant resistor...???

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The "LDR" part name is actually quite confusing, I was meaning "LeD Resistor", and realize later on it can be confusing! It is just the current limiting resistor for the LED!

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I made your PCB version and it is great! Thanks so much.

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What output buffer would you guys recommend?

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