The Hood Preamp

21 February 2009

The Hood is one of my first tube preamp designs. The Hood evolved from my medalings with low voltage 12AU7 tube stompbox designs. The Hood is a low voltage preamp that uses two 12AU7 tubes, four cascaded common cathode Class A gain stages. The Hood is essentially a big glorified stompbox/preamp which offers a lot of unique tones.

The Hood features an all tube signal path. In the signal path it has controls for Volume, Gain, Body, Tone and Presence. The Body controls essentially affects how scooped or flat the mid range is. The Tone, Body and Presence control evolved from experimenting with the Big Muff Tone Control and the results at AMZ.

The Hood also has a toggle switch labeled Clean/Dirty. This switches selects different cathode resistors on select gain stages and biases them differently. The bias of each stage essentially determines how the signal passes/clips as it goes through the stage. The Clean/Dirty switch either gives you a nice clear tone or a semi dirty gritty tone.

The Gain control is also pulls out for a gain boost. This switch adds a bypass cap to one of the un-bypassed cathode resistors and increases the AC frequency gain of the stage. When engaged this switch gives you a crunchy saturated tone.

The Hood also has a relay based bypass system (true bypass). What this means is that when using a small external footswitch plugged in at the back you can bypass the preamp. This allows the user to have the option configure the Hood as a stompbox. As well as switching between bypass and the preamp the relay also turns on a bright red LED mounted on the front panel. The LED only turns on when the signal goes through the preamp, thus making it clear when the preamp is active. The bypass is done remotely by a small footswitch that connects to the back of The Hood with a standard 1/4″ guitar cable.

Build:

Schematic:

the-hood-schematic1

the-hood-schematic2

 

 

 

 

 

Results:

I am very pleased with the tone of The Hood. The controls are all very interactive, especially the volume and gain controls. The Hood is capable of having headroom and a nice clean tone while on the other hand it gets nice and thick and dirty with the right settings. The Clean/Dirty switch adds a nice bit of grit and bite when you need it. On top of the Clean/Dirty switch the push-pull gain pot adds a nice tubey crunch sound. The Body and Tone controls provide endless combinations of sounds. The Presence is subtle yet very useful control that allows you to dial in just the right amount of high end detail, perfect when you need to tame that distorted tone to something a bit softer.

 

{ 3 comments… read them below or add one }

Younes Meknassi Lagraoui July 27, 2015 at 16 h 10 min

Hey man,

First, Kudos for the designs and very clean finishes, you’re doing an impressive work. I’m a guitar player, mainly playing blues and classic rock and I started to be very interested in DIY. Have 2 very horrible solid state amps (Fender Frontman 15G and Marshall MG30DFX) which both sound very ugly. I’m very interested in building the Hood Preamp as it looks the simpliest, yet powerful, for me to start DIY on Tube guitar gear.

For the first build, and in order to push simplification to the maximum, I intent not to build the power circuit. I’ll just use an AC to DC 12V 2.5A transformer I have. I will also ignore the switch circuit. Finally, I’ll also wire the output to a HI-FI amplifier or use line-in in one of my cheap amps. Do you think this will still provide a good tone? Also do you think this is a great example to learn how guitar preamps work?

Finally, can you share some advices about the parts that needs to be chosen to avoid noise and have better sound quality? I assume parts I find in the electronics shop nearby will not necessarily be the ones adapted for this project.

Thank you
Younes M.

Reply

cmoir August 7, 2015 at 0 h 28 min

You could use the tube preamp into the power amp of your solid state amps. It’s the preamp of those solid state amps that is sucking your tone. A SS poweramp is not always such a bad thing. So for simplicities sake I would run The Hood into your amps power amp in (you might need to modify your amp slightly to do this).

Audio circuits for guitar are lo-fi by nature and don’t require fancy expensive parts. Decent resistors and poly caps are good starting points. Spend extra money on good quality hardware (jacks, switches, pots, etc.)

Reply

Younes Meknassi Lagraoui August 7, 2015 at 1 h 34 min

Thanks for your help !

Reply

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