I first saw the Valvecaster pedal on the now defunct Beavis Audio site you can still find it in the Internet Archive. It was originally designed by a fellow named Matsumin. When I first put this thing together, it made awful noises, so I decontructed it and today I started all over. I also did some modifications in the wiring of the Gain pot and the Coupling capacitors. Now beware its looks gross but sounds just amazing! Lets see how I built this Sh*t!
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Step 1: The Power Supply: Parts 'n Stuff
My first build suffered from noise, I belive it was because I used a crap wall wart power supply. I found this cute little transformer which I measured. It put out 13,2V AC without load connected. After I wired the tube heater the voltage dropped to 12,8V which is just fine for the filaments. Then I connected a bridge rectifier I took out from a bad LCD monitor, the rectified voltage is around 17V DC what makes the anode more happy than the 9 Volts the original design uses. I used a 2200 uF filter cap. The noise levels are now lower, but I possibly add more caps later. Remember: the ground pin on the connector is connected to the chassy only! Safety first! Also notice that the AC filament wires are twisted together to cancel out hum.
Update: I created an artfical center tap for the heater supply to have it grounded. Less noise.
Here's the list of parts I used in the supply:
- The chassy of an old PC power supply
- IEC connector
- A 230V-->12V transformer
- A diode bridge rated at 2A (more than enough)
- 2200 uF electrolytic cap
- 2x220 ohm resistor
Step 2: Wiring + Mods
Now the wiring is a mess built on the tube socket alone. I can't really show you how it's done, but I show you the orginal schematic then my modified one. I also explain the mods. It is critical that you must ground the whole circuit to the chassy at one point (mine is at the input jack) otherwise you get nasty ground loops.
The first mod I did was to relocate the Gain pot. In the original schematic the Gain pot was put between the first triode's cathode and ground. By twisting the pot you change the bias of the tube, which is not a very good idea in my oppinion. Instead I wired a classic Gain/input level pot right after the input jack.
The second mod is about the tone of the circiut. The original tone is very muddy with almost no highs. So I looked up a few schematics: a Marshall Randy Rhoads Plexi and a Tube Screamer TS9. (They all sound good, don't they?) What I found that both of them use 22nF coupling caps. I put in two 22 nf caps in the input and the output, I got a 33 nF in the middle position because thats what I found in my collection. I also took out the original tone control as some folks on the internet said that it sucks the highs off. This thing don't really need a bypass switch as the sound beg for full time use :) Also without Volume control it farts most guitar amps, it likes power amps better.
That's it for now, the thing absolutely rocks! I may add some improvements later like an Output Level control and a cathode bypass cap. I will also upload sound samples later. Bye!
- Noval tube socket
- 12AU7 = ECC82 tube, mine is a Tungsram ECC82
- 50K log. potmeter for Gain
- 100K linear potmeter for Volume
- two 22 nF capacitor
- one 33 nF capacitor (ceramic will do or some pro audio caps if you have a lot of money)
- Resistors: 1M, 2.7K, 220K, 470K, 100K
Step 3: Sound Samples
Okay at the first I cheated a little bit: it's finger picked.
What you hear is the California channel of a Behringer GM108 practice amp. It is set on Clean wich is crunch.
- The first sample is the clean sound of the amp without the pedal
- The others are with the pedal
All clips are recorded with a cheap MSN mic and Audacity.