I give you the "Whisper of Screams" - a trace of the Kasha OD Guitar pedal. One of the best, oddest, yet simplest, circuits in guitar pedals I've ever encountered.
This post is my contribution to the pedal building community in resurrecting this rare find.
This thing rips and has only two active components - a pair of JFETs. No diodes were used or harmed in the creation of this circuit. Yet, it can can deliver a clean 3db boost, all the way up to pleasant, punchy overdrive with about an 18db total gain - depending on the settings.
The provided schematic does not show any stompswitch wiring as that can be decided at build time. Same for any power protection and\or filtering.
Wait! There's no "tone" control! It doesn't need a tone control. It is quite transparent with regard to reproduction of a guitar's tone.
Unity-gain varies with component tolerances and the position of the 'Level' control.
The settings are described thus:
• Calm: This setting is bright, and sounds like a low-wattage tube amp but you don’t lose any of your lows. A nice 3db clean boost with the 'Whisper \ Scream' switch in the 'Whisper' position.
• Sullen: This setting is a simple boost stage, which ups your gain about 6dB. If you toggle the 'Scream' switch, then you get a full 13dB to start really working your amp.
• Moody: This setting hits your amp with 11dB of gain, and gives you added bass. The tone from this channel is absolutely HUGE! Then you flip the Scream switch, and WOW!
• Angry: With this setting you get a heavy metal type sound. The voicing gets a big, but tight bottom-end plus a few more dB’s of gain.
The original Kasha OD Guitar pedal was in a simple, tight 1590B sized enclosure with just a 'Volume', 4-way rotary and a SPST toggle switch. In my initial build, I replicated the 1590B enclosure approach. Then I had second thoughts and thought it would be a good idea to put the 'Whisper - Scream' toggle under a footswitch instead. (NOTE: the 4-way switch must be wired backwards to match the labels of the enclosure artwork.)
It sounds great thru a solid-state amp, but it really shines when used in front of a tube amp.
After the initial trace, I grew more curious about JFETs & general FET-based audio circuits and started digging into other FET circuits. Of course the phrase "Fetzer-valve" showed up in my searches. After examining "Fetzer-valve" tech, it occurred to me that applying the phrase 'Fetzer-valve' to any FET circuit is much like saying a brushless motor is "digital". So that phrase shall not be used in any of these descriptions.
I was pleasantly surprised & amused when I discovered the the Drain and Source pins can be swapped without issue. The original circuit used BF862 (2AW) FETs. Very hard to find, I started a deep-dive into auditing several different FETs for this build. Without changing a single component value, I first learned that NXP, who manufactured the BF862, also still produces the BF861C FET. These work well in this circuit with no need to change any values.
Continued auditing revealed that (genuine) J201s work perfectly in this circuit - and IMO, better than the BF861C or any other FET. I also found 2N5457s and 2SK170 (not to be confused with the BS170 - they are NOT the same) work very well in the unaltered circuit.
Other FETs (J112, J113, 2N3819 and some others) will work, but the Source resistances must be changed to higher values in order to work as nicely as J201s. But I found they don't achieve as much boost as J201s. The proper Source resistances have to be found to avoid gating and more volume. To be honest, there is a lot of technical jargon that revolves around the differences that I am not able to just recite off the top of my head. But the auditing process is pretty much fool-proof.
So, there you have it. I hope that those of you who elect to try this circuit find it as exciting as I do.
