Chuck D. Bones
Circuit Wizard
Another FET-based tube amp emulator pedal! I like this one. Here's a preview schematic, courtesy of PedalPCB. The first two stages (Q1-Q3) emulate a Marshall preamp. The tone contols are a Fender stack with the MID control hardwired at zero. After playing with it a bit, I bumped C7 up to 1nF, C11 up to 15nF and changed the TREBLE pot to B250K. I had to disconnect C4 on my breadboard because at higher gain settings it oscillated between 10KHz and 20KHz. The schematic calls for 2N5457, but you have to select ones that put Q1 and Q2's drain voltages between 5.0V and 5.5V. I used 2SK193s and lowered R3 & R7 to 22K to dial-in the drain voltage. I tried PF5102 and PN4393, but they have too much gain and too much capacitance which choked off the tone. The switch between Q4 and Q5 tightens up the bass a little when it's ON. Its effect is noticeable, but subtle. The PENTODES control is a second gain control that affects the distortion in the last stage (Q5 & Q6). The last stage is intended to simulate the phase inverter in a push-pull amp. At moderate settings of PREAMP and PENTODES, you get smooth, symmetric clipping. Even with PREAMP and PENTODES at zero, you have to back off the guitar volume a bit to play clean. It's pretty easy to dial-in the desired tone. Good range of tones and the pedal responds well to touch and guitar controls. Subdecay has a good description of the operation and control settings on their website. I tried two different transistor sets for Q5 and Q6. First, I installed a silicon dual transistor. The two transistors in one 6-lead package are matched and track each other with temperature. Their HFEs were over 300. The second transistor set I tried was a pair of MP38As from the batch I just bought. Their HFEs were between 50 and 60. I tried a few 'til I found a pair that were pretty well balanced. I checked the balance by measuring the voltage from Q5-C to Q6-C. With the dual transistor, the difference in collector voltages was about 220mV. With the MP38A's, the difference was closer to 800mV and wandered around. In this circuit, HFE does not matter much. The two transistor sets did not sound all that different. I would recommend that anyone building this should match the transistors for Vbe. It's too bad Subdecay didn't put the spare opamp to use as a DC servo to keep Q5 and Q6 in balance. I'll give that a try to see if it does any good.
I will definitely be building one of these on a pedalPCB board.
I will definitely be building one of these on a pedalPCB board.
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