Guitars of Love

Oscilloscope screen grabs for the GuitFET


Oscilloscope screen grabs for the Diason class 'A' amp



The new 2016 GuitFET:

The new one sounds great and very close to the original, but has a lot more tone control variation, and a bit more drive. It also has electronic switching between two FET overdrive circuits, a mild single FET overdrive and a double FET high OD. This is done with a bistable circuit operated with a momentary foot switch. It also sports a treble boost option which is done on the input FET, giving a much warmer sound than the original GuiFET treble boost, which was done on a regular BJT. Another feature is a bass cut / OD boost option which is done by cutting off the negative feedback to the bass control circuit. This has the effect of turning the bass control into a middy boost to create a very drivy type of overdrive. Warm and funky. As with the original unfortunately I was left with no room for a true bypass switch so this has to be done separately.

What does it sound like? I just recorded the OIL's instrumental 'Wedding Cake Island' with it:

Download a pdf of the schematic here:

After recently gaining further insight into how to maximise the performance of the FETs, I have changed the value of the source resistors from 680 ohms to 1K2 (1,200 ohms). This produces a more even waveform top and bottom and sounds better with less odd harmonics. It aso produces a more stable DC voltage in the FETs which is desirable to prevent the blocking sound when driven into overdrive.

Note that the connections for the LEDs are not included in the schematic. These are facilitated by the toggle switch's spare terminals and knock down resistors to each LED from the DC supply.

NOTE: Initially I had a hum problem with the PS but this has been fixed. It's preferable to use low ESR caps on the raw DC input from the diode bridge. 2 x 3300uF/25V low ESR electrolytic caps to give a better quality 9V DC. Also reccommended is a larger than spec power transformer as this will reduce PS noise as well.

A pdf of the Vero board design for the new one is here:. The power supply and bistable switch are on a separate board; the PS board being half the size of the main board after trimming.

Testing the boards for full functioning after completion. Everything worked 1st time!


Assembling the circuit boards in the case after initial testing. Now it's a matter of placing the pots in the appropriate sequence 'I/P level / BASS / TREBLE / OD LEVEL / OD MATCHING AND VOLUME OUT'. There will also be a treble boost switch; the momentary foot switch for OD selection and the mid-boost which bypasses the feedback in the bass tone circuit giving extra drive.


All assembly complete. The picture depict the PS section before the addition of extra DC filter caps at the front end of the PS.


All assembly complete. The final device with the extra PS filter caps gaffered to the top. I also recommend using an oversized power transformer to facilitate a smoother DC supply - around 300mA


The finished unit ...