I decided to quickly put together a circuit today but found an issue that has sometimes happened when making circuits with TL072.
Briefly, the circuit is a non-inverting amp with about 34dB gain in the audio region, including some switchable clipping diodes in the feedback loop. Since supply is 9V, the input is biased with Vref.
Vref is set to be roughly 6V. This is because of the common mode input range of the TL072 being 4V to 9V when the supply rails are 0 and 9V. However, when I power it on and input a signal (active guitar, <500mV peak), the output comes much weaker and heavily distorted, the gain knob starts working in reverse and the bias voltage drops to roughly 1.6V. If I increase the gain to max, it is very quiet until the input is very high where it suddenly outputs a burst. There was a brief moment when I had disconnected everything and reconnected with low gain that it worked fine and had the correct bias, but increasing the gain suddenly pulled down the bias and the problem returned.
I am completely at a loss as to what could be causing this, what am I missing? Thanks in advance!
Edit: To add, also when the circuit was reset and had the correct bias, when i then measuring the voltage with multimeter at the non-inverting input, it suddenly dropped down to 1.6V and exhibited all the above mentioned issues.