The pentode preamp tube limits the options. That leaves feedback to the 6V6. Some Fenders use feedback to the power tube grid, but they are push pull amps IIRC and the Volume control presents a problem. The last option is feedback to the 6V6 cathode. Disconnect the cathode resistor from ground and connect it to the speaker. Don't try it unless you have an oscilloscope. It may oscillate no matter which way you change the phase.