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09-03-2020, 06:32 PM
Hi Tomislaw
Keep in mind that in Fender's expected use of this amp there would NEVER be a 66V signal of any proportion at the input to the reverb driver.
The PA has a closed-loop gain of 55, so a 20V output at the speaker requires just 365mV at the PA input. This is where the 3M3||10pF meets the 470k grid-leak and the 470k from the reverb recovery stage. The attenuation through the dry path is about a factor of seven, so only 2v5 is needed from the preamp for the 20V at the speaker.
Is there a signal threshold where the reverb oscillation starts?
A later version of the Princeton (019320) has a 220k in series to the reverb driver and a 560pF from cathode to plate. They added a tube so the reverb and trem could match the concurrent models and most circuitry was standardised even though it meant the Princeton would be more expensive to build.
The ground groupings look questionable:
V1A,B are cascaded stages and even Fender did not tie their grounds together. They did however tie the reverb driver cathode to the V1B ground, which is not problematic for their intended use of the amp and "reasonable" signal levels.
V3A reverb recovery okay
V3B trem should ground to Vs cap along with bias since the amp uses bias-modulation.
V4 is the PA gain and concertina. NFB and reverb pot can be tied here. The concertina really should have its own filter since it provides a supply-referenced signal plus a ground-referenced one.
Have fun
Keep in mind that in Fender's expected use of this amp there would NEVER be a 66V signal of any proportion at the input to the reverb driver.
The PA has a closed-loop gain of 55, so a 20V output at the speaker requires just 365mV at the PA input. This is where the 3M3||10pF meets the 470k grid-leak and the 470k from the reverb recovery stage. The attenuation through the dry path is about a factor of seven, so only 2v5 is needed from the preamp for the 20V at the speaker.
Is there a signal threshold where the reverb oscillation starts?
A later version of the Princeton (019320) has a 220k in series to the reverb driver and a 560pF from cathode to plate. They added a tube so the reverb and trem could match the concurrent models and most circuitry was standardised even though it meant the Princeton would be more expensive to build.
The ground groupings look questionable:
V1A,B are cascaded stages and even Fender did not tie their grounds together. They did however tie the reverb driver cathode to the V1B ground, which is not problematic for their intended use of the amp and "reasonable" signal levels.
V3A reverb recovery okay
V3B trem should ground to Vs cap along with bias since the amp uses bias-modulation.
V4 is the PA gain and concertina. NFB and reverb pot can be tied here. The concertina really should have its own filter since it provides a supply-referenced signal plus a ground-referenced one.
Have fun
