Princeton Reverb tone stack capacitors outside foil orientation

[Lemon]

Hello everyone,

I noticed that the orientation of the 0.047uf tone-stack cap in Rob's suggested mods layout is different than in his original layout. Which do you think is correct?


Here's the area on the board-
Original:

Suggested mods:

 

[sds1]

The one where both caps are oriented the same way is the correct one.

You want the outer film as close to ground as possible, which is almost always via the power supply. In this case it's thru the slope and plate resistors and then finally thru the D node's filter cap to ground.

 

[Lemon]

Looking at the schematic, shouldn't the "easiest" way to ground be through the 6.8K mid resistor?

 

[robrob]

The .047uF Mid cap has 6.8k Mid resistor to ground and 200k through the Tone Slope and plate load resistors to the power supply AC ground so we put the outer foil on the Mid resistor end.

The .1uF Bass cap has 250k Mid pot (set at max) + 6.8k Mid resistor to ground but only 200k to the power supply AC ground. Higher freqs can bypass the Bass pot through the .047uF Mid cap + 6.8k Mid resistor but since hum is a low 50 or 60Hz we can ignore that. Since we rarely run the Bass pot at max I decided to attach the Bass cap outer foil to the Bass pot.

In reality, since the two impedances for the Bass cap are so close it's really a toss up and makes no difference in noise suppression.

 

[sds1]

Looking at the schematic, shouldn't the "easiest" way to ground be through the 6.8K mid resistor?

What Rob said, but to put another way don't forget AC paths (and impedance) are frequency-dependent. So the answer to your question depends on what frequency we are talking about.

 

[Lemon]

Thanks for the great info, sds1 and Rob. I'll be sure to report results in my build thread.