Reverb bypass with 3.3Meg, 10pF?

[chipaudette]

Hi,

I've got a Fender Deluxe Reverb Reissue (DRRI) and, to seperate the send and return on the reverb, there is a 3.3 Meg resistor with a 10pF capacitor in parallel.

I could easily be pursuaded that the 3.3 Meg resistor is to cut down the signal level to properly mix with the return from the reverb circuit. But, what is the purpose of the 10pF capacitor? It looks like a hi-pass filter. Wouldn't the cutoff frequency be REALLY high, though?

Check out the attached PDF. The schematic is on page 3 or so. I'm looking at capacitor C13 in the middle of the page where the signal branches off to the reverb. Why is that capacitor there? What does it do? What would happen if I cut it out?



Chip

[CarlZ]

It really doesn't do much of anything except pass a little more highs through to the power amp. What will happen if you remove it? the amp will still work fine but it'll sound duller and wont have that glassiness and spank those amps are noted for. But then some people think fender amps are a bit too bright. This is one of the places some amp guys do their tweaking....fiddle with some values and see what you like. Really not much else to say here.

-Carl

[chipaudette]

Thanks for the view.

I'm having a hard time figuring out what the cutoff frequency of this filter is...there are several paths to ground and I'm not sure which contribute to the calculation of the filter and which don't.

Any guidance on how to calculate this value (or maybe you know the cutoff value already!)?

Thanks,

Chip

[Steve A.]

Thanks for the view.

I'm having a hard time figuring out what the cutoff frequency of this filter is...there are several paths to ground and I'm not sure which contribute to the calculation of the filter and which don't.

Any guidance on how to calculate this value (or maybe you know the cutoff value already!)?

Thanks,

Chip

In my very unscientific opinion, the values in the traditional Fender "reverb splitter" circuit are so small that the customary RC frequencies do not really apply. FWIW Mesa Boogie would often replace the 3M3 resistor with 2M2, which would allow more of the signal to get through. And
just changing the 10pF ceramic cap to silver mica can make a big difference in the sound of the amp.

I believe that they use a 3M3 resistor there to isolate the reverb feed from the reverb return so that you don't get feedback from the reverb circuit.

If you want to try tweaking this circuit, you may have trouble finding anything lower than 10pF except maybe a 5pF cap. However, you could try adding a resistor in series with the 10pF to temper the brightness a little bit.

Steve Ahola