"Pop" in a preamp cathode switch

[chocopower]

Hi!!

How could i eliminate the noise in a center off, 3 position cathode switch, locate in a preamp valve, pin 8...?

Is a 25uf - blank - .68uf.

[Enzo]

Every time you switch caps in or out, they have to charge up to the cathode voltage at first contact of the switch. The trick is to keep them charged with a resistor, but make the resistor large enough that it doesn't allow any effect from the cap.

This is easy. Use a high value resistor - I use 1 meg since I have tons of them, but 100k wouod likely work fine too - and connect it to the switch end of each cap. That is two resistors, one each cap. Now the switch more or less shorts across the resistor to engage the cap. A convenient place for these resistors is across the switch terminals.

I am guessing the center terminal of the switch is grounded, yes? and the two ends are wired to caps. So connect two resistors to the center, and the free ends each go to one end terminal of the switch.


The resistor keeps the cap charged up to voltage, but reduces the caps effect to almost nothing. SHorting across teh cap doesn't affect its charge, but it does allow it to have full effect.

[chocopower]

Thanks!!


I,ll chek it in 10 min....

[bloom]

Enzo, can you add a little sketch? a text reply with character can do as well. I'm visual.. Thanks

[Enzo]

I'm not artistic, but it is simple. If you have a bypass cap you want to switch in and out, just wire a high value resistor in series with it. The use your switch to short across the resistor. Do that instead of switching the cap in and out.

When the switch is open, the resistor keeps the cap charged up, but prevents it from having any affect on your sound. Then when the switch is closed, the cap becomes part of the circuit.

Here, look how they switched the resistor and cap in the cathodes of several of these Mesa preamp stages

[soontobe1968]

pop fix picture..

I think I got this fix from Enzo quite a while back for a Dave Hunter 2 Stroke and it works great. Here's a picture if that helps.

1M resistor is connected on the switch between the two caps.