Passive pedal problem

[Mark Keogh]

I’ve noticed that when I use a Fender tube amp there is a slight scratchiness often only for the first half hour of playing associated with the Goodrich L120 passive VP. It does not occur when using a Session 400 or NV112, only the Princeton and Vibrosonic. I’ve got a new pot in the pedal so that’s not the issue. I’’ve got a Sarno BB mini before the VP but turn it off when I run the tube amps. S’got me knackered. Any ideas?

[Mike Auman]

Your Peavey solid state amps have a blocking capacitor at the input jacks, your Fender tube amps don't. If the first preamp tube in the Fender is leaking a little voltage to it's grid, that grid has a direct DC connection to the input jack tip, and therefore to the output jack of your volume pedal. That can put enough DC potential across the pot in your volume pedal to make it sound scratchy. If the scratch gets better over time, the preamp tube might be leaking less after it works for a while.

Possible fixes: change the first preamp tube, or put a buffer pedal between the volume pedal and the tube amp to block DC. You could also move your BB after your volume pedal, but that would defeat the purpose of the BB.

[Jim Palenscar]

Now that's an answer!!

[Ron Wood]

Is it possible to add a blocking capacitor at the input to your Fender amp?

[Mark Keogh]

Thanks Mike and yes as Jim says, terrific response I’m always enamoured and grateful for the advice and generosity of people on this forum. I’ll start by replacing the pre amp tube.

[Mark Keogh]

Your Peavey solid state amps have a blocking capacitor at the input jacks, your Fender tube amps don't. If the first preamp tube in the Fender is leaking a little voltage to it's grid, that grid has a direct DC connection to the input jack tip, and therefore to the output jack of your volume pedal. That can put enough DC potential across the pot in your volume pedal to make it sound scratchy. If the scratch gets better over time, the preamp tube might be leaking less after it works for a while.

Possible fixes: change the first preamp tube, or put a buffer pedal between the volume pedal and the tube amp to block DC. You could also move your BB after your volume pedal, but that would defeat the purpose of the BB.

Thanks so Mike