Transformer Buzz in Bright Switch Circuit

[aftergear]

Hello Trendsetters, i have added a seperate bright switch circuit to my Hot Rod Deluxe that works on the Drive channel only, it links to the pole on SW1 that connects directly to C4 with the other wire connected to pin 9 on relay 1, a toggle switch is mounted through the chassis near the fuse.
I have used shielded wire and have grounded the earth to the chassis at one end only.
I have tried some different switchs but there is a continual loud buzz when the drive channel is selected that has become apparent since i added this circuit.
Any thought suggestions to get rid of the buzz would be appreciated.
Cheers
Darryn

[Steve Conner]

Two ideas:

Don't ground the shield of your wire to the chassis, ground it to whatever ground reference the audio circuit uses.

Put a Faraday shield between the toggle switch and the fuse, so the audio wiring can't "see" the AC wiring. I use copper foil tape similar to what is used for shielding guitar control cavities. Be careful not to break any electrical safety codes.

[soundguruman]

The buzz will subside when you use silver mica capacitors instead of regular ceramic disks.
The ceramic disk caps act like antennas, and induct the 60 cycle, then introduce it into the audio path.
Also get rid of the ceramic disk cap on the treble control, in the tone stack, and replace it with silver mica.
OR any other ceramic disk in the audio path, especially in the first gain stages of the preamp.
The silver mica should be 500 volt or 1000 volt. Recommend you use USA made instead of cheap Chinese made.
Cornell Dublier is a good USA silver mica Cap.
Keep the grid wires on the preamp tubes (pin 2 and 7) away from the filament wires (pins 4,5 and 9).
Route the filament wires to the rear of the amp and the grid wires to the front of the amp...

Also something to WATCH is that you have ALL the covers, and shielding in place when you test it for noise.
The rear cover to the chassis is shielded, and when the cover is removed, the circuits are sensitive to AC fields,
and you will get quite a lot more BUZZ when the cover is OFF.

Some people are quite surprised to learn that capacitors are inductive, at lower frequencies. The ceramic disk cap
does a great job of absorbing 60 cycle fields, and injecting the noise right into the signal path.

[aftergear]

Thanks Steve and Soundguruman,

Soundguru, i have exchanged the treble cap and others before this mod to good effects.

Steve, attaching to the Audio circuit ground makes sense, my limited knowledge of electronics on this PCB makes it hard to identify where i should ground it to. Maybe something like the Reverb shield ground???? im not sure! Should i somehow ground the switch as well or is there a better switch type to use?
Also the switch is close to the transformer, maybe this is a factor?

Cheers
Darryn