Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mebond11 on December 03, 2019, 07:37:00 pm

Title: Lacing turret board and RF interference
Post by: mebond11 on December 03, 2019, 07:37:00 pm

Hello all, I recently completed a Marshall 18 watt amp with isolated inputs.   The amp works great, but I could faintly hear radio stations on the TMB input...obviously a strong signal in my area.   I was able to eliminate the problem by connecting a .01 uf capacitor from the input ground to a solder lug grounding washer attached to the input.   Worked like a charm.

I laced the turret board in a figure 8 pattern, for all turrets, including the turrets that eventually connected to the preamp ground.   The ground for the TMB input was connected to one of these turrets, about 4 inches from the preamp ground.   The guitar cable was acting as an antenna and the distance from the isolated jack to ground was allowing RF interference to propagate.  The cap connected directly to ground at the jack killed the high frequency coming through the cable.

 My question is this...Does the lacing pattern enhance RF interference?  It looks great, and is very secure, but would a straight bus wire be more functional?  The chassis is shielded. 

Thanks to all for your input. 

Title: Re: Lacing turret board and RF interference
Post by: PRR on December 03, 2019, 07:49:44 pm

Criss-cross or straight, no real difference.

A radio bypass cap AT the input is very common defense against strong local radio.

Title: Re: Lacing turret board and RF interference
Post by: mebond11 on December 03, 2019, 08:02:08 pm

Thanks, starting a new project and will lace this as well.