Hi all.
I have just completed the amplifier section of the 5f1 fender champ, and what a good sounding piece of history this is. However there is one problem, a hum/buzz that increases as you turn the amp up. to me it sounds like a grounding issue. the amp is grounded in several places all of them to the chassis. The connection points are as follows, preamp to input jack, power caps to PT bolt 6.3vCT to PT bolt HT CT to PT bolt and of course mains earth to PT bolt.
I was reading somewhere that it is possible to put a resistor between the CT and ground. I think this was used in the marshall jcm900 circuit however they used two diodes a resistor and a cap all in parallel to ground.
does anyone have a suggestion for me as to whether this will work and what resistor value should I use? any other considerations?
or am I just blowing bubbles?
cheers
I have just completed the amplifier section of the 5f1 fender champ, and what a good sounding piece of history this is. However there is one problem, a hum/buzz that increases as you turn the amp up. to me it sounds like a grounding issue. the amp is grounded in several places all of them to the chassis. The connection points are as follows, preamp to input jack, power caps to PT bolt 6.3vCT to PT bolt HT CT to PT bolt and of course mains earth to PT bolt.
I was reading somewhere that it is possible to put a resistor between the CT and ground. I think this was used in the marshall jcm900 circuit however they used two diodes a resistor and a cap all in parallel to ground.
does anyone have a suggestion for me as to whether this will work and what resistor value should I use? any other considerations?
or am I just blowing bubbles?
cheers
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The first thing I would do is to float the input jacks off the chassis and to bring their signal grounds to the bottom of R4 and C7. At the input jacks, I'd also connect the jacks' grounds to chassis with a small capacitor, maybe .001 to .01, something to shunt off stray RF.
To get really serious, I'd add a Zobel across the input of 100pF and 75 Ohms series'd. Or maybe not, but I'd try it and keep it if I couldn't hear any tone issues, as extra insurance.
All good fortune,
Chris
To get really serious, I'd add a Zobel across the input of 100pF and 75 Ohms series'd. Or maybe not, but I'd try it and keep it if I couldn't hear any tone issues, as extra insurance.
All good fortune,
Chris
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