A friend of mine gave me a tube amp for inspection. It makes noise (not 50 Hz hum). On a scope it looks like spikes with some ringing, the interval between two is 10 ms and it seems each two start in the opposite direction. It is a fairly standard 6L6 cathode biased amp. I suspect that the noise source is the ss rectifier in connection with poor filter caps - there are two 100uF in series - and I thing their high frequency impedance makes things worse. How can I get rid of this noise? Could bypass with a film cap help? Or should I rather bypass the AC side of the bridge rectifier? Any thoughts appreciated!
I would be looking more at:
1. Power line filtering- not just fluorescents, but CFLs, computers, switching supplies, and other equipment on that power line, perhaps in other parts of the house. An effective power line filter will handle common-mode noise as well.
2. Grounding- if diode switching noise is present, it is almost certainly because of poor grounding practice, a fundamental design flaw that is seen all too often. Simple bypassing will not fix this, and changing diode types is a band-aid, avoiding the actual source of the problem. This is especially critical in the high current loop formed by the rectifiers and the filter caps of the raw supply.
1. Power line filtering- not just fluorescents, but CFLs, computers, switching supplies, and other equipment on that power line, perhaps in other parts of the house. An effective power line filter will handle common-mode noise as well.
2. Grounding- if diode switching noise is present, it is almost certainly because of poor grounding practice, a fundamental design flaw that is seen all too often. Simple bypassing will not fix this, and changing diode types is a band-aid, avoiding the actual source of the problem. This is especially critical in the high current loop formed by the rectifiers and the filter caps of the raw supply.
SY, as far as I can tell it is not noise coming from outside, there is no noise in my other amps and I think I can detect this kind of noise by ear, this sounds different. I guess you may have hit the point - grounding looks a bit suspicious - it seems that there is a try for star-grounding and I am not sure how good it is. The spikes are synchronized with mains.
Changing diodes seems as a good idea - there is a round bridge rectifier of the cheapest kind. Which diodes would you suggest?
Changing diodes seems as a good idea - there is a round bridge rectifier of the cheapest kind. Which diodes would you suggest?
1. Connect wires from the secondary to the bridge,
2. Connect outputs of the bridge directly to the capacitor,
take power from this capacitor assuming the star ground is exactly at it's negative leg.
However, if your filter is not limited by this one capacitor, and the main current is drawn from another capacitor connected through a choke, now this after-choke capacitor will be a reference for B+ and for the star ground. Never solder anything else to wires going from bridges to capacitors, even though you may save some length of a wire!
2. Connect outputs of the bridge directly to the capacitor,
take power from this capacitor assuming the star ground is exactly at it's negative leg.
However, if your filter is not limited by this one capacitor, and the main current is drawn from another capacitor connected through a choke, now this after-choke capacitor will be a reference for B+ and for the star ground. Never solder anything else to wires going from bridges to capacitors, even though you may save some length of a wire!
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