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previous: Ed I was having a problem with 60hz & ... -- 9/22/2004 11:29 PM

Re: DC Heater Wiring

10/3/2004 4:35 PM
Brutus	Re: DC Heater Wiring To resolve your 120 hum problem, temporaily clip in a 50mfd or greater filter cap across the current PS filter cap to see if the hum goes down. Do this separately for each filter cap in the PS feeding each of the stages separately. If the hum diminishes at all while doing this, then you simply need better filtering. Also, I find that 120hz hum radiates electromagnetically a little better than 60hz. Meaning that you should keep all wiring with 120hz AC components on it away as far as possible from other audio carrying components. I once had a severe 120 hz problem in an amp where I placed the PS filter supply cap too close to a tube that was microphonic. The hum stopped immediately when I moved the cap a half inch further away from the tube. You can easily diagnose problems like this by simply taking a nonconductor like a wooden stick and prying various components to see if the hum increases or decreases as you move them slightly. Same technique is good for diagnosing parasitic problems. Also, is your 120 cycle hum really 120 cycles and not really maybe 150 or 180? What I'm getting at is that you may have a parasitic oscillation problem without knowing it. Some parasitics sound an awful lot like 120 cycle hum. Beating an audio signal generator set to 120 hz against the hum will quickly tell you whether you are dealing with 120 cycles or not. Brutus

10/3/2004 4:35 PM

Brutus

Re: DC Heater Wiring

To resolve your 120 hum problem, temporaily clip in a 50mfd or greater filter cap across the current PS filter cap to see if the hum goes down. Do this separately for each filter cap in the PS feeding each of the stages separately. If the hum diminishes at all while doing this, then you simply need better filtering.

Also, I find that 120hz hum radiates electromagnetically a little better than 60hz. Meaning that you should keep all wiring with 120hz AC components on it away as far as possible from other audio carrying components.

I once had a severe 120 hz problem in an amp where I placed the PS filter supply cap too close to a tube that was microphonic. The hum stopped immediately when I moved the cap a half inch further away from the tube.

You can easily diagnose problems like this by simply taking a nonconductor like a wooden stick and prying various components to see if the hum increases or decreases as you move them slightly. Same technique is good for diagnosing parasitic problems.

Also, is your 120 cycle hum really 120 cycles and not really maybe 150 or 180? What I'm getting at is that you may have a parasitic oscillation problem without knowing it. Some parasitics sound an awful lot like 120 cycle hum. Beating an audio signal generator set to 120 hz against the hum will quickly tell you whether you are dealing with 120 cycles or not.

Brutus

Replies:

loudthud The 120Hz problem could just be a m... -- 10/5/2004 4:19 PM