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My standard answer for FP Hum...

Posted by Marshall W (A) on November 29, 2004 at 09:16:38

In Reply to: Gotta get this off my chest. posted by olddude55 on November 29, 2004 at 06:39:56:

is DC Filaments. I tried all the usual methods but could not get rid of the hum in my FP until I switched to DC supply for the filaments.

Here’s how I changed the tube filaments on my Foreplay to DC:

1.Went to Radio Shack and bought a DC power supply, the kind that plugs into the wall. I think the one I bought is rated for 6 volts and 800 mA.

2.Snipped the green wires that go from the transformer to tube “B”, about half way between the transformer and tube.

3.Insulated the ends of the wires from the transformer with heat shrink tubing since they are no longer needed.

4.Stripped the ends of the green wires going to tube “B”.

5.Cut off the connector on the wires at the end of the DC supply and stripped them.

6.Slipped some heat shrink tube over the wires, soldered the power supply wires to the green wires going to tube “B” and shrunk the tubing to insulate the connection.

One additional note- The polarity needs to be correct when attaching the DC supply. I can't remember the pin number on the tube, but the negative from the DC supply needs to go on the pin with an existing path to ground.

After I decided I liked the change, I installed a separate on/off toggle switch on the chassis plate for the DC filaments so I wouldn't have to unplug the wall-wart each time I turned the Foreplay off.

This is not a terribly elegant solution, but it worked for me. The actual DC voltage measured on the filaments of my tubes is 6.5 volts, a bit high, but the noise is gone and I am now enjoying the music instead of focusing on the noise.

This post is made possible by the generous support of people like you and our sponsors:

Schiit Audio

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