PT bias tap

[xtian]

I'm using a Weber WPT100 power transformer for my "plexi" build. The transformer as a 45VAC bias tap off the HV winding:

[img:615:379]http://taweber.powweb.com/store/wpt100sch.jpg[/img]

Why not use this for my bias voltage? This is demonstrated in the Orange 100 clone schematic from Weber's site:

[img1504]https://taweber.powweb.com/store/6o100_schem.jpg[/img]

This uses only four components to get bias voltage instead of seven. Any reason NOT to go this route?

[JazzGuitarGimp]

This looks like a reasonable solution to me. A few suggestions:

1. Add a 10nF ceramic snubber cap across the diode to suppress diode switching noise.

2. As wired, the pot will take your grids all the way to GND, or 0V, which will cause the output valves to red plate. Once you have the operating point at hand, you might consider placing a resistor between the bottom leg of the pot and GND. Choose a resistor value that will limit the power dissipation in the output valves to about 85% of maximum for the valves you're using.

[xtian]

Just reporting back: This worked really well. Ended up with a 3.3Kohm resistor in series with the bias pot to prevent redplating. I hear some buzzy hash when the gain is dimed, and tried adding snubber caps across the diode, and also disconnecting the power LED, but made no difference in noise.