PT taps question

[gui_tarzan]

A friend has an Ibanez TSA30-H that was very unstable and he was tired of it working for a while a crapping out, so he brought it to me and said "make something hand-wired, your choice, do with it what you will".

The transformer has a 53(ish) volt tap, two white wires, and I can use that for the bias circuit since the voltage is perfect for the pair of 6L6GCs it will have.

My question is, can I run one lead to ground and use the other for the positive lead or do I need to make an artificial ground? My gut says I can ground one of the leads, but I wanted to make sure before I try it. I'm using the attached bias and power tube circuit.

[martin manning]

Is this white-white winding connected to any others? If not, you can ground one side and run the other into a typical fender-type bias circuit. Note in the schematic you posted the bias circuit is capacitively coupled to the one side of the HT, which is only necessary with a FWB rectifier.

[gui_tarzan]

Nope, they're on their own. So with a separate winding I don't need the initial cap.

[martin manning]

It's the FWB rectifier that makes the cap necessary. You can just copy a Fender bias circuit as used with FW rectifier.

[gui_tarzan]

Ok, just so I understand why, would you explain the difference between the two? I looked at several sites that explain how bias works (I know the why but not why the difference) and none of them showed a circuit with a cap on the front end.

[martin manning]

In the FWB arrangement the AC from the transformer is all above ground (give or take a diode drop), so there is no significant negative voltage available. To use that to generate a negative bias voltage, a cap is connected first, and then a resistor to ground immediately after to get an AC voltage that has both positive and negative swings. Then you can rectify that with a reversed diode and get a negative DC voltage.

[gui_tarzan]

Ok, that makes sense. And a Fender style already has the negative ground so that's not necessary. I haven't been able to find a schematic for this amp so I wasn't sure how to make this one work. If I read the circuit board correctly one of the white leads goes to ground, the trace goes to pins 8 of the power tubes. The other lead goes to a fuse, then a jumper, then to the bias diode so I don't need the cap after all.