PT with multiple secondarys question

[Bob-I]

I'm using this PT for my Princeton/Dumble build.

http://www.classictone.net/40-18027.pdf

There are 2 secondary taps allowing either 400V or 330V output. I'd like to put a switch on the back to select either secondary for different power levels and feel.

The question is on the bias. There's no bias tap on this PT so I need to tap from one of the secondary leads. Should I tap from one of the secondaries directly, or should I switch the bias tap when I switch the secondary?

My concern is that I may not have enough range on the bias pot to handle switching secondary voltage.

Thoughts?

[John_P_WI]

Simple solution. Use a DPDT On-On switch, the "330" leads go to one set of contacts, the "400" leads go to the other set of contacts. The center "common" contacts go to your diode string, take the bias off of both leads (keeps things balanced) before the main rectifier diode string. Use a high quality full size Carling switch. Your bias will adjust with the change in high voltage.

[martin manning]

How about a DPDT on-off-on switch in the standby hole? Center position is standby, and the other two are high and low voltage. I agree take the bias from the center of the switch, either half- or full-wave.

[John_P_WI]

How about a DPDT on-off-on switch in the standby hole?

Good question Martin, I try to avoid that as the bias voltage is at 0 v when switched, and IMHO the full bias voltage should be present WHEN the B+ supply is switched (as found with the "conventional" circuit position of the Standby switch.

[Bob-I]

Simple solution. Use a DPDT On-On switch, the "330" leads go to one set of contacts, the "400" leads go to the other set of contacts. The center "common" contacts go to your diode string, take the bias off of both leads (keeps things balanced) before the main rectifier diode string. Use a high quality full size Carling switch. Your bias will adjust with the change in high voltage.

I'm confused, are you saying to take the bias off of both the 400V and 330V taps? Wouldn't that then connect these taps together?

[Bob-I]

How about a DPDT on-off-on switch in the standby hole? Center position is standby, and the other two are high and low voltage. I agree take the bias from the center of the switch, either half- or full-wave.

Um... I won't do that. No bias on standby

[martin manning]

How about a DPDT on-off-on switch in the standby hole?

Good question Martin, I try to avoid that as the bias voltage is at 0 v when switched, and IMHO the full bias voltage should be present WHEN the B+ supply is switched (as found with the "conventional" circuit position of the Standby switch.

Ah, very good point! Two switches are better than one, but even then as the switch is thrown the AC to the bias supply is cut.

[John_P_WI]

I'm confused, are you saying to take the bias off of both the 400V and 330V taps? Wouldn't that then connect these taps together?

No, the 330 / 400 v transformer wires go to the switch, from the switch center poles two wires go to the rectifier diode string. Connect the bias (dropping resistors and diodes) at the junction of the two wires and two B+ diode strings.

Sorry if I confused you when I was proposed using both legs of the transformer for the bias, not a single (like marshall) to keep the balance on the power transformer winding.

[Bob-I]

Sorry if I confused you when I was proposed using both legs of the transformer for the bias, not a single (like marshall) to keep the balance on the power transformer winding.

No problem. This is what I drew up as you were replying...

I'm gonna redraw... hold on

[Bob-I]

Like this?

[John_P_WI]

Yes, just make sure the down stream components (pot and resistor) etc are connected otherwise the cap will see the full rectified voltage of the 330 / 400 (which ever is selected) vac windings.

[Bob-I]

Yes, just make sure the down stream components (pot and resistor) etc are connected otherwise the cap will see the full rectified voltage of the 330 / 400 (which ever is selected) vac windings.

Good point, I think I'll change that cap JIC

[Bob-I]

Here's the rectifier/bias board I built. Whatcha think?

[martin manning]

Yes, just make sure the down stream components (pot and resistor) etc are connected otherwise the cap will see the full rectified voltage of the 330 / 400 (which ever is selected) vac windings.

I haven't seen it done before, but a voltage divider could be placed in front of the cap to protect it. This would only require adding a resistor across the cap to ground.

[Bob-I]

Yes, just make sure the down stream components (pot and resistor) etc are connected otherwise the cap will see the full rectified voltage of the 330 / 400 (which ever is selected) vac windings.

I haven't seen it done before, but a voltage divider could be placed in front of the cap to protect it. This would only require adding a resistor across the cap to ground.

That's a 350V cap, even if it gets hit with 400 briefly it should survive.