question on hammond PT salvaged from old organ(Ao-35)

[imo1]

Hello friends,

I have a PT and OT from an AO-35 circuit. I have already, in the past, built an AC15 out of this circuit.

I am using these parts to build a Deluxe style build for my younger son. Single channel, no tremolo.

I have a question on how to best utilize the transformer. It has HV 290AC with CT. 5v winding, 6V winding, and another winding for the tone wheel(17V to CT, 35V total)

I am going to be using solid state diodes to rectify the HV, as I would like to get 415-420, plus I want the reliability for my kid. The HV has a centertap, so i know it won't work with a traditional bridge rectifier. I will use 2 4007s per side for this.

My question is with the 35V winding. This is perfect for my bias needs. I am not using the CT and am using either side to get 35V aC(45V DC max to bias pot) I have tried a bridge rectifier for this and it seem to work fine, but I was wondering if that would casue problems in any way with the rest of the transformer, since you aren't supposed to use a bridge rectifier for the HV windings if there is a centertap(which there is)

I am still contemplating best moves forward before I commence the build in earnest

would love to hear any thoughts.

Ian

[martin manning]

My question is with the 35V winding. This is perfect for my bias needs. I am not using the CT and am using either side to get 35V aC(45V DC max to bias pot) I have tried a bridge rectifier for this and it seem to work fine, but I was wondering if that would casue problems in any way with the rest of the transformer, since you aren't supposed to use a bridge rectifier for the HV windings if there is a centertap(which there is)

You can use a FWB on a center tapped winding designed for use with a full wave (two phase) rectifier, but the current available will be reduced. With the FWB, the entire coil will be conducting (and producing heat) on both swings of the 360 degree cycle. A bias supply requires very little current, so you should be fine. I'm assuming that you have confirmed that this 35V CT winding is not connected to any of the other windings.

[imo1]

Would there be any benefits in doing this over simply using 2 diodes? I feel like I am ok current wise on the transformer but don’t want to push it where I don’t have to.

[martin manning]

Wiring it as a full wave (two diodes) with the CT grounded you will only get 17.5 x 1.4 = 24.5 VDC, minus 0.7V diode drop, 23.8.
A FWB (four diodes, CT unused) will get 35 x 1.4 = 49 VDC minus 2x 0.7V diode drop, 47.6.
Another option is a half-wave rectifier (typical Fender bias circuit with one diode) using the entire winding with one end grounded. That would get 35 x 1.4 = 49 VDC minus one diode drop, 48.3.
You only need a few mA of DC, so that should be fine.

[imo1]

I have a bridge rectifier mounted, and actually from a build/space standpoint it is easier to implement.

I think i will use the bridge rectifier with the CT not used and then use the HV taps into series diodes on each leg. It sounds like that should work in theory.

[imo1]

Just to wrap up this thread, I’m pretty sure the PT was fried by my incorrectly attempting to use the bridge rectifier for the bias voltage.
My primary is testing really low in terms of resistance. I think the wires melted. Lesson learned. Thankfully I have another Salvaged PT that I was able to drop in

[martin manning]

FWB on an isolated winding with CT unused should not have caused a problem unless there was some error. Possibly incorrect grounding?