Power Transformer wiring

[Amled87]

I recently bought an Antek 325v toroidal power transformer. I'm just wanting clarification I have my wiring correct before applying voltage.

Here's what I believe is correct :
#1 Wire the primaries in parallel for 110v.
#2 Wire the 325v secondaries in parallel for 325v 0.8A. Which would be yellow to yellow, and white to white, the white then goes to ground?

This is for a preamp I'm working on. So, the 325v will then be rectified, filtered and the voltage dropped before being applied to my preamp stages. The 6.3v secondaries will then power my 12AX7 heaters.

 

[Rusan]

I'd say that since there aren't any phasing dots on the schematic, you could logically assume that like colors paralleled together will put the windings in the proper phase, and keep the magic smoke where it belongs.

That's a WHOLE lot of transformer for a few 12AX7s! Being toroidal, the inrush current at power-on will be significant, especially with the HV secondaries in parallel. And, the off-load voltage will be very high, not that a couple of 12AX7s will pull it down much anyway. So, it might perhaps be a good idea to add some carbon comp current limiting resistors in series with the HV secondary legs? What type of rectification device(s) are you using?

If you're not runniing the heaters on DC, your raw AC heater voltage will also be much too high on the 4A rated winding, especially when running 120V on a trafo designed with a 115V to 6.3V step-down ratio. A neat trick is to put a pair of BIG, beefy Schottky diodes paralled in anti-phase in series with each leg of the heater string. You still have AC operation, minus two diode drops of voltage, which in the case of Schottkys is usually just about right. Plus, Schottkys won't introduce a bunch of reverse recovery spike RF switching hash into the works.

 

[Amled87]

Since there aren't any phasing dots on the schematic, you could logically assume that like colors paralleled together will put the windings in the proper phase, and keep the magic smoke where it belongs.

That's a lot of transformer for a few 12AX7s! Being toroidal, the inrush current at power-on will be significant, especially with the HV secondaries in parallel. And, the off-load voltage will be very high, not that a couple of 12AX7s will pull it down much anyway. So, it might be a good idea to add some current limiting resistors in series with the secondary legs. What type of rectification device(s) are you using?

Well, I picked the transformer up for 65$ shipped and I figured it would allow me to add a PI and output stage later if I chose. But, for now it's just the preamp stage I'm focusing on.

I had planned to use a full wave diode bridge network using 1N4007's, 100uf 500v filter caps and 10k 2-watt voltage drop resistors.

 

[Rusan]

Well, I picked the transformer up for 65$ shipped and I figured it would allow me to add a PI and output stage later if I chose. But, for now it's just the preamp stage I'm focusing on.

I had planned to use a full wave diode bridge network using 1N4007's, 100uf 500v filter caps and 10k 2-watt voltage drop resistors.

Ahhhh, gotcha! I was just wondering, since a bridge rectifier here would give you well over 450V DC N.L. and there's no center tap to do a C.T. fullwave rectifier, which wouldn't give you much voltage wiggle room in the other direction for 12AX7s.

 

[Amled87]

Ahhhh, gotcha! I was just wondering, since a bridge rectifier here would give you well over 450V DC N.L. and there's no center tap to do a C.T. fullwave rectifier, which wouldn't give you much voltage wiggle room in the other direction for 12AX7s.

I think my actual idea here was to use a voltage divider first (pair of 10k resistors) to drop the voltage back down to 230v. Then after rectification that would give me roughly 322v which would be easier to drop to the 12AX7's voltage limit.

The rectifier schematic I planned to use was something like the picture I'm posting.

 

[Bo Deadly]

Be careful. Voltage is what stops your heart from beating and 300+ volts is pretty high.

No, you would not connect white to ground. One of the rectifier outputs defines 0V of your circuit. Then the 0V at the last filter cap is your "star" ground which should also be connected to a bolt on the chassis where earth ground of mains in is also connected.

Note that there are lots of ways to do heaters. Because heaters are one of the greatest sources of noise, you might elect to wire them in series, rectify and filter to 12.6VDC and then also use a resistor divider to elevate the heater supply relative to some point within your HT supply. Although not all tubes can be wired for 12.6V heaters.

And I'm not a tube guy so I could be talking out of my rear.

 

[Rusan]

I apologize, I'm not picking up what you're putting down here. The schematic is a bridge rectifier-derived bipolar supply, (EDIT: I looked at it wrong!) and if I understand correctly you wish to use a voltage divider in the AC portion of the circuit?

In my finite way of understanding things, I'd either:

1. Use a standard bridge rectifier.

2. Tie the white wire of the "top" HV secondary to the yellow wire of the "bottom" one to make a center tap referenced to ground. Then, connect a pair of 1N4007 diodes in series to the remaining yellow wire, and another pair in series to the remaining white wire. Ties their outputs together to begin your power supply rail.

Either way, you're gonna have about 475V of raw B+.

I'd use a 100uF first cap, then drop the voltage through multiple RC pi filter stages to drop voltage and provide adequate filtration. A small, 10 to 15 Henry choke for the first pi stage would be even better, you'd just have to be mindful of IR² heat gain in the high DC resistance of the windings if you later run a phase inverter and power tube screen grid feeds through it.

A voltage divider would indeed would work to drop the voltage on a hard Class A amplifier stage that never exhibits an increase in delta plate current, sure, but it doesn't provide much needed ripple filtration, which you're going to need a lot of to have a quiet preamp stage. And, if you later add power tubes and run the screen grids from that supply, the rail voltage is gonna oscillate like crazy in response to the screen current fluctuation.

 

[Amled87]

I apologize, I'm not picking up what you're putting down here. The schematic is a bridge rectifier-derived bipolar supply, and if I understand correctly you wish to use a voltage divider in the AC portion of the circuit?

In my finite way of understanding things, I'd either:

1. Use a standard bridge rectifier.

2. Tie the white wire of the "top" HV secondary to the yellow wire of the "bottom" one to make a center tap referenced to ground. Then, connect a pair of 1N4007 diodes in series to the remaining yellow wire, and another pair in series to the remaining white wire. Ties their outputs together to begin your power supply rail.

Either way, you're gonna have about 475V of raw B+.

I'd use a 100uF first cap, then drop the voltage through multiple RC pi filter stages to drop voltage and provide adequate filtration. A small, 10 to 15 Henry choke for the first pi stage would be even better, you'd just have to be mindful of IR² heat gain in the high DC resistance of the windings if you later run a phase inverter and power tube screen grid feeds through it.

A voltage divider would indeed would work to drop the voltage on a hard Class A amplifier stage that never exhibits an increase in delta plate current, sure, but it doesn't provide much needed ripple filtration, which you're going to need a lot of to have a quiet preamp stage. And, if you later add power tubes and run the screen grids from that supply, the rail voltage is gonna oscillate like crazy in response to the screen current fluctuation.

Sorry, new to design and just trying to make sure I have everything correct.

Say I went down to a same spec and wiring 240v transformer (roughly 311v after rectification) I'd then connect the primaries in parallel for 110v operation.

With this transformer I'd tie the white/gray 220v tap and 240v together and the yellow would be my zero correct? Then use a bridge rectifier for my ac to DC? Here is the transformer data sheet and the bridge rectifier I plan to use.

My first filter cap would be between the +/- of the bridge rectifier and the - of the cap would go to the chassis ground? Did I get all that correctly?

That all seem correct?

 

[Rusan]

No, the gray 220V lead is a tap on the 240V winding, you definitely don't want to tie taps together. Give me a minute, and I'll draft a rough schematic of what I'm talking about.

 

[Amled87]

No, the gray 220V lead is a tap on the 240V winding, you definitely don't want to tie taps together. Give me a minute, and I'll draft a rough schematic of what I'm talking about.

If possible use the transformer with the 220v tap. I'm not sure what to do with the 220v tap though, but for 37$ shipped, I bought the transformer.

 

[Rusan]

Okay, here's a rough schematic I drew really fast. I think everything's in order.

You have two a bridge rectifier with RC pi filter stages, with "ballpark" values that should give you something close to what you're needing, and sufficient wattage ratings for the current draw of 2 or 3 12AX7s . You can tweak one of the values to get exactly what you need.

 

[Amled87]

Okay, here's a rough schematic I drew really fast. I think everything's in order.

You have two a bridge rectifier with RC pi filter stages, with "ballpark" values that should give you something close to what you're needing, and sufficient wattage ratings for the current draw of 2 or 3 12AX7s . You can tweak one of the values to get exactly what you need.


View attachment 92617

Thank you! The 33k and 22k resistors will drop the voltage correct? After the 22k should the voltage be close enough to use with the 12AX7's? And for the unused 325v and 6.3v I can just wire those to my breadboard somewhere so they're out of the way and not use them?

 

[Rusan]

I don't know exactly what voltage you'll require, but nailing it after you've built the circuit is just an Ohm's Law calculation and a resistor change away. Though not essential, I'd recommend placing a filter choke after the bridge rectifier output (with the 100uF cap moved to there) and before the first RC pi filter (with a 40uF cap there replacing the 100uF in the schematic) for much greater ripple reduction. but there's a huge variable involved in doing that.

Before selecting a filter inductor, you'll need to decide for sure whether you're gonna add a phase inverter and finals later on, and whether there'll be one or two pairs of power tubes in said finals. The reason is that the choke will then have 100% of the screen grid current flowing through it, plus the current draw of all the small signal tubes. That requires a much larger choke than the minuscule plate current of a few 12AX7s, and too small of a choke will go into core saturation (especially on hard transients when the screen current rises drastically) and this will create a strong 120Hz harmonic in your power rail, and the B+ rail voltage will oscillate wildly.

If you use that same big choke rated for say, 7H@150mA with only a pair of 12AX7s drawing 4mA collectively, its actual reactance will be much smaller at such low current, because it's insufficient to magnetically excite the choke up to near its full inductance. Thus, you're not be getting much ripple reduction benefit.

Right now though, I'd just focus on building the circuit and taking time to learn the theory concerning the hows and whys.

As for your unused secondaries, NEVER connect unused secondary wires to ANYTHING WHATSOEVER. If they came pre-stripped from the factory, cut the stripped ends off and tape or wire nut them individually, then tape them together and bend them over out of the way.

Here it is again, same thing but with the 240V secondary transformer.