Reverse PT for auxiliary supply

[K O'Connor]

Hi Guys

There are situations where we need an extra voltage that we cannot derive directly from the main power transformer, such as when adding a bias supply to a fixed-biased amplifier, or if we are adding a tube to a solid-state circuit. This is often referred to as a"back-to-back" connection as the secondaries of the two PTs tie together with the nominal primary of the added PT used as an output.

The reverse-wired auxiliary PT is usually low-VA which results in a voltage loss of twice the regulation rating. Tiny PTs are a compromise of design and even when used with forward connections, the loss is significant and certain adaptations are made to get useful throughput of energy.

In the case of a bias supply in a tube power amp, the aux-PT powered from the AC heater voltage, which can be tied to ground, floating, or on a DC-stand-off. The transformer provides galvanic isolation to its output, which can be rectified and filtered and used how we wish. For a bias supply the positive end is ground. The aux-PT may be for other uses, such as to support switching circuits, relays, opamps, or whatever else.

Similarly, if we have a solid-state circuit to add a tube AND the stock supply can support the heater current, then we can use the aux-PT to generate the plate voltage for the tube. The reverse connection is more lossy and is very useful in some cases to achieve a more controlled output - see below In the bias supply context, it has sufficient output and remaining VA capacity to do the job well. For the plate supply, the series-connected primaries of the aux-PT will produce about 200Vdc instead of the expected  320Vdc.

The reduced output of the backwards PT is only an issue with small PTs, say 7VA or less. If you use a higher-VA part even though you do not need the extra power, the effective loss is reduced because the basic design of higher-VA PTs is less compromised than for low-VA devices. We can use this factor to our advantage. For example, say we use a  small PT with dual primaries and dual secondaries and all windings are rated for 115Vac. We wish to make a tube plate supply, so the output windings are series connected. If we use a forward connection for the 6VA part, the output is way in excess of the 320Vdc expected, due to the high regulation figure of 30% for this size device. We get  over 400Vdc unloaded and may need a regulator to rein things in. If we reverse the PT, we get half as much voltage.

The back-to-back connection requires that the main PT that is tied to the mains be able to handle all the power needed by the circuit. Generally, the main PT will be used to generate the low voltages required, say for opamp supplies, and to support the tube heater. For example, say the main-PT output is 12-0-12Vac. We can rectify and filter this to produce about +/-16-19Vdc. Best to use it to support regulators making clean DC of lower value and allow margin for line drops. If we are using 12V heater tubes, then the heaters can be connected to +/-12Vdc or be tied directly to the 12Vac.

The reverse-wired PT must have its secondary windings tied to the AC secondary of the main-PT, since transformers only work with AC.

Ideally, the main-PT has each primary fused, and then each secondary fused. The aux-PT only needs one fuse between it and the main-PT and then one or two fuses on the output depending on how the transformer is used. For a bias or plate supply, a single fuse can protect the output. We have four fuses around the main-PT and two for the aux-PT, so six fuses in this example. If the main-PT is only generating a single DC output apart from supporting the aux-PT, then we have one less fuse.

Compare the example of B2B PTs to using two forward-wired PTs. In the latter, each PT must have both primaries fused, then their respective outputs fused as needed. In the case of +/-12Vdc plus a plate supply, we have four mains fuses plus three secondary fuses, so seven total. However, depending on how much power we need for the various supplies, the compromise of one more fuse may allow us to use devices that are available, and certainly we can use a main-PT that is slightly lower-VA.

[K O'Connor]

Hi Guys

The transformers used in any application should be used in a manner that takes advantage of their capabilities without introducing loading imbalances that cause in increase in EMI (electromagnetic interference) noise output.

For example, a toroidal transformer is considered to be about as ideal as we can manufacture. One rule for winding them is that each winding must completely cover the core for there to be minimal noise escaping. Each winding acts to shield the core. Some windings may require so many turns of wire that it goes around the core more than one, but each layer must meet the requirement of covering the core completely.

If there is a tapped winding, then each side of the tap, or any portion between taps constitutes a separate winding and is made as such, then the related wire ends are connected together to wire lead-outs. Because each portion of the tapped winding fully screens the core, the loading on each portion can be independent of the other sections. For example, a center-tapped winding can have widely different loads on each side of the CT without a noise penalty.

There will be gaps in the core screening where the lead-outs are made, so there will be some EMI.

Semi-toroidal transformers typically use a core that uses square-U laminations that can be inserted through bobbins holding the windings. The Us are inserted from both ends to make a complete squared circle. This takes advantage of standard EI winding methods and can be very economical.However, there will be two large gaps in the core coverage, so a semi-toroid has inherently higher EMI than a true toroid.

For lowest EMI output, the semi-toroid must have balanced loadings through its two wire groups held within the two bobbins. Each group will have a primary and secondary, requiring that both primaries be used as primaries all the time and always used all the time - wired in series or parallel to suit the mains input - and that the secondaries must be used all the time at equal loading.

Hobbysists wanting to build a small PSU to support a tube may see a clever way to iwire a standard semi-toroid to generate both plate and heater voltages. This only works in North America or Japan where mains is 120Vac or 100Vac, respectively. One primary ties to the mains. The second primary is used to provie high voltage. The two low-voltage secondaies support the heater. For hobbyists, the noise outpt may be acceptable to them, and they may be used to using / building noisey equipment? But for a discerning ear, this is not as good as it could be and it is of no use for the rest of the world using 240V mains.

Traditional EI transformers are inherently noisy and often use half-turns to generate the correct output voltage. Shielding the core is not a big concern as the windings are never extremely tight anyway. There are limits to how tight you can wind the wire without it breaking during the assembly, or breaking later in use. magnetic leakage will cause end-bells (metal coverings over the windings) to vibrate and buzz, but to what extent depdends on the manudfacturer's attention to detail and execution of design.

Most auxiliary function PTs will be smaller in their power rating (VA). The magnetising current is often of a similar order to the full-load current and core losses can be a few watts of heat coincident with a steep temperature rise. In general, EMI is in step with the core temperature and is something most hobbyists and even some designers do not notice or know about. Cooler operation at full load requires a larger, heavier, more expensive transformer, so you are buying a larger part to have better performance through cooler running, or specifying a custom PT with a low-temp rise.

Adding a small PT to an existing amp, or building a preamp using small PTs, we are lucky that "small" can be critically-sized or generously-sized for very similar cost.