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Reverse PT for auxiliary supply
#1
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.
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