The AA8V Wingfoot 813 Amplifier
High Technology Of The 1950s In The 2000s
by Greg Latta, AA8V

Transformer T1 Primary Circuit Description and Schematic Diagram

A Comment on Phase Notation:
In many cases "polarity", or more properly, "phasing" does matter when connecting transformer primaries and secondaries, just as it does when connecting batteries. However, because transformers operate on AC, we cannot talk about "positive" or "negative" connections to a particular primary or secondary. Instead, we label the connections based on the instantaneous polarity of a particular connection. Those connections that are is phase with each other (simultaneously positive or negative) are marked with a black phasing dot, as shown in the diagram below:

Transformer T1 Primary Phasing Diagram

Note that the black, blue, and orange leads above are marked with a black phasing dot. This means that if, at a particular point in time, the black lead is positive with respect to the brown, then, at that same point in time, the blue is positive with respect to the grey and the orange is positive with respect to the yellow.

Primary Wiring to Transformer T1: Transformer T1 has two 120V primary windings. The two primary windings can be placed in parallel (OPERate) or series (TUNE) by mode switch S2. When placed in series the output of T1 is essentially cut in half. Transformer T1 also has a primary buck/boost winding that can be connected in phase (LOW) or out of phase (HI) with the two major primary windings by HI/LOW switch S1. This gives a total of four possible output voltages from the secondaries of T1. Switch S1 (HV HI/LOW) determines whether the buck/boost winding is connected in phase (buck) or out of phase (boost). When switch S1 is in the HI position, the winding is connected out of phase, effectively lessening the number of turns on the primary and raising the output voltage. When switch S1 is in the LOW position, the winding is connected in phase effectively increasing the number of turns on the primary and lowering the output voltage. Switch S2 (OPER/TUNE) places the transformer primaries either in parallel or series. When placed in parallel (OPERate Mode), full output is obtained from the power supply. When placed in series (TUNE Mode), the output of the power supply is greatly reduced. This greatly reduces the plate dissipation of the 813 while the amplifier is being tuned up.

OPERate Mode - Primary Current Flow: OPER/TUNE in OPER) For normal operation, switch S2 is placed in the OPERate position. This places the 120V transformer primaries in parallel. If you follow the arrows in the figure at right, you will see that for each transformer primary current flows from the AC neutral connection (BLUE), through the primary winding, and back to the AC Hot connection (RED) via the HI/LOW switch and the buck/boost winding. Each transformer primary is thus connected to 120 volts, and the transformer primaries are in parallel. This yields full output from the power supply.

TUNE Mode - Primary Current Flow: OPER/TUNE in TUNE) For tune up operation, switch S2 is placed in the TUNE position. This places the transformer primaries in series. If you follow the blue arrows in the figure at right, you will see that the AC current flows from the AC neutral (BLUE) connection, through the first primary, through S2 to the next primary (GREEN), and then back from the second primary to the AC Hot connection (RED) via the HI/LOW switch and the buck/boost winding. The two 120V primaries are thus in series and in phase. By placing the primaries in series, the voltage applied to each primary is cut in half from 120 volts to 60 volts. This cuts the secondary voltages in half and cuts the output of the plate supply in half. TUNE mode doesn't cut the total output of the power supply in half because the screen supply (using T3) is not affected by the position of S2. In fact, S2 has a center off position, where the plate supply can be completely turned off. In that case the total power supply output is reduced to about 450 volts.

HI (Boost) Mode - Primary Current Flow: ( HI/LOW in HI) In normal operation switch S1 is set to HI (boost) to place the buck/boost winding out of phase with the 120V primaries, effectively reducing the number of turns on the primary and raising the output voltage. If you follow the green arrows in the figure at right you will see that the current flows from the Grey connection of Primary 2 through the HI/LOW switch to the yellow lead of the buck/boost winding. The Primary 2 winding and the buck/boost winding are thus connected out of phase. (Whether S2 is set to OPERate or TUNE does not affect this connection.) The Buck/Boost winding is thus connected out of phase overall with Primary 1 and Primary 2, effectively decreasing the number of turns on the primary, raising the output voltage.

LOW Mode - Primary Current Flow: ( HI/LOW in LOW) If lower plate voltage is desired, S1 can be set to LOW (buck) to place the buck/boost winding in phase with the 120V primaries, effectively increasing the number of turns on the primary and lowering the output voltage. If you follow the green arrows in the figure at right you will see that the current flows from the Grey connection of Primary 2 through the HI/LOW switch to the orange lead of the buck/boost winding. The Primary 2 winding and the buck/boost winding are thus connected in phase. (Whether S2 is set to OPERate or TUNE does not affect this connection.) The Buck/Boost winding is thus connected in phase overall with Primary 1 and Primary 2, effectively increasing the number of turns on the primary, lowering the output voltage.

Click here for pictures and information on the matching Wingfoot 813 Amplifier

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