The Heart of Push-Pull Power: Understanding Output Transformer Wiring

Jul 25, 2025 | 0 comments posted by Vincent Zhang

Published by IWISTAO

If you've ever delved into the world of tube amplifiers, you've likely heard the term "push-pull" output stage. It's the workhorse design for achieving high power and low distortion. And at the very center of this design lies a crucial component: the push-pull output transformer.

Connecting this transformer correctly is fundamental to an amp's performance, safety, and very existence. Today, we'll demystify the primary-side terminal naming conventions and show you exactly how to connect it to your power tubes.

The Three (or More) Key Terminals

A push-pull output transformer's primary winding is center-tapped, meaning it has a wire connected to the electrical midpoint of the coil. This gives us three primary terminals, with names that vary slightly by manufacturer and era but always mean the same thing:

B+ / HT / C.T. (Center Tap):
- This is the high-voltage DC power input. B+ is the classic term for anode voltage, HT stands for "High Tension," and C.T. means "Center Tap." They all refer to this single, critical point.
P1 / Plate 1 / A:
- This is the plate connection for one half of the push-pull pair (often the "upper" tube).
P2 / Plate 2 / B:
- This is the plate connection for the other half of the push-pull pair (the "lower" tube).

For example here, the diagram of vacuum tube FU50 and its pull-push power amplifier presented as below.

1-cathode, 2-grid, 3-screen grid, 4-filament, 5-suppressor grid, 6-anode, 7-empty pin, 8-filament.

A diagram of vacuum tube pull-push power amplifier

From the diagram, you can see B1 to connect high voltage 500V. The sixth pin ( anode) of upper FU50(VT1) to connect P1 of pull-push output transformer, similar with the other FU50(VT2).

For Ultra-Linear Taps: Many modern transformers support Ultra-Linear operation, a clever design that blends the power of pentode mode with the linearity of triode mode. These transformers have two additional terminals:

SG1 / G1: Connects to the screen grid (G2) of the first power tube.
SG2 / G2: Connects to the screen grid (G2) of the second power tube.

There is an example for vacuum tube EL84 pull-push amplifier diagram, its output transformer is UL tap.

Pins 1-2 of the EL84 are connected as the signal grid. The cathode of pin 3 is internally connected to the suppressor grid. Pins 4-5 are the filament. Pins 6-8 are empty. Pin 7 is the anode, and pin 9 is the G2 screen grid.

There is the related pull-push output transformer with Ultra Linear tap as below

IWISTAO 23W Tube Amplifier Output Transformer 1 Pair Pull-Push Z11 Silicon Steel Shield Cover for 6P1 6P6P 6P14 FU29 6AQ5 6F6

IWISTAO 2X12W HIFI Tube Amplifier 6N1x2 Preamp 6P1x4 Class A Pull-Push Amplifier Circuit

How to Connect to the Power Tubes

The push-pull circuit works by having two tubes (VT1 and VT2) alternately conduct and amplify the positive and negative halves of the input signal. The transformer's job is to combine these two "push" and "pull" actions into a single, powerful waveform at the speaker.

Let's look at the two main connection schemes.

1. Standard (Pentode/Tetrode) Connection

This is the most straightforward wiring method.

The B+ terminal is connected to the high-voltage supply from your power supply's filter capacitor.
The P1 terminal is connected to the plate (anode) of the first power tube (VT1).
The P2 terminal is connected to the plate (anode) of the second power tube (VT2).
The screen grids (G2) of both tubes are typically tied together and connected to a lower, stabilized voltage source (often through a dropping resistor from the B+ line). Crucially, they do not connect to the output transformer in this mode.
The cathodes are usually tied together and connected to ground through a shared cathode resistor (often bypassed with a capacitor).

A simplified visual guide:

          +B (High Voltage from Power Supply)
           |
           |
       (Primary Winding)
       /        \
     P1          P2
      |           |
    VT1 Plate  VT2 Plate
      |           |
     ...         ... (Other tube elements)

In words:

B+ goes to your high-voltage supply.
P1 goes to the plate of VT1.
P2 goes to the plate of VT2.

2. Ultra-Linear Connection

This method feeds back a portion of the output signal to the tubes' screen grids, reducing distortion and output impedance.

The B+ terminal still connects to the high-voltage supply.
The P1 terminal connects to the plate of VT1.
The SG1 terminal connects to the screen grid (G2) of VT1.
The P2 terminal connects to the plate of VT2.
The SG2 terminal connects to the screen grid (G2) of VT2.

A simplified visual guide:

          +B
           |
           |
       (Primary Winding)
       /    |    \
     P1    SG1   SG2    P2
      |     |     |      |
    VT1-P VT1-G2 VT2-G2 VT2-P

In words:

B+ goes to your high-voltage supply.
P1 and SG1 go to the plate and screen grid of VT1, respectively.
P2 and SG2 go to the plate and screen grid of VT2, respectively.

Critical Considerations for a Successful Build

Phase is Everything (!!):
- The entire push-pull concept relies on the input signals to the two power tubes being perfectly out-of-phase (180 degrees apart). This is the job of the preceding phase inverter circuit.
- Getting P1 and P2 backwards is a catastrophic error. If you swap them, the tubes' output signals cancel each other out in the transformer. The result is extremely low power, high distortion, and a thin, nasty sound. Always double-check your schematic.
Winding Balance:
- A quality transformer ensures the winding from P1 to B+ is perfectly balanced with the winding from P2 to B+. This symmetry allows the push-pull stage to effectively cancel even-order harmonic distortion and prevent DC core saturation.
Safety First:
- Warning! The voltages at the B+ point and on the tube plates are lethal—often in the 400V to 500V range or higher. Always ensure the amplifier is completely powered off and the main filter capacitors have been safely discharged before touching, measuring, or soldering any of these points.

Summary Table

Terminal Symbol	Common Names	Purpose	Connects To
B+	HT, C.T.	High-voltage DC power input	Power supply filter capacitor (High Voltage)
P1	Plate 1, A	AC signal input for push-pull side A	Plate of Power Tube VT1
P2	Plate 2, B	AC signal input for push-pull side B	Plate of Power Tube VT2
SG1	G1 (Ultra-Linear)	Screen grid feedback point for side A	Screen Grid (G2) of Power Tube VT1
SG2	G2 (Ultra-Linear)	Screen grid feedback point for side B	Screen Grid (G2) of Power Tube VT2

Understanding these connections is a rite of passage for any tube amp enthusiast. By respecting the terminology, the phase, and most importantly, the high voltages, you can harness the classic power and tone that only a well-executed push-pull amplifier can deliver.

As always, the final authority for any specific amplifier is its official schematic. Happy building, and stay safe.

blog tags: EL84 FU50 IWISTAO pull push output transformer pull-push tube amplifier shuguang tube amplifier diagram UL tap Ultra-Linear Vacuum Tube