2A3 and 300B Push-Pull Output Transformers: A Practical Selection Guid

2A3 and 300B Push-Pull Output Transformers: A Practical Selection Guide

Sep 19, 2026 | 0 comments posted by Vincent Zhang

PUBLISHED BY IWISTAO · Tube Amplifier Engineering

Plate-to-plate impedance, DC balance and full-power bandwidth for directly heated triode amplifiers.

A useful specification for a 2A3 or 300B push-pull output transformer begins with the amplifier’s operating point, speaker load and required output power. The tube name alone is insufficient. Hammond explicitly makes its tube suggestions conditional on the circuit, bias and operating conditions. ^[3]

1. What the transformer does

In a conventional transformer-coupled push-pull stage, two output tubes receive opposite-phase drive. Their plates connect to opposite primary ends, with the center tap feeding the supply voltage, B+. The secondary delivers the signal to a much lower-impedance speaker load. Both tubes conduct throughout the cycle in Class A; Class AB moves toward partial-cycle conduction at larger signals.

Balanced idle currents produce opposing DC magnetization in the core. The windings still carry those currents, so copper heating remains a design constraint. A conventional single-ended transformer must accommodate unbalanced DC; substituting a push-pull unit solely because its impedance matches is inappropriate. ^[5]

Figure 1. Original simplified illustration of the conventional center-tapped arrangement. Filament supplies, bias circuits and driver are omitted. See [5] for manufacturer wiring examples.

2. 2A3 and 300B: electrical differences

Both are directly heated power triodes, but their electrical ratings differ. This comparison uses the RCA 2A3 sheet dated October 15, 1947 and Western Electric’s published 300B specifications. Check the exact manufacturer and variant before applying these values. ^{[1], [2]}

Selected tube data; not a push-pull operating recipe
Parameter	RCA 2A3	Western Electric 300B
Filament supply	2.5 V / 2.5 A	5.0 V / 1.2 A nominal
Maximum plate dissipation	15 W	40 W
Published characteristic point	250 V; −45 V grid; 60 mA	300 V; −61 V grid; 60 mA
Plate resistance at that point	800 Ω	700 Ω

The Western Electric average-characteristic point above uses a 5.0 V AC filament supply. Preserve the manufacturer’s filament reference convention when interpreting plate and grid voltages; DC filament operation with a different return connection requires the corresponding data-sheet adjustment. ^[2]

RCA’s ratings use its design-center convention; Western Electric labels its limits as non-simultaneous. Plate resistance is a tube characteristic, not the required transformer primary impedance. Similarly, 15 W or 40 W plate dissipation describes heat in the tube, not speaker output. The differing filament requirements alone rule out a simple 2A3-to-300B tube swap. ^{[1], [2]}

3. Understanding plate-to-plate impedance

Raa denotes the load across the complete primary, from one plate connection to the other. For an ideal transformer:

Raa = (Np / Ns)² × Zspeaker

Np is the full-primary turns count; Ns is the turns count of the selected secondary connection. For a resistive 8 Ω load and Raa = 5,000 Ω, the ratio is √(5,000 / 8) = 25:1. This agrees with the 5 kΩ/8 Ω ratio in Monolith’s BA-8/5K sheet. ^[6]

Figure 2. Calculated impedance reflection, not a measured frequency-response plot. Moving to the correctly rated 4 Ω connection changes the turns ratio and restores the intended primary load.

Half the primary has half the turns, so its isolated reflected impedance is Raa/4. That is different from the active load-line slope of each tube while both tubes contribute signal: the ideal balanced Class-A value is Raa/2. When only one tube conducts, it becomes Raa/4; Class-AB analysis must account for the transition. ^[5]

For 5 kΩ plate-to-plate, these values are 2.5 kΩ and 1.25 kΩ respectively. Do not label a 5 kΩ center-tapped primary “2.5 kΩ + 2.5 kΩ” as though impedance simply followed turns. Specify the full plate-to-plate value. These calculations assume ideal coupling and omit winding losses.

4. Manufacturer selection examples

RCA 2A3: two Class-AB1 operating examples

RCA’s October 15, 1947 sheet gives these selected conditions for two tubes. The examples use different bias arrangements, loads and distortion levels. ^[1]

RCA Class-AB1 examples — selected data, not a complete amplifier design
Parameter	Fixed bias	Cathode bias
Plate voltage	300 V	300 V at zero signal
Total idle plate current, two tubes	80 mA	80 mA
Plate-to-plate load	3 kΩ	5 kΩ
Power output	15 W	10 W
Total harmonic distortion	2.5%	5%

The quoted current is for the pair, not each tube. The 15 W example uses 3 kΩ; it does not establish 15 W at 5 kΩ. These tube-stage figures are not measurements of either the Hammond or IWISTAO transformer. ^[1]

For 2A3: Hammond lists the 1615 for two-tube push-pull service, including 2A3: 5,000 Ω center-tapped primary, 15 W audio rating, 100 mA maximum DC per side, and 4/8/16 Ω secondary connections. This is a documented candidate to evaluate, not proof that every 2A3 amplifier will produce 15 W. ^[4]

For 300B: Monolith lists B-8/5K, B-8/6K6 and B-8/8K for push-pull 300B applications. Those 5, 6.6 and 8 kΩ alternatives demonstrate why “the correct 300B impedance” needs a specified circuit. ^[7]

The separate BA-8/5K AmorphCore sheet describes 5 kΩ primary loading and a typical 400 V, 20 W Class-A1 application. Treat this as a manufacturer application example: the cited passage does not supply a complete bias and driver design. It is not a universal 300B output-power guarantee. ^[6]

Choose the operating point first. Then evaluate candidate loads against tube curves, drive capability, dissipation and the intended distortion limit.

5. IWISTAO 40 W / 5 kΩ: a practical product example

The IWISTAO WHFTR-PPOPT5K push-pull output transformer pair puts the preceding selection criteria into context. It is listed for 300B and 2A3 push-pull projects requiring a 5 kΩ plate-to-plate load. Evaluate it against your circuit’s operating point and power requirements. ^[10]

IWISTAO WHFTR-PPOPT5K push-pull output transformers shown on the product listing

Figure 3. Product photograph from IWISTAO’s WHFTR-PPOPT5K listing. The package is offered as one pair. View product details.

WHFTR-PPOPT5K — seller-published specifications, checked September 20, 2026 ^[10]
Parameter	Listed specification
Transformer type / package	Push-pull output transformer / one pair
Rated power	40 W PP
Primary impedance	5 kΩ plate-to-plate
Secondary connections	0–4–8 Ω (common, 4 Ω and 8 Ω)
Primary inductance	35 H across P1–P2, as listed by IWISTAO; measurement conditions not specified
Frequency response	28 Hz–39 kHz (−1 dB)
Core	86 × 50 specification; grain-oriented silicon steel
Winding / treatment	Oxygen-free copper enamelled wire; sectional/interleaved construction; controlled drying and vacuum varnish impregnation
Overall dimensions	88 × 100 × 75 mm
Component weight	Approximately 2.4 kg each; 4.8 kg per pair

IWISTAO’s specification table lists 35 H, while its feature list says “up to 35 H.” Treat this as a seller-listed value, not a guaranteed minimum inductance. ^[10]

How to apply these specifications

The 5 kΩ primary matches the worked impedance example above: the ideal full-primary ratio is 25:1 for 8 Ω and approximately 35.36:1 for 4 Ω. These are calculated ratios, not winding measurements of this product. Use the corresponding speaker connection to retain the intended nominal primary load.

The 40 W transformer rating does not promise 40 W from a pair of 2A3 or 300B tubes. Amplifier output still depends on operating class, plate voltage, bias, available drive and the chosen distortion limit. A lower-output amplifier can use a higher-rated transformer when its other specifications suit the circuit.

6. Power, bass and bandwidth

A power rating needs a frequency and test conditions. Hammond specifies its 1608–1650 series response at full rated power: 30 Hz–30 kHz, within ±1 dB relative to 1 kHz. That statement does not establish the same output capability at 20 Hz. ^[3]

Monolith’s BA-8/5K sheet separates a −3 dB bandwidth of below 2 Hz to 175 kHz at 1 W with the secondary grounded from a core-saturation point of 21 Hz at 30 W RMS. Neither number is a 30 W, 2 Hz low-distortion specification. The document also identifies its case layout as preliminary. ^[6]

For a calculated ideal example, 20 W into 5 kΩ requires √(20 × 5,000) ≈ 316 V RMS across the full primary. A balanced half-primary then carries approximately 158 V RMS of signal. Actual design must allow for losses, the DC operating point and available tube swing.

Ask suppliers for measured distortion at your lowest required frequency and power, plus the source impedance and load used. Small-signal extension is useful, but it cannot substitute for a full-power bass test. Compare transformers under matching conditions.

7. DC balance and winding details

DC carrying capability and tolerance of unequal DC currents are different specifications. A rating of 100 mA per side does not permit 100 mA of imbalance. Nor are all push-pull cores completely ungapped: Lundahl’s cited family uses a small 25 µm gap for limited imbalance. ^{[4], [5]}

Confirm actual current balance after warm-up using the amplifier maker’s service procedure. Tube matching helps establish suitable pairs, but bias still requires attention; Western Electric distinguishes adjustable fixed bias, non-adjustable fixed bias and cathode bias when assessing compatibility. ^{[2], [8]}

Winding construction also matters. Lundahl explains that sectioning can reduce leakage inductance and that winding arrangement affects capacitive coupling. Request frequency and phase-response evidence instead of judging performance from core material or transformer weight alone. ^[9]

Follow the exact connection drawing. Hammond notes that both secondary windings must be engaged in its cited series to meet specifications. The 2A3 and 300B have no screen grid; unused ultralinear screen taps on a compatible transformer require individual insulation, not a connection to the control grid. ^{[1], [2], [3]}

8. A practical ordering checklist

For a supplier inquiry, provide:

Circuit: exact tube make and variant, tubes per channel, push-pull or parallel push-pull, Class A/AB, and bias method.
Operating point: plate-to-filament reference voltage, idle current per tube, B+ and available driver swing.
Load: required plate-to-plate impedance and speaker connections.
Performance: output-power target, lowest full-power frequency, distortion limit and intended feedback arrangement.
Construction: DC ratings, allowable imbalance, insulation ratings, connection drawing, dimensions and mounting requirements.

Use the supplier’s response to resolve missing specifications before ordering. For replacement work, retain the amplifier maker’s required load and verify stability after any transformer change. Tube equipment contains hazardous voltages; internal measurements and wiring belong with a qualified technician. ^[8]

Frequently Asked Questions

Can one output transformer work with both 2A3 and 300B?

Possibly, if its primary load, power, DC and insulation ratings suit both complete circuits. Sharing a nominal impedance does not establish compatibility. ^[3]

Will the IWISTAO 40 W transformer make my amplifier deliver 40 W?

No. The listing’s 40 W value is a transformer rating. Tube operating points, drive, supply and distortion limits determine amplifier output; confirm the transformer’s rating conditions with IWISTAO. ^[10]

Can I connect a 4 Ω speaker to an 8 Ω connection?

In the ideal 5 kΩ/8 Ω example, this reflects only 2.5 kΩ to the full primary. Use the specified 4 Ω connection unless the amplifier manufacturer authorizes another load.

Is a small-signal bandwidth figure enough for bass selection?

No. Request full-power low-frequency capability with a distortion criterion. The Monolith example explicitly gives different conditions for bandwidth and saturation. ^[6]

Can I use a single-ended transformer in a push-pull circuit?

Do not assume interchangeability. The winding connections, core gap and DC specification must support the intended circuit; select the appropriate manufacturer configuration. ^[5]

Shop IWISTAO 40W 5K 1 Pair Push-Pull Tube Amplifier Output Transformer for For 300B 2A3 PP Amplifiers →