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Power Transformers for 2A3 Tube Amplifiers: Voltage, Filaments, and Sizing
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posted by Vincent Zhang
A practical guide to B+, rectifier choice, 2.5 V filaments, and transformer loading for a stereo single-ended design.
A useful 2A3 power-transformer specification starts with the amplifier schematic: operating point, bias arrangement, rectifier, filter, and filament supply. This guide uses a stereo single-ended amplifier with one 2A3 per channel as its worked example. Its calculations are design illustrations, not a tested amplifier or a universal purchase specification.
1. Start with the tube and circuit
RCA specifies a 2.5 V, 2.5 A filament and lists a single-tube Class-A1 example at 250 V plate voltage, −45 V grid bias, and 60 mA plate current. The stated output is 3.5 W into a 2,500 Ω load. These are tube operating conditions; the mains transformer supplies the rectifier and heaters, while the separate output transformer matches the tube to the loudspeaker. [1]
Check the exact tube version. Sophia Electric specifies 15 W maximum plate dissipation for its listed 2A3; JJ specifies 40 W for its 2A3-40. Neither figure is the amplifier’s audio output, and the JJ rating must not be applied to every 2A3. [2], [3]
Figure 1. Original functional diagram based on references 1, 6, and 8. *Ratings are at each 2A3 filament; DC supply input windings need separate sizing. Branches denote separate windings, not electrical connections.
2. Calculate the required B+
With cathode bias, plate-to-filament voltage differs from plate-to-ground voltage. For this illustrative design, assume 250 V from plate to the effective filament midpoint, a +45 V filament midpoint, and 20 V lost in the output-transformer primary at the selected current. Then the supply feeding that primary needs:
B+ = 250 + 45 + 20 = 315 V DC
The 45 V term corresponds to the RCA bias example; the 20 V term is our assumption, equivalent to approximately 333 Ω at 60 mA. Replace it with the chosen output transformer’s winding resistance and actual current. Any upstream choke or dropping-resistor loss must be added separately. Fixed-bias designs require their own voltage budget. [1]
3. Match voltage to rectifier topology
All AC voltages below are RMS. A two-wire winding feeding a bridge and reservoir capacitor has an ideal peak of approximately √2 × VAC. A conventional two-diode, center-tapped supply uses each half winding alternately: 300-0-300 V means 600 V end-to-end, with a roughly 424 V ideal capacitor peak. A bridge across both outer ends instead sees 600 V and a roughly 849 V ideal peak. These are calculated peaks before losses, not promised operating B+. [4]
Figure 2. Original voltage comparison calculated from RMS-to-peak conversion and the rectifier arrangements in reference 4. Actual no-load voltage may be higher if the winding is rated at full load.
Choke input gives a different result. Sowter’s guide implies approximately 0.90 × VAC for a bridge with a true choke-input filter. The choke must maintain the required conduction conditions; light-load behavior needs checking. Choose the topology before ordering the winding, and allow for rectifier and filter losses. [5]
For a valve rectifier, use its own curves and limits. JJ’s 5U4GB requires a 5 V, 3 A heater and lists a 40 µF capacitor-input limiting value. That value does not replace checks on source resistance, surge current, or the specified operating conditions. Its heater supply also needs insulation suitable for its potential relative to ground. [8]
3.1. The 290-0-290 V winding option
290 V–0–290 V AC (580 V CT) is another high-voltage winding option to evaluate. Each half supplies 290 V RMS relative to the center tap. In a conventional two-diode CT rectifier with a reservoir capacitor, the calculated ideal peak is 290 × √2 ≈ 410 V, about 14 V below the 300-0-300 V case at the same primary conditions. This is a calculated comparison, not a measured B+ specification.
| AC winding | Capacitor input: ideal peak | Choke input: ideal average* |
|---|---|---|
| 290-0-290 V (580 V CT) |
≈410 V | ≈261 V |
| 300-0-300 V (600 V CT) |
≈424 V | ≈270 V |
*The choke-input estimates use approximately 0.90 times the half-winding RMS voltage and assume continuous conduction. Real voltages depend on load, line voltage, rectifier drop, winding regulation, and filter resistance. [5]
For the worked circuit’s 315 V / 135 mA DC target, treat 290-0-290 V as a candidate for a capacitor-input supply with the selected rectifier and downstream filter. The roughly 95 V difference between 410 V ideal peak and 315 V must be accounted for by the complete loaded supply model. A conventional choke-input supply at approximately 261 V before losses would fall below this example’s B+ target. Retain the separate filament and heater requirements, and ask the transformer supplier to confirm sufficient winding capacity for the intended 135 mA DC load under the specified rectifier/filter conditions.
Use the exact valve rectifier’s curves to determine its loaded output. [8] A bridge across the full 580 V winding would instead have an approximately 820 V ideal peak; it is a different supply arrangement, not a direct substitute for the CT connection. [5]
4. Choose AC or DC filament supplies
Two conventional 2A3 filaments consume 2 × 2.5 V × 2.5 A = 12.5 W. With direct AC heating, a separate 2.5 V winding for each tube makes individual bias referencing practical. Specify 2.5 V across the complete filament: a center-tapped 2.5 V winding is 1.25-0-1.25 V. Verify the voltage at the socket under load. [1]
The 2A3 is directly heated, so its filament supply participates in the cathode circuit. DC heating can help address filament-related hum, but supply grounding and ripple still matter. TentLabs’ documented DC arrangement uses one floating supply and a separate transformer winding per tube, referenced according to the bias circuit. Follow the selected supply’s instructions rather than grounding its negative terminal by habit. [3], [6]
A regulated 2.5 V DC output generally needs a different AC input specification. Rectifier drops, ripple valleys, and regulator dropout consume voltage headroom. For illustration, a linear pass element dropping 5.0 V DC to 2.5 V at 2.5 A dissipates approximately 6.25 W per tube, excluding other losses. Select the AC winding and heatsink from the regulator’s requirements; this calculation is not a recommendation for a particular input voltage. [7]
5. Build a current and VA budget
Assume two output tubes at 60 mA each, 10 mA total for drivers, and a 5 mA bleeder. The illustrative B+ load is 135 mA DC. A push-pull Class-AB amplifier needs a separate maximum-signal current calculation; multiplying its idle current is insufficient.
| Load | Calculation | Result |
|---|---|---|
| Output tubes | 2 × 60 mA | 120 mA DC |
| Drivers + bleeder | 10 + 5 mA, assumed | 15 mA DC |
| Total B+ load | 120 + 15 mA | 135 mA DC |
| Bridge-capacitor winding current | 135 × 1.61 | ≈217 mA RMS |
| Direct AC 2A3 filaments | 2 × 2.5 V × 2.5 A | 12.5 VA |
The bridge-capacitor conversion uses Sowter’s approximate 1.61 current factor. A 250 mA RMS winding would provide about 15% above that preliminary estimate, subject to supplier approval for the actual waveform and thermal conditions. This factor does not transfer unchanged to center-tapped or choke-input supplies. [5]
For a two-wire secondary, winding VA is VRMS × IRMS. An assumed 250 V, 0.25 A winding therefore contributes 62.5 VA; adding direct AC output filaments gives 75 VA before driver heaters and any valve-rectifier heater. This arithmetic is a partial budget, not proof that 250 V AC produces the target 315 V B+. Agree total simultaneous loading and rating conventions with the manufacturer.
6. Read a real transformer specification
Hammond lists the 302AX as a 198 VA model with a 600 V CT high-voltage secondary at 230 mA, two separate 2.5 V CT / 2.5 A windings, a 5 V CT / 3 A winding, and a 6.3 V CT / 6 A winding. Its 300 Series supports specified universal primary connections and 50/60 Hz operation. [9]
This is an example of a useful winding set, not a verified match for our worked circuit. Its 600 V CT secondary means 300-0-300 V; the loaded B+ still depends on rectification and filtering. Confirm how the supplier defines its high-voltage current rating. The 2.5 V windings suit direct AC filament ratings; they do not establish compatibility with an arbitrary regulated DC module.
7. What to specify when ordering
- Primary: actual mains voltage, frequency, connection drawing, and permitted line range. For a 50 Hz installation, require an explicit 50 Hz or 50/60 Hz rating. [9]
- High voltage: full or half-winding voltage, load at which it is specified, CT arrangement, and current-rating convention.
- Filaments: AC heating or exact DC module, independent windings, driver-heater loads, and rectifier-heater requirements.
- Construction: dimensions, mounting, insulation requirements, intended ambient temperature, and simultaneous continuous loads.
Ask for regulation data and an agreed thermal limit. Core shape, potting, and an “audio grade” label alone do not demonstrate acceptable hum or temperature in your chassis.
8. Verify the finished supply
Have a qualified technician verify wiring, protective earthing where required, fusing, insulation, capacitor discharge provisions, and safe access before energizing. These supplies contain hazardous voltages. [6]
Record filament voltage at each socket, loaded B+, residual hum, and stabilized transformer temperature with the intended enclosure and loads. Check startup and high-line/light-load voltage against component ratings: transformer regulation can raise capacitor voltage above its loaded value. [4] Treat these measurements as acceptance evidence; this article makes no measured noise, temperature, or audio-performance claim.
Frequently Asked Questions
Is one universal transformer suitable for every 2A3 amplifier?
No. Tube count, operating point, bias method, rectifier, filter, mains frequency, and filament supply determine the winding specification.
Does 2.5 V / 5 A replace two separate 2.5 V / 2.5 A windings?
It provides the same nominal filament VA, but ties the filaments to one electrical supply. Use it only if the schematic permits the shared bias reference; independently biased or floating supplies may require separate windings.
Can I rectify a 2.5 V AC winding to obtain regulated 2.5 V DC?
Do not assume so. Check the ripple valley after rectification, diode losses, regulator dropout, and RMS winding current against the chosen module’s documentation. [4], [7]
Is a larger VA rating always better?
It can provide loading margin, but voltage regulation, physical size, inrush, and actual circuit voltages still need assessment. Specify winding duties rather than selecting solely by total VA.
Can a power transformer upgrade guarantee better sound?
No. Validate the resulting supply voltages, hum, and temperature in the amplifier. Transformer specifications alone do not establish an audible improvement.
Find More
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References
- RCA, 2A3 manufacturer data sheet, historical scan (pp. 51–52). https://frank.pocnet.net/sheets/021/2/2A3.pdf
- Sophia Electric, 2A3 Vacuum Tube Specification. https://sophiaelectric.com/pages/pages-se-2a3-specification
- JJ Electronic, 2A3-40 data sheet. https://www.jj-electronic.com/images/stories/product/power_tubes/pdf/2a3-40.pdf
- Hammond Manufacturing, Design Guide for Rectifier Use. https://www.hammfg.com/pdf/5c007.pdf
- Sowter, How to Calculate Your Mains Transformer HT Voltage. https://www.sowter.co.uk/rectifier-transformer-calculation.php
- TentLabs, Directly Heated Tubes Supply: application and safety. https://www.tentlabs.com/Products/Tubeamp/Tubefilament/index.html
- Analog Devices, AN-140: Basic Concepts of Linear Regulator and Switching Mode Power Supplies. https://www.analog.com/en/resources/app-notes/an-140.html
- JJ Electronic, 5U4GB data sheet. https://www.jj-electronic.com/images/stories/product/rectifying_tubes/pdf/5u4gb.pdf
- Hammond Manufacturing, 300 Series: primary options and 302AX winding table. https://www.hammfg.com/electronics/transformers/classic/300
Sources checked October 26, 2026. Diagrams and worked calculations are original explanatory material; no product-specific performance testing is claimed.
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