Formula Used
The calculator uses practical transformer planning formulas. They are estimates for early design and comparison.
- Turns per volt = 10000 / (4.44 × frequency × flux density × core area)
- HV VA = desired B+ voltage × B+ current × rectifier VA factor
- Heater VA = heater voltage × heater current
- Total secondary VA = HV VA + heater VA + rectifier heater VA + bias VA + auxiliary VA
- Input VA = total secondary VA / efficiency
- Design VA = input VA × design margin factor
- Primary current = input VA / primary mains voltage
- Required window area = sum of turns × wire area / fill factor
How to Use This Calculator
- Enter your mains voltage, frequency, and low line allowance.
- Add your desired loaded B+ voltage and expected B+ current.
- Select a rectifier and filter style.
- Enter all heater, bias, and auxiliary winding loads.
- Add core size, window size, flux density, and current density.
- Press Calculate to view VA, turns, wire, core, and window checks.
- Use CSV or PDF download for records.
Example Data Table
| Input Item | Example Value | Purpose |
|---|---|---|
| Mains voltage | 230 V | Primary winding voltage |
| Frequency | 50 Hz | Turns per volt calculation |
| B+ voltage | 350 V DC | Loaded tube plate supply |
| B+ current | 120 mA | High voltage load current |
| Main heater load | 6.3 V at 4 A | Tube heater winding load |
| Core area | 16 cm² | Magnetic core sizing |
| Flux density | 1.1 T | Core operating limit |
| Design margin | 20% | Thermal and build allowance |
Tube Power Transformer Planning
A Careful Start
A tube power transformer feeds every high voltage and heater rail in a valve amplifier. It must handle steady load, warm up stress, ripple current, insulation limits, and line variation. A small mistake can cause heat, hum, sag, or failure.
Why VA Matters
VA is the main sizing value. It combines voltage and current for each winding. The high voltage winding needs extra allowance because rectifiers draw pulses. Heater windings are easier. Their VA is voltage times current. Bias and auxiliary windings also add load. The total load is then divided by efficiency. A design margin is added for safer operation.
Core And Turns
The core area, flux density, and frequency set the turns per volt. Lower frequency needs more turns. Higher flux density reduces turns, but raises heat and hum risk. Conservative tube equipment often uses moderate flux. The calculator estimates primary turns, secondary turns, and an empirical EI core VA check. These values are useful for early design, comparison, and rewinds.
Copper And Window Space
Current density controls wire size. Lower current density needs thicker wire and more window space. This lowers copper loss, but increases build size. The window check estimates the winding area needed after fill factor. Real winding space also depends on insulation, bobbin thickness, tape, lead exits, and winding method.
Rectifier Effects
A capacitor input supply usually needs more transformer VA than a choke input supply. Current flows in shorter peaks. Tube rectifiers also drop voltage. Solid state rectifiers drop less voltage, but can raise no load B+. The calculator lets you compare these conditions before choosing a secondary voltage.
Practical Use
Use this tool for planning, not final safety certification. Enter realistic currents from the tube data sheets and circuit design. Add all heater currents, including pilot lamps. Choose a margin when the amplifier may run hot or continuously. After building, verify temperature rise, output voltage, insulation rating, fuse size, grounding, and mechanical clearance. Work on tube supplies only with proper training. Stored capacitor energy can remain dangerous after power is removed. Keep notes from each build. Compare estimates with measured values. This improves future choices and helps spot wiring or loading errors before damage occurs during early testing.
FAQs
What is a tube power transformer?
It is the mains transformer that powers tube circuits. It usually provides high voltage B+, heater voltage, rectifier heater voltage, bias voltage, and sometimes auxiliary supplies.
Why is high voltage VA multiplied by a rectifier factor?
Rectifier supplies do not draw smooth current. Capacitor input supplies draw short current pulses. This increases transformer heating, so a VA factor gives a safer planning estimate.
What flux density should I use?
Many conservative tube designs use moderate flux density. Lower values need more turns and copper. Higher values may increase heat, hum, and saturation risk.
What does window fill factor mean?
It is the usable portion of the core window for copper. Insulation, bobbins, tape, and winding gaps reduce the real space available for wire.
Does this calculator replace transformer standards?
No. It is a planning tool. Final transformer work must consider insulation class, creepage, clearance, fusing, temperature rise, and applicable safety rules.
Why does low line current increase?
When mains voltage drops, the transformer must draw more primary current to supply similar output power. This can increase heating under poor line conditions.
Can I use this for guitar amplifiers?
Yes, it is useful for early guitar amplifier planning. Enter real tube currents, heater totals, rectifier type, and a margin suitable for stage use.
Is the PDF download library free?
This file creates a simple PDF directly. It does not require an external library. The report is basic, but useful for saved calculations.