Bench tool
Fuse Rating Calculator
Work out a sensible primary mains fuse rating from your transformer's total VA, or build it up from each secondary winding.
From Transformer VA
If your transformer is rated in total VA (volt-amps), this is the quickest route to a fuse value.
Slow-blow (time-delay) fuses tolerate the brief inrush from transformer magnetization, tube heater cold-resistance, and filter cap charging — that's why the headroom above is modest. A fast-blow fuse in this spot would need much more headroom and likely nuisance-trip anyway.
From Individual Windings
No VA rating on hand? Build up the total from each winding's voltage and expected current draw — typically the HT (B+) secondary and one or more heater windings.
| Winding | Voltage (V) | Current (A) |
|---|
Total VA is just the sum of each winding's V×I. This treats the transformer as roughly lossless for sizing purposes, which is the same assumption most amp schematics make when they quote a single VA figure.