How to Test an Output Transformer
Got an old output transformer with no markings or a transformer in an amp that your are not sure about? Maybe one used for a 100v line system? Don’t toss it just yet — with a few simple tests, you can figure out what it is, how it’s wired, and whether it’s usable for your amp build. This guide walks you through the process using a multimeter, a low-voltage AC source, and some basic calculations.
Do not do this with the amp plugged in, with the power tubes in, or with the speaker connected. Remember - dangerous voltages in amps... make sure the filter caps are drained of DC before touching anything.
1. Identify the Windings
If it's in the amp, this will be simple, but otherwise, a multimeter can be used to find the different windings:
- Primary winding (plate to plate): typically has a resistance of 50–300Ω.
- Secondary winding (speaker side): usually very small, maybe even less than 1Ω.
If there's a centre tap on the primary, it's likely a push-pull transformer. The measurement between the centre tap and the two plates is half the total plate to plate value.
2. Measure the Turns Ratio
Apply a known low AC voltage (like 1V) to the secondary and measure the resulting AC voltage across the primary. Then calculate:
Turns Ratio = Vprimary / Vsecondary
Example: If you apply 1V AC to the secondary and measure 22V across the primary, then the turns ratio is 22:1.
You can also do the reverse, and apply a larger voltage across the primary (like 20v) and measure the secondaries. This is probably safer, as the AC voltage will be stepped down, not up. I normally use 15v, cause that’s heading towards the upper limit of one of my signal generators.
3. Estimate the Primary Impedance
Once you know the turns ratio, you can estimate the reflected primary impedance using:
Zprimary = (Turns Ratio)2 × Zsecondary
Example:
Zprimary = 222 × 8 = 3,872Ω
4. Check for Shorts or Opens
Use a multimeter to confirm continuity across each winding. Also make sure there's no continuity between any winding and the core or mounting hardware. If there is, the transformer may have an internal short and should not be used. Also watch out for any high values - if it reads a few meg, that’s a bad sign.
5. Check Frequency Response (Optional)
If you have a signal generator and oscilloscope, you can sweep an audio signal through the transformer and observe the output for frequency roll-off or resonant peaks. Note that a low e on guitar is about 80hz and most speakers drop off pretty fast after 5kHz. A lot of Hammond transformers intended for guitar amps list 75hz to 15kHz as their frequency response
What you are looking for here is significant changes to the size of the sine wave displayed on the scope. i.e. when the wave decreases in size, the AC voltage is dropping, meaning that the frequency response is changing by a noticeable amount. It's not as accurate as a spectrum analyser, but it's close enough for a guitar amp, considering the kind of speaker and cab that it will be hooked up to.
6. Visual Clues
- A center tap on the primary usually means it’s for a push-pull amp.
- A gapped core is a sign it’s for single-ended operation.
- Larger transformers with thicker wire usually handle more power and lower impedance loads.
Reference: Typical Load Impedances
| Tube Configuration | Typical Primary Impedance |
|---|---|
| 6V6, single-ended | 5k – 8kΩ |
| 6V6, push-pull (pair) | 8k – 10kΩ |
| 6L6, single-ended | 3.5k – 5kΩ |
| 6L6, push-pull (pair) | 4k – 6.6kΩ |
| EL84, push-pull (pair) | ~8kΩ |
| 6L6, push-pull (quad) | 1.7k – 2.2kΩ |
Tips from the Old Masters
“A 1-volt signal applied to the speaker winding should give you a good idea of the turns ratio just by measuring the voltage on the primary.”
— Jack Darr, Electric Guitar Amplifier Handbook
The Radiotron Designer’s Handbook also provides detailed transformer theory, including reflected impedance formulas, equivalent circuits, and response testing using low-voltage AC signals — all foundational concepts that support this method of transformer identification and testing.
Transformer impedance is reflected from secondary to primary in proportion to the square of the turns ratio:
Reflected Impedance = Load × (Turns Ratio)2The handbook also discusses transformer response across frequency and how to analyze bandwidth using -3 dB points, supporting AC test methods for audio applications.
Source: Radiotron Designer’s Handbook, 4th Edition
Transformers can produce dangerously high voltages, even when powered by a seemingly low-voltage AC source. The windings in tube amplifier transformers are capable of stepping voltages up or down significantly. Always assume exposed leads may carry lethal voltages, and take proper safety precautions. Use insulated tools, never work on live circuits unless qualified, and ensure your test setup is isolated and safe.
No comments:
Post a Comment