Output Transformers for Tubes
Why we need output transformers?
Output Transformers for Tubes : Many engineers attribute tube amplifiers’ unique sound to their output transformers, but these components are also costly, discouraging DIY builders. Output transformers are essential because vacuum tubes and speakers have incompatible electrical properties.
A very popular brand of output transformer is that of Hammond, head quarters being in Canada. Manufactured chiefly in Canada, the USA, Mexico, India, Italy.
Essential functions of the output transformer
Impedance Matching
Vacuum tubes have high plate resistance (typically 2,000–10,000 Ω), while speakers present low impedance (4, 8, or 16 Ω).
Maximum Power Transfer: Power is transferred most efficiently when the source and load impedances are matched.
Impedance Transformation: The output transformer converts the speaker’s low impedance to the high impedance needed by the tube, ensuring efficient power delivery.
Voltage and Current Conversion
Tubes: Produce high-voltage swings (hundreds of volts) but very low current.
Speakers: Require low voltage but high current to physically move their voice coils and create sound.
The Transformer’s Role: It acts as a step-down transformer, converting the high-voltage/low-current signal from the tubes into the low-voltage/high-current signal needed by the speaker.
DC Isolation and Safety
Blocking DC: Tubes operate with high DC plate voltages (often 300V to 500V+). If this DC current were sent directly to a speaker, it would quickly overheat and destroy the voice coil.
Galvanic Isolation: Because transformers transfer energy via magnetic induction rather than a direct wire connection, they physically isolate the dangerous high voltages inside the amp from the user-accessible speaker terminals.
Considerations in Design
- Air-Gapping: In single-ended amplifiers, the transformer always carries DC current, which can saturate the iron core and cause distortion. An “air gap” (a physical break in the core) is used to prevent this saturation.
- Frequency Response: Unlike mains transformers (optimized for 50/60 Hz), output transformers must maintain a flat response across the audio spectrum (20 Hz – 20 kHz). This requires specialized core materials and interleaving (layering) of the primary and secondary windings to reduce parasitic capacitance and inductance.
Output transformers and vacuum tubes - Paul McGowan of PS Audio
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Key differences in power transformers and output transformer
- Function: Mains PTs step down high mains voltage for circuits; OTs convert high-impedance tube signal to speaker-level, low-impedance AC.
- Impedance Matching: OTs have precise primary-to-secondary turns ratios (e.g., 2k:4, 5k:8) to match tubes to speakers; mains units don’t have this, causing severe mismatch.
- Core Design: Mains PTs use solid cores; OTs need an air gap in the primary for single-ended (SE) amps to handle DC plate current without saturating and distorting, a feature mains transformers lack.
- Frequency Response: Mains transformers aren’t designed for audio frequencies, leading to poor bass, muddy mids, and harsh highs; OTs are built for wide, flat (or shaped) frequency response.
- Power Handling: Mains transformers are for continuous power delivery, not the dynamic, varying impedance/current of an audio signal.
More information covering circuitry
An interesting aspect to tube amplifiers is the amount of DIY research and reuse of mains transformers as output transformers in tube gear. Although transformer operation remains similar for the two this is about where the resemblance disappears – mains transformers do not like any form of DC through them which causes saturation, heat and distortion. Enough should be understood about output transformers to understand the importance of air-gapping, reflected impedance, plate or anode (plate to plate) voltages, breakdown, interleave, frequency response and core size before embarking on any ambitious project.
. Because loudspeakers are low-impedance loads requiring high current to function, an output transformer is necessary to match these conflicting impedances [3, 4]. This transformer also provides critical electrical isolation between the high-voltage DC circuitry and the speaker [3]. In these stages, tubes typically operate in a common-cathode configuration, where the signal enters the first grid (𝑔1) and is tapped from the anode [5].
- Vacuum Tube Basics
- Impedance Matching in Tube Amps
- The Role of Output Transformers
- Loudspeaker Impedance Explained
- Common Cathode Amplifier Configuration
Common cathode amplifier – input to control grid, output from anode or plate. The input and output waveforms are 180 degrees out of phase.
Voltage Gain
The voltage gain of the above device, a triode, is calculated by knowing the transconductance (gm) in milliSiemens, (ex- mho) shown in the spec sheet and the Ra or anode resistance and is the product of the two. Voltage gain or mu = gm*ra.
In pentodes, such as the EL84 and EL34 other parameters need to be taken into account, the screen grid voltage being one of them. The manufacturer has spec sheets covering their tubes and will give them for all natures of operation.
The EL84
The EL84 or 6BQ5 is a very popular tube amongst DIYers and can deliver approximately 6W into a loudspeaker under the proper conditions, more popular however is the push-pull arrangement which delivers approximately 17W of power.
An EL84 push-pull audio amplifier, the +Vs being the screen grid supply, usually fed from +Va or anode supply. (Note ultra linear schematic below). The push pull amplifier requires two exactly same signals but 180° out of phase. Designing such an amplifier relies on two factors, one being the gm or transconductance of both tubes being exactly the same and of course, for good quality reproduction, a well designed and constructed output transformer.
Simple "Concertina" phase splitterdriver for push pull amplifiers
The simplest splitter known to mankind, the “Concertina”. Often found used to drive EL84s in push pull configuration.
And in Summary
The tube is a high impedance device, the anode load being a few kilo-ohms and needs to be perfectly matched to the load, the loudspeaker.
There are many circuits in the public domain which use air gapped mains transformers for the output transformer. I have not personally built an amplifier with a toroidal mains transformer in the output but have heard one rebuilt guitar amplifier with this “modification” and the reproduction was good enough.
The objective is to get a closest match turns ratio for good reproduction without the core being magnetised. The input or primary impedance is a reflection of load impedance at the secondary on the primary, in other words a mains transformer with a turns ratio of 20:1 would yield 12V at the secondary if 240V was to be applied to the primary. In impedance terms an 8 Ohm load would reflect 20²:1² (impedance follows square law) or 20² * 8 or 3.2 kilo-ohms at the primary.
Where a user may have damaged an output transformer and requires a quick low cost fix one would need to know firstly the manufacturer recommended anode load impedance at a specific voltage. Assuming the Mullard spec states 5.2k, the user would work on the loudspeaker load requirement and work back. Using sqrt(5200/8) if the load is to be 8 Ohms we get the result 25.495. Applying a voltage of 9V at the secondary we would get approx. 230V. Push pull requires two such loads, one for each output device. A 30W 230V 9-0-9 transformer might prove to be a good replacement but do realise that the transformer needs to be air gapped.
Mains transformers are designed for a specific frequency and core size is increased as the frequency is reduced. They can be surprisingly good at getting higher frequency responses.
Further reading:
Gamma Electronics – using air gapped mains transformers for audio output.
Lenard Audio Education – a primer on output transformers for audio
Vintage Radio – audio output stages