Output transformer alternatives – the long and the short
This is a topic often coming up in audio circles and yes, we can use a mains transformer to drive a speaker and no, it’s not a drop in replacement.
Getting down to the basics, a tube audio amplifier comes in two flavours – high fidelity, getting the best out of vinyl and secondly, dedicated guitar amplifiers which serve one purpose – distortion. I am of the brigade which believes that one gets what one pays for, a good quality audio amplifier relies on an exceptional output transformer. Tube amplifiers designed for distortion do not rely on huge, bass performing quality transformers. They do however need to be reliable.
A single tube in the output section of an amplifier will be drawing a standing current which eliminates distortion so as to amplify the total input signal. This standing current when passed through a “standard” mains transformer will cause early saturation of the core and produce heat and distortion. Experimenters have used a second secondary winding as a DC injection circuit to offset the standing current. Look at further reading, DIYaudio below.
Mains transformers are wound secondary on primary (bobbin of EI core), the audio output transformer, inter-leaved, secondary over primary over secondary etc. This improves frequency response. Typical section layout is five primary and four secondary.
Mains transformers are designed to perform best at the mains frequency. A 50Hz transformer will operate at 60Hz but depending on core size a 60Hz transformer design will overheat at 50Hz. Audio transformers are designed to perform well over the hearing spectrum, e.g. 20Hz to 20kHz. Large transformers are required for good bass response. Mains transformers use thicker lamination.
Mains transformer winding insulation breakdown voltage is lower than audio output transformers, one factor being the interleave design in output transformers with lower voltage higher current windings situated closer to the high voltage low current primary. Tube amplifiers are prone to transformer flashover under no load conditions.
The input impedance of an audio transformer is a reflection of load impedance on it’s output. Multi-tapping is very important to create proper matching.
The Common Cathode
A quick word on mu, gm and ra
The triode audio amplifier in common cathode mode such as in figure 1 exhibits high input impedance and medium to low output impedance. Load-lines and operating points in this configuration are given in the spec sheets of the manufacturers manual. (likewise pentode operation in the case of pentodes, beam tetrodes etc).
In the case of a triode, mu or voltage amplification is a product of the transconductance or gm and plate or anode resistance, ra. Transconductance is a calculation based on anode current change over a change of anode voltage or delta Ia/delta Va. The unit for transconductance is the reciprocal of the Omega sign, Ω, the ℧ in mhos, now millSiemens.
If in the spec of a tube we come across the ideal load impedance as being 5.2kΩ we could by manipulation determine which mains transformer would be ideal. In this case if a load impedance of 8 Ohms is desired we use sqrt(5.2kΩ/8Ω) or a transformer with turns ratio 25:1. If the mains supply is 230V RMS we are looking at an output voltage of 9V RMS. In a push-pull amplifier as below, figure 3, we would be looking at 460V-230V-0V primary if the required load was to be 5.2kΩ per tube anode. This is not a common transformer by any means, 230V-115V-0V primary being the more common. The requirement therefore would be a transformer with 4.5V output.
Experimentation users stick to the 220-240 primary and 9 – 12 secondary EI and toroids. They are common and fairly cheap.
In the circuit above the output transformer could therefore be a 230/115 mains transformer if the primary was designed for 230V / 115V input 30VA. and 4.5V * 2 secondary BUT as mentioned before, there is a trap.
There is a standing current through the tubes which would pass through the primary winding, causing early saturation of the core. DIYer types strip the transformer and pull the I lamination apart a very short distance from the E core to create an air gap. Likewise this could be a gap in a C-core transformer. In both cases this reduces core saturation.
In the author’s view if one were to strip the transformer it would be relevant to rather re-assemble with an interleave on the bobbin than reinstall the secondary over primary design.
Look for old tube sets, often the output transformer is still OK. Possibly apply a vacuum and re-resin.
Output Transformers by Hans Beijner
Dick’s Website – Coil and Inductance Calculator
Audio Transformer Design Manual – Robert Wolperd
Turner Audio – Output Transform winding
DIYaudio – tubes/valves, self nulling DC in transformation
Deer Lovers Site – Transformers