Tube Output Transformers – that tube sound
Output Transformers - why we need them?
Tube output transformers for many is THE definition of THAT tube sound. Many of the tube junkies out there are amateur radio hams or have been in the industry for long enough to have collected a small arsenal of tube amplifier parts for RF and AF. Over the years they have garnered enough knowledge and wisdom to to know which parts are collectables (rare), commonly used and for audio or modulation purposes, matching transformers.
Matching transformers are by no means mystique, they can be hand wound if one knows the parameters and follows solid working practices. A matching transformer in our article will apply to audio output transformers, expensive and/or time consuming to hand wind.
Z and Turns Ratio
An output transformer has no impedance as such, it is the reflected load impedance on the secondary winding on the primary winding.
A matching output transformer on a single ended power output tube.
The Tube: Output impedance
Although it is out of the scope of this article to cover load lines there are some interesting aspects to tube parameters which one must be aware of. But first, courtesy Mullard/Phillips we have some graphs, figure 1. showing 5.2kΩ load and figure 2. push-pull with anode to anode load 8kΩ. The spec sheet is obtainable here for the EL84 or 6BQ5.
Pentode EL84 with load 5.2kΩ
Pentode EL84 in Push Pull – Load anode to anode 8 kΩ
Looking at the graphs above and in the spec sheet we become overwhelmed at the amount of criteria given, electrode voltages vs current, the effects of load on gain, the effects of electrode current vs voltage, gain in triode mode versus pentode and distortion figures in triode, pentode and push pull mode.
For our purposes and those servicing tube equipment there are three main values which we need to consider:
There are some values which need to be known, called explicits:
- gm or transconductance the amount of current through the tube / plate voltage measured in mSiemens (previously mho)
- rp, the dynamic plate (anode) resistance and
- mu, the product of these two values, the implicit value: the voltage gain.
Tubes derive their amplification factor then by mu = gm * rp
As gm increases in value, rp decreases proportionally.
Video on You Tube by David Beard: Vacuum Tube Gain & Gm Calculations & Some Insight Into Transformer Inductance and Impedance
It is significant to note that voltage gain or µ or mu is related more to triode operation, gm more to Pentode operation when checking tube performance.
Operating Points
If one takes a common and garden step down transformer which is rated at 240V to 12V we know that the turns ratio is 20:1. If one knows that the output load impedance is 8 Ohms and turns ratio is now 20²:1² or 400:1 , the impedance reflected on the primary is 400 * 8 or 3 200 Ohms.
One of the most commonly asked questions by newbies to the tube arena is why a tube needs an output transformer, the most expensive part to an audio amplifier and secondly, is there an alternative solution?
To answer this properly one needs to understand the principles of tube operation and how and where the tube is used. For instance a tube can be used in many configurations, the most frequent being “common cathode” whereby the input and output signals have one common signal path, the cathode. Shown below, the common cathode has a very high input impedance and medium to low output. We have the common grid, used for RF and the common anode or plate (also known as cathode follower) which offers very high input impedance and low to medium output impedance. In tube and audio amplification the common cathode and cathode follower are the most popular.
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Loudspeaker voice coils have a very low impedance, possibly 3 to 16 Ohms which is driven by high currents, much more than the tube can produce by driving the load through a capacitor.
Specifications of most tubes are easily obtainable on the internet and in audio work tubes rarely draw more than 100mA of plate current which for our purposes would be termed relatively high power if the plate or anode is sitting at 400V. (40W). The EL84 is designed to handle 48mA max at 250V dissipating 12W or in the real world, 6W plate dissipation in class A mode. (typical pentode operation using an EL84). There is an internal resistance Ri
Heater currents are high though and once you get to grips with the dynamics of tube audio one would quickly realise that these beasts are very inefficient.
Although one can find tube load resistances through the manufacturer data sheets bear in mind that tubes are also used in single ended triode mode, parallel, push-pull, and parallel push-pull which means a variety of transformers are used, matched for a particular design and purpose.
For our purposes we will be looking single tube pentodes in Class-A and push-pull class B (or AB).
Just for the record:
Something else to consider:
The pentode came about because of (a) Miller effect, the capacitance between grid and anode of the triode, hence the fourth electrode, the screen grid and (b) the secondary emission in a tetrode lead to the suppressor grid.
The fourth electrode, the screen grid also has an affect on the amplification of the tube – screen grid voltage variation causes anode current variation.
If one had to look at the general specifications of a tube, for instance the pentode EL84, pinout as below:
EL84 – looking at bottom of tube.
Anode voltage = 250V, anode current 48mA , peak plate power 12W, class A 6W.
Furthermore, Rp or dynamic plate resistance is 38kΩ and the gm 11mA/V or 11mHo (now in Siemens or here, milliSiemens). Remember that the gm and Rp are explicits and vary according to operating point of the tube.
How, why and what?
If one needs to know what the load impedance needs to be, i.e. the transformer primary impedance.
This is found in the specs manual, usually the load resistance for a set configuration. In Class A, if the anode voltage is 250V and max plate current is 48mA this would be in the vicinity of 5000Ω (V/I)
In order to have the same gain on more than one channel the mu is important, as is dynamic plate or anode resistance.
In audio work we would usually find a Pentode at the output stage such as the EL84 or bigger brother EL34.
The values below are given by spec sheet from Mullard.
At plate voltage (va) of 250V and Ia = 48mA peak available power = 12W, dissipates 6W in Class A. (Some sources give peak 300V at 64mA but power dissipation must never be higher than 12W).
The gm = 11mA/V. (as in Ohm’s law, resistance = V/I the mho or now Siemens is the reciprocal or I/V).
Max ig2 or screen grid = 5.5mA to 11.2mA at 250V
The g1 or control grid voltage = -7.3V (pentode operation).
The rp = 38kOhm (Internal resistance)
As Pentodes are often used in the output stage the impedance of the output transformer now plays a role in getting the mu, so in Pentode operation with output transformer the gm is more relevant in matching output devices.
Without re-writing the internet, excellent write up on the augustica.com pages to calculate EL34 gain in a push pull circuit showing transformer primary Z.
Output Transformers – slicing the budget
Getting to the meat of the article an oft asked question is “can a mains transformer be used as an output transformer for a tube amplifier?”
Well, the simple answer is yes and if good reproduction quality is not the objective repair personnel often have good results in either winding their own using air gapped EI mains transformers or toroidals. (the last thing one wants is a magnetised core). A very good write-up on audio transformers is to be found on the Lenard Audio website.
A standard rule of thumb with mains transformers is that a 50Hz transformer is better at handling 60Hz than a 60Hz transformer at 50Hz. (in fact its something that you should not do – if you HAVE to do it you need to derate the transformer input supply by 50/60). So the more bulky an output transformer the more likely the design was for getting to frequencies below 50Hz, possibly 20Hz , often seen in output transformers with a larger core than the mains transformers. The problem here is in heat generation.
I will get back to Output Transformers in a follow up January 2026 sometime.