EML 45B. How to use this tube

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Part1) How 45B differs from the 45

The 45B tube resulted from the many conversations I had with Roger Modjeski, who unfortunately passed away in 2020. Roger spend his entire life and career on designing classical tube amplifiers, combining high output power with long tube life. This is respected as much by real professionals, as it is ignored by bad tube companies and others who are not interested in long tube life. Roger had an outstanding memory for details. He pointed out to me, the original RCA 45 tubes were intended to use at positive grid current, though this was never clearly said so in data sheets, but it be seen if you read the details. Yet this wears out the tubes faster, or can even burn the grid. His desire for a 45 tube with higher plate dissipation was always present. Another thing Roger always did, was to use tubes possibly at a higher voltage and lower current. He always thought such old data sheets seemed to be commercial, like making the use of the tubes look easier as it really is. The main "trick" with that kind of presentation, is to pretend the tubes can work nicely at lower voltage. That makes of course any design simpler and easier. But for getting maximum tube life, you need to work at lower current and higher voltage. To Roger's opinion that was so, and I fully agreed with him always. If done right, this gives more lifetime and more output power at the same time. He was very successful with this with many of his amplifiers, but the original 45 tube was already positioned in a "higher voltage, lower current" area by RCA. So they knew it, and there was little to achieve here. But today, can we not get a step further with that? Get more power and more life time out of (almost) the same tubes? The idea for a 45 tube with higher plate dissipation, and higher possible working voltage was born, in the year 2015. WIth the general larger size tubes of EML, this was just asking for it.

The RCA 45 tube was designed for speakers which produce sufficient loudness at 2 Watt. For this, two classical operating points have been in use ever since, and these can not be improved. The 1.6 Watt circuit was intended for USA radios, which typically run at 250V DC Supply Voltage. The 2 Watt circuit is running at 275 Volts, and was recommended for HiFi use. The tube output power is always specified before the transformer. Given a normal transformer loss is 6...11%, the 2 Watt circuit delivers 1.8Watt at the loudspeakers, at low distortion. Designed 85 years ago, this circuit is a perfect balance between a few difficult choices, such as output power, distortion, damping factor and lifetime of the tube. After a careful study of the old RCA data sheets, using today's computer simulation software, we have to praise respectfully the good work by the RCA company. Meaning those working points were chosen perfectly, and distortion data was published totally correct by RCA. (Much unlike the Western Electric 300B data sheets, some of the "unbelievable" working points in there are fake)

The 45B tube

The classic 45 was designed for speaker systems needing less than 2x 2Watt of RMS power. Such speakers exist, but many other still excellent speaker systems, require up to 2x 4 Watt output power. Here is where the 45B comes in. The 45B can be used such to keep the sound character of the 45 tube, but do so at higher power. For this, some carefully chosen operating points are recommended below. These will all come near to the original 45 sound, in terms of harmonics development and damping factor, using similar transformer impedance too. For this reason you will see comparable distortion figures as with the genuine working points, only with the 45B that will achieved at higher output power. Additionally, some 45B working points are recommended with relatively high transformer impedance of 9k2, which aim for lowest possible distortion, lower as the classical 45.

The 45B tube will NOT give higher output power when the amplifier is not modified. However the 45B can be used normally instead of a standard 45, and will behave identical. So the modification of the amplifier can be done at a later time if required.

Following Roger Modjeski, the best result comes from increasing the voltage and not so much from increasing the current. As a side effect, this allows us to use the existing SE transformers. With great care, Roger set up the 45B working points for this.

Distortion in percent or Decibel (dB)

In electronics, distortion can be expressed as percentage of the signal, or in dB. Using dB, is more related to human hearing, meaning a similar difference in dB, is experienced the same. This is why many technicians prefer dB. (See also Note6). As as rule of thumb it can be said, a 3dB change can be heard by anyone, but below that needs a trained ear.

The 26dB rule of thumb: -26dB of 2nd harmonics is the level at which most human ears just do not hear this distortion. So at -25dB or more, you will experience this as minimal distortion. Whereas harmonics added with a levels of -20dB or more, sounds not pleasant any more.

 

With SE amplifiers, the harmonics will be mainly 2nd harmonics. The Ratio of 2nd. harmonic to 3rd. harmonic is an amazing factor 350 with the 45 tube, and the 4th harmonic or higher is virtually not present at all. So we need to look only at the 2nd harmonic.

Distortion and (modern) Amplifier Design

Historical bias settings give us a 1.6 Watt operating point, and a 2 Watt operating point. The 1.6 Watt circuit has the advantage of a somewhat lower distortion, but it needs really a loudspeaker system which requires no more than 1.6 Watt. The advantage of this circuit seems only the low heat development in the tube, and lower working voltage. It has to be said that the lifetime of historical (NOS) 45 tubes is probably much higher in the 1.6 Watt circuit, as these tubes are very small dimensions, and they get quite hot in the 2Watt circuit. Yet EML 45 has better cooling, and lifetime is not reduced at maximum dissipation.

Unlike in the 1930's, high voltage capacitors were expensive, and had short life time. So a higher working voltage was an issue in the early days of HiFi, but today capacitors are low cost and reliable. This makes the 2 Watt circuit the first choice today. When the output signal of the 2Watt circuit is only 1.6 Watt, distortion drops even 2dB below that of the 1.6 Watt circuit, so -30dB is achieved with the 2Watt circuit at 1.6 Watt only.

Still, with loudspeakers requiring more than 2 Watt, the standard 45 tube reaches it's limits, and we know it does become a problem for many owners of loudspeakers. Initially 2Watt may seem sufficient, but after some time this may appear not so.

The EML45B gives more options. This tube can be used anywhere between 6Watt and 22Watt plate dissipation. In case plate dissipation is below 6 Watt, like in driver circuit, the classical 45 or 45M would be the better choice.

Transformers: Several working points in the table below will have just 10% higher DC current compared to historical bias points. This allows in many cases to use Single Ended 45 transformers that may already be available. With some other working points, up to 55mA it will probably require another transformer.

Low cost transformers: Please keep in mind, transformers, have copper loss, iron loss, and stray loss. All together this can be 6...12% depending on the construction. A low cost transformer will atypically have a smaller core, and in order to get enough windings on such a core, the wire needs to be thinner. So there will be higher copper loss, additional to the iron loss. For this reason, only finest transformers will be better than 94% efficiency, whereas some others can be as low as 89% efficiency. However there should be awareness, that the SE transformer can loose 5...11% of the output power, depending on it's quality. Given what a tube amplifier costs, it is not a good idea to save on the transformer, and loose the sound energy, which was so expensively (!) generated, to something silly like copper resistance of the windings, while saving 20$ on the transformer. So instead of generating speaker movement, you generate heat in the transformer. Moreover it is a question, if such transformers that convert the tube output power into transformer heat, are good sounding. Interesting, this trade off can be observed best when comparing the Lundahl LL1623 and LL1663, in the below table. LL11663 a fine transformer, but it is smaller dimensions. So you can see the difference in efficiency when comparing to the larger size LL1623 or others at the left column. Yet, the LL1663 is not even a bad transformer, it is medium performance, like for instance the Hammond and Electra Print. Just, better products do exist, like the LL1623 or Bartolucci.

 

Ultimate transformers: To avoid copper loss, the wire needs to be thick. In order to get still enough windings on the core, it will become larger size. On top of that, the wire length will increase by the larger core, and more wire thickness is needed to compensate that too. So low loss transformers can become very large dimensions. For SE transformers, the first consideration is a good frequency range. When there is a trade off between anything and else and frequency behavior, the frequency will have to come first. So SE transformers are often surprisingly large, just to get a few Watts out of them. As an example, a transformer like the LL1623-060 as used in the below table, has a core with dimensions of a 250 Watt mains transformer, and yet in LL1623 construction it produces only 3.4 Watt, Single Ended.

Output power. There is great confusion about how many output power a classical 45 amplifier delivers. In the historical data sheets is always given 1.6 Watt or 2.0 Watt, but this means at the tube, before the transformer. After the transformer only 1.4 or 1.6 Watt is left. In the following table with operating points this can be seen on lines 1...3.

Conclusion: At the same distortion level, the EML45B is capable of delivering more than twice the output power. The historical bias point delivers 2 Watt at the tube, or 1.8 Watt at the transformer output with -27dB harmonics. The EML45 B at -27dB harmonics can deliver 4.4 Watt at the transformer output. This is a factor 2.2.

Part2

Link to: Data Sheet