This article is the natural continuation of the previous one “How an SB-LAB audio transformer is made“, which is not only an informative article but also an answer to certain trolls who are unable to distinguish a cheap little product from a quality one. I wanted to waste time doing this competition, so, ladies and gentlemen, here is SB-LAB “6K6PP34” VS “Mummietta”…

A transformer that dares the troglo-phone and weighs 27 kg against a little jewel that weighs little and holds up well (maybe)… You’ve guessed it: in this article I will talk about the size of a transformer. Do they matter or not? These two transformers being compared are theoretically both 6600 ohm, rated for 25 watts, designed for a nice EL34 push-pull. According to some, the majority of transformer manufacturers in Italy (therefore also me) use terrible laminations, obtained by melting scrap metal, while a chosen few use sublime and pure materials extracted directly from the earth. Still according to some, those who use higher-quality laminations can achieve a better result with a smaller and lighter core. Finally, those who charge little (because it’s a factory and sells you something that is worth little) are honest, while those who charge more for a superior product get covered in mud. So, will Mummietta really perform better than 6K6PP34? I didn’t rig any test because I have morals (something certain people lack), and if I had been the loser in the tests, I simply wouldn’t have published anything.

To drive the tested transformers, I used the PushPull Transformer Tester 2.1, which, coincidentally, mounts a pair of EL34 biased at 240 volts of plate voltage and 70 mA of bias current. Let’s remember that no instrument, no matter how expensive, will ever completely simulate the real operating condition of the transformer. As load I used my audio wattmeter, in order to perform the measurements at the same power levels.

Premise: I already know that some guru-philosopher will start saying these instruments are not calibrated or certified and blablabla… Well then: if I have a meter that measures 99 centimetres, it will measure everything with an error of -1%, but it certainly won’t tell me that Sultan Kösen is shorter than He Pingping. Two transformers, same measurement setup, so no excuses. I will also publish the video of the passband acquisition because I am always accused of using Photoshop to retouch my graphs…

The damping factor at 1 watt, zero feedback, for both is around 0,2, but the driving is done by pentodes to pentode, with a very high Ri of 15k of the EL34, and the turns ratio of both transformers is the same; so this parameter, in this case, is irrelevant. I still point out that damping must be measured at 1 watt or at a power far below the maximum achievable by the circuit. Otherwise, if you do like some racing donkey and measure damping at full power, the result is completely skewed because, the moment the load is disconnected from the amplifier output, it cannot increase its output voltage any further because it is in saturation; hence, you get an optimistic measurement, much better than the real one.

Frequency response of Mummietta measured at 6.5 watt on 8 ohm load – Zero Feedback

In short: about 25 Hz/55 kHz at -1 dB. Taking 1 kHz as reference, the low-frequency attenuation levels off at 600 Hz… So far it seems acceptable, apart from a hint of resonance at 30 kHz which would be better if it wasn’t there. Let’s look at the sine wave shape at 20 Hz and 5 watts…

Uh oh, what do my eyes see: saturation at 5 watts! For me, everything could have ended here, but for the readers I will go on…

Square wave at 100 Hz

Now let’s move to my 27 kg transformer (it doesn’t really weigh 27 kg). We start with the passband, still 6.5 watt, still on 8 ohm load…

In short: -0,6 dB at 9 Hz and -1 dB at 40 kHz, still at 6.5 watt. Taking 1 kHz as reference, the lower frequencies level off at 90 Hz (not 600). Let’s go take a look at the sine wave at 20 Hz…

I could go lower in frequency until seeing the ugly saturation distortion also on the 6K6PP34. Let’s try…

Here it is: the SB-LAB transformer distorts at 5 Hz, while Mummietta distorts at 20 Hz. Still at 5 watt. Below is the square wave at 100 Hz for completeness…

Conclusion

By a simple rule, a transformer core halves its low-frequency response when power doubles. This means that the 6K6PP34 transformer will have that ugly unacceptable distortion at 5 Hz with 5 watts, at 10 Hz with 10 watts, at 20 Hz with 20 watts, at 40 Hz with 40 watts. Meanwhile, Mummietta distorts at 20 Hz with 5 watts, at 40 Hz with 10 watts, at 80 Hz with 20 watts. Doing the inverse, Mummietta could show a distortion equal to that of the 6K6PP34 at 5 watts but at 1.25 watt. The SB-LAB transformer is correctly sized, while Mummietta is a useless piece of iron that you cannot profitably use even for a 6V6 or PCL82 push-pull, because its nominal impedance is not optimal for these small valves that want 8/10 kohm, always obviously unless you settle for a miserable result of distortion-phone with poor bass. Undersized even by 1950 standards. Mummietta is not even worth the 40€ they charge for it and should be thrown into the scrap metal bin. Moreover, the test performed with your V2.1 tester doesn’t even push the transformers in power and there is no feedback in the circuit, because in that case many more issues could emerge regarding that resonance of Mummietta, the high current density it works with, phase rotations and who knows what else. The parasitic capacitance of both transformers is about the same; Mummietta’s is only slightly lower, but given its size it could be less than a third, a sign that they used power transformer materials and it is not built as an output transformer should be. I opened it and there is practically no sectioning: two primary halves with a secondary in the middle and that’s it. The point is that to build a tube amplifier you must spend on all the components, it requires work to build it, and if in the end you save on the most important component just to get a mediocre result, you are throwing money away. Beggars shouldn’t criticize those who aim for the best; in HiFi, once a certain point is reached, small improvements require big efforts.