I started to work on one tube project and the first question was about choosing the transformer for HV and filament power supply for the tubes. The case in which I am going to install this transformer is not very tall (only 6cm height!) and the space for installing it is quite limited (looking from potential interference point) too. It is a bit challenging but that is what I have 🙂
Good old EI-shaped transformer
I spent couple evenings browsing eBay for the tube transformers. At the beginning, I looked into traditional EI-shaped transformers (via eBay). However, most of them sellers were from Asia and I read that it is not uncommon that these transformers are under-winded or/and even marked for 220V (which is lower than normal voltage in IE). Some people were complaining in the forums that these transformers were getting hot without overloading them. That was a clear indication of under-winding the transformers too. Adding to this, EI transformers can be quite noise and create the interference with the rest of your audio circuit. At the end, it might become pretty challenging to fight the last one 😉
R-core transformers from Asia
Another type of the transformers which caught my eyes was R-Core transformer (via eBay). According to some sources, they merge the advantages of EI-core and toroidal transformers. One of their distinctive design is that “made from continuous silicon steel strip, and the cross section is round, like O core transformer”. The core has no (air) gaps and is continuous. So, they supposed to have low noise, low flux, leakage, low no-load current, low loss, and high efficiency. Many low* 😀
It sounds as a dream for each DYIer! I tried to find how good they are in different forums like dyiaudio.com etc. However, I found only few reasonable comments on this. E.g. in Russian forum mentioning that these R-core transformers come with over-declared output currents values and you could simply divide at least by two 🙂 At least it looked that they are not under-winded which was semi-good sign. So, I decided to extract whatever information is possible about them.
Trying to extract the real specs of the R-core transformers from eBay
Depending of what kind tube stage you are building you might need a different voltage transformer. Here is a summary of the R-core transformers models (via eBay), which I found on eBay and calculated their total power.
So, what can we say about these R-core transformers from Asia?
First, each model has encoded (potentially true?) power of the transformer into the name of the model.
Second, the total power for each transformer looks like over-declared (some peak?) power. It looks that this over-declaration is varying from one model to another, which is not so surprising with e.g. Chinese products.
Third, the transformer and its core dimensions (not given in my table above) are higher for bigger power transformers. Technically, that should allow to use thicker wire (if it is not saved!) to be used for winding. Plus, you can check the weight. Yes, the seller might give a wrong value of the weight. But still it might be an additional indication of how much of copper might be used to make the transformer.
Last, most likely these transformers might come with not thick enough copper wire according to the declared output currents (best scenario, if you account for this before buying). It is not very likely but it is possible that these transformers can be under-winded too (worst scenario, the voltages would be lower that you plan them to be). But you can open the case on eBay if the voltages are too low.
In general, if you are looking for the transformer of X power, the transformer with X*2 (via eBay) might be quite safe choice. That is what I chose as criterion.
I found also these more conservative (and more realistic?) values of power, dimensions and weight on James transformers website:
Power A×W×X(mm) E×F×G(mm) Weight(kg)
5VA 58×47×26 45×33×4 0.2
8VA 70×61×33 53×44×4 0.3
10VA 75×61×37 68×48×4 0.4
20VA 82×70×39 56×46×4 0.5
25VA 83×72×42 70×55×4 0.6
30VA 97×77×42 70×60×5 0.7
40VA 99×80×44 70×60×5 0.9
50VA 102×90×48 75×65×5 1.0
80VA 125×90×52 90×70×5 1.4
100VA 126×103×55 100×80×5 1.7
These values (e.g. of the power) might be a good indication of the total power based on the transformer’s dimensions assuming that the transformers are not under-winded.
End of story
If these transformers are well made they really have the advantage over EI-shaped transformers and even might have advantage over toroidal transformers too. After analysing all info, I decided to get R26-06 transformer for myself and to test it (via eBay), which is should to arrive pretty soon and I might do some testing on it 😉