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My R-core transformer arrived!

February 8th, 2018

After waiting for a month+ , my R-core transformer finally arrived  🙂 It was well packed to survive a long trip from China. The declared parameters of the R26-09 R-core transformer’s (via eBay) were supposed to be:

Primary: 0-115VAC x2;

Secondary: 0-220VAC x2 with max total output current of 50mA and and 6.3VAC x2 with the total of 0.8A.

According to given currents, calculated total power of the transformer should be either 16W or 32W. It depends either declared currents were per winding pair or per each winding or the power calculations are simply different in China 😉



I asked the Chinese sellers on eBay few times about the total power but I got the replies ranging from “It is designed for tube amp” up to whatever you might want to get 🙂

So, I was pretty doubtful if the real values will match to declared. But OK, let’s see 😉 I performed few tests of the transformer total power handling and separate winding plus the inter-winding capacitance etc.

First let’s determining the phase of the secondary winding

The phase is normally either marked on some transformers or you could find this in their datasheet. In my case, there is no datasheet and the phases on the wires are unmarked on the transformer itself except only the colour (6VAC or 220VAC), so I have to distinguish them myself.

There are at least two methods for doing this. You could use the oscilloscope or a simple multi-meter. I will describe the method with the multi-meter.

First, you can connect the 6.3VAC secondaries in series with one connection point between them (just below 12.6VAC label) like this:R-core transformer Matching the phases

Depending on the phases direction, this should give you either 0V or 12.6VAC. If you get the voltage around a double value e.g. 12.6VAC (in 6.3VAC winding case), then your secondary winding are connected like it is shown in the picture above.



Once you know the phases of secondary winding, you can connect the secondaries in parallel (the wire marked with the dot with the same wire from another winding etc.). The same applies to 220VAC winding. Only be careful, the voltages are high and dangerous!

Testing the output of secondaries

Straight after connecting to the mains, it appeared that not declared 220VAC comes from the secondary winding but 270VAC (without any load)! That is a surprise… The heaters voltage is around 7.8VAC what is quite OK.

But what about the output currents? I loaded both pairs of the secondaries with the load. First, parallel connected 6x 1w resistors with the total 23.3k resistance were connected to 270VAC winding group. Such load should produced 11.6mA current. To second 7.8VAC winding group, my adjustable 30R rheostat was set to 10R load. Both secondaries 270VAC and 7.8VAC were loaded at the same time.

Loading both groups of the winding should allow to test not only separately the secondary winding but also the total power of the transformer.

The results

With the 23.3k loading (~12mA) the 270VAC dropped only up to 265VAC and that is only 2% drop. Hmm, it is a good sign. It mean that there is a reserve for loading more. Let’s try with 11.6k which would produce 22mA current. The voltage dropped up to 260V. Ok, it is almost 4% voltage drop this time. I read somewhere that the voltage drop should not be more than 5%. Otherwise, you might end up with the overloading and overheating of the transformer.

At the same time, the parallel 7.8VAC winding group was loaded with 10 Ohms rheostat which translates into 0.78A current with the voltage drop up to 7.4VAC. This is around 5% voltage drop, so we have reached the reasonable max here.




l also measured the inter-winding capacitance and leakage inductance as it is described here. The results of this is following for the groups of the winding:

Sec II(270V) 148pF (short circuit) 32mH
Sec II(7.8V) 158pF (short circuit) 0.1mH

Ok, we have a full set of the specs for the R-Core transformer R26-o9, which I got from China (via eBay).

Some conclusions

Let’s sum up. I have just got 6.75W(HV)+6.24W(Heaters)=13W R-Core transformer, which is not as advertised 40W, but these 13W should be more than enough for most of simple tube line stages with couple tubes (e.g. SRPP stages or other). Of course, you should take this (over-declaration) into account if you would like to use it for  more complex project.

At the same time, the transformer (via eBay) runs cold and silent with the loading of 270VAC @25mA and 7.8VAC @0.8A. It is time to start building a regulated HV power supply.