• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Chinese Output transformer for 300B SET

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hello from Japan ! This is my first post.

Recently I had a business trip to ShenZhen, China where I bought a pair of Output Transformer for 300B SET, as shown in the attachement.

I have no plan to build 300B SET but it is for my study to investigate Chinese trend.

Primary Imp. : 2.5 Kohm
Secondary Imp.: 4,8,16 ohm
Primary DC resistance: 136 ohm
Secondary DC resistance: 1.1 ohm @ 8 ohm tap
Primary Inductance: 14 H (@all other terminals open)
Core: EI, 95mm x 80mm, Laminate Sickness: 50mm, approx.

I think secondary DC resistance is too high. Is it intensional design ??

Primary low inductance of 14 H indicates the recent trend in Asian Industries'.

This part is used in their product named BL-90 shown in their Web page below.


'73 de JA2DHC


  • china-opt.jpg
    31.2 KB · Views: 2,221
Joined 2004
Paid Member
A dcr of 1.1 ohm on the 8 ohm tap is actually a pretty good result, I've not seen many that were better including the Tamura F-5002 (about 1 ohm) I used in some projects. I use 4 wire (kelvin) connection with a lab grade Keithley for very low resistance measurements.

The 14H primary inductance seems rather low, but may be an artifact of your measurement technique, to get a valid reading you need a relatively robust current through the primary to take care of the excitation losses in the core otherwise you will get rather low inductance readings.

One thing you ought to measure would be the leakage inductance, this will tell you something as well about the quality of transformer construction. IMO something less than 1mH per H of primary inductance would be a fair to good result.

Just curious as to why you don't plan to build an amp with these transformers? Just looking at importing these commercially into Japan? Building an amp and comparing a couple of different transformers under otherwise identical conditions would be a good way to know just how good these are compared to the usual Hashimoto/Tamura/Tango suspects.
Gents, Thanks so much for your replies.

I could get it by 100USD for a pair. Because of low price, I immediately paid money without thinking to build an amp using this. I am a pure DIY hobbyist of tube audio.

OK, I will measure the leakage inductance and report it here.
The measurement of the primary inductance was done by handy digital LCR meter without DC bias to the primary winding. Measuring frequency is 200Hz which is described in its manual.

My next plan is to build E-Linear using EL34 based on Pete Millett's design with my modifications. After that, I may use these output transformers for a SET.

'73 de JA2DHC
I have measured the frequency response of OPT as attached instead of leakage inductance.

Measuring Condition:
Primary Driving Impedance: 3.9 Kohm
(it is mis-matching to 2.5K ohm of the nominal impedance of OPT)
Secondary Load: 8 ohm @ 8 ohm tap
0dB= 0.775Vrms @ 8 ohm

Result is shown in the attached photo. The higher frequency range shows big bump which causes difficulty of over all NFB. In net, this transformer could perform good characteristics in No-NFB amplifier.


  • f-response.jpg
    38.2 KB · Views: 1,965
Joined 2004
Paid Member
Hi ja2dhc,
You need to drive the transformer with a source impedance comparable to the rp of the tube it was designed to be used with - in this case between 600 - 800 ohms would be reasonable. Despite driving it with a wildly wrong source impedance the response looks better than I expected. Even better rig up a current source so that you can test it with a static dc current superimposed on the ac source. (Use a large blocking cap for the ac component.)
Hello kuton,

In ShenZhen, there is famous electronics shopping town named HuaQiang North Road which is similar to Japanese AKIHABARA. You will get off the subway at HuaQiang Road. Then walk into HuaQiang North Road about 300m. You will find WanSahn Electronics Building on your right. On 2F and 3F in the building, there are more than 20 shops who are selling tube amps. Some of them sell tube components, such as OPT.
from the specs of OT it seems that the unit is more suitable to work with 2A3 tube...
the Lp of 14H is too low value for the 300B even for 2A3...
and zp of 2.5K is lets say common standard for 2A3 tube (3.5W output)
Measurements of the transformer only is good but not telling us almost anything real.
Try to take some measurements in real circuit with the tube 300B.
I am shure that You will have much bigger loss in low fr...
Joined 2004
Paid Member
Joao Pedro said:

Isn't .5mm lamination too thick for and output?

I though one would never build an audio trafo with at least .35 lamination (M6)

The frequency response will be limited by the core lamination, I don't think they can sound very good...

Hi Pedro,
I looked and looked for a reference to the lamination thickness and did not find it. ja2dhc does mention 50mm (5cm) but I think that was for the stack thickness itself..

I also wouldn't assume that M6 was used, without checking carefully.

The HF performance looks ok considering the source impedance used to drive it - might give some hint of the behavior with a pentode output tube, still not all that bad for the money IMO.. Just tricky to compensate.
Joined 2004
Paid Member
ja2dhc said:
Hi, kevinkr and all,

Thanks for the clarification. Yes, 50mm is the stack thikness.

I am planning to use it for my next SET amp using 4E27 triode connection which has low plate impedance.

Hi ja2dhc,
This sounds like a very interesting project, I look forward to reading all about it. Hopefully when the amplifier is done you can post some pictures.

I think the results will be pretty good based on what I have seen so far.

It will be useful for someone

Any more info? Would you put em in say, James category?

I mean near to Tango XE20S category.

For my project I have chosen Daburu DA-35X OT model. This transformer contains high quality Double C-core Z11 Silicon Steel (legendary Tango Japanese steel quality). For bass amount and quality I am thankful to this transformer.

For frequency response measurements see the details
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.