Chinese Output transformer for 300B SET

[ja2dhc]

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.

http://www.boleyx.com/_private/001.html

'73 de JA2DHC

[kevinkr]

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.

[Salas]

Has anybody used Daburu yet? I think I will give it a try (DA-35X). Looks promising for the money.

[kmtang]

My friend tried them out in his 300B and 2A3 SE amplifier.

I would say they are not bad at all. However, I believe I would go for those well known manufacturers who knows how to build good transformers.

Johnny

[Salas]

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

[ja2dhc]

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

[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.

[kevinkr]

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.)

[kuton]

I might have a business trip to Shenzen soon and am wondering where you got the output transformer. It's cheap enough to try to experiment with.

[ja2dhc]

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.