Too small hFE rank for replacement transistors

Thread Starter

Roidsson

Joined Nov 17, 2020
7
What is affected if the hFE rank of the replacement transistors is too small? Sound power in watts? Sensitivity? Idle power consumption? Noise?

I accidentally ordered a hfe rank of 50-100 while the originals are apparently in the 70-140 rank.
 

Ian0

Joined Aug 7, 2020
3,508
Base current.
If you treat a bipolar transistor as a voltage-controlled device. Collector current varies with the exponential of base voltage.
hfe tells you how much base current will flow as a fraction of the collecor current.
More base current means that the stage will have a lower input impedance and draw more current from the previous stage.
 

Wolframore

Joined Jan 21, 2019
2,352
Is this for an audio amplifier? It really depends on where and how it’s being used. It may or may not matter. 50-100 is tighter than 70-140.
 

Ian0

Joined Aug 7, 2020
3,508
Is this for an audio amplifier? It really depends on where and how it’s being used. It may or may not matter. 50-100 is tighter than 70-140.
If it’s part of a long-tailed pair, replace both.
but 50-100 isn’t tighter than 70-140, they are both the same. Both +/-40% from the centre value which is 71 or 99.

If you can measure any changes to the amplifier, the you must also be able to measure HFe.
 

Thread Starter

Roidsson

Joined Nov 17, 2020
7
Is this for an audio amplifier? It really depends on where and how it’s being used. It may or may not matter. 50-100 is tighter than 70-140.
Yes, it is Sony TA-F530ES.
https://www.hifiengine.com/manual_library/sony/ta-f530es.shtml

https://www.semicon.sanken-ele.co.jp/sk_content/2sa1215_ds_en.pdf
https://www.semicon.sanken-ele.co.jp/sk_content/2sc2921_ds_en.pdf

Both for L+R channels ordered. I just measured the old ones with multimeter, hFE is about 110-115.
 

Wolframore

Joined Jan 21, 2019
2,352
I found the schematics. I would try it, they are the output transistors, it may produce some distortion (THD) at higher output but there’s usually some room for variations. I would match vbe and beta as close as practical, it’s not as touchy as the front end differential.

also there’s an even higher category Y which are 90-180
5DCD35C9-21A7-4F43-8D8D-5BC833FCEBBA.jpeg
 
Last edited:

Ian0

Joined Aug 7, 2020
3,508
A good designer would have designed it for hfe=70, so you would probably count yourself unlucky if the new ones were less than that.
It‘s doubtful that it would affect the bias and thus the distortion, but you could trim that with RT351 (which you should do anyway after changing the output transistors). The only thing which could be affected is the maximum output power Into the minimum impedance load.
 

DickCappels

Joined Aug 21, 2008
7,897
Dependence upon highly variable factors such as HFE very small in a well-designed product and the difference you describe is in itself very small. Reasons for having designs largely independent of such factors is product performance consistency, which relates to production yield of the finished product and reliability.
 

Thread Starter

Roidsson

Joined Nov 17, 2020
7
New Sankens arrived. Weight is ok about 18,6g, case looks genuine, but these are sanded and painted. I was able to remove paint and markings with acetone. Genuine markings is not possible remove with acetone.

hFE values are good 17,118, 120, accidentally very close of my originals, except one wich have 159.

How I can be sure, what models these really are? There is many models whit this MT-200 case with similar hFE ranks.

https://www.semicon.sanken-ele.co.jp/sk_content/2sa1216_ds_en.pdf
https://www.semicon.sanken-ele.co.jp/sk_content/2sa1295_ds_en.pdf
https://www.semicon.sanken-ele.co.jp/sk_content/2sa1493_ds_en.pdf
https://www.semicon.sanken-ele.co.jp/sk_content/2sa1494_ds_en.pdf
 

Thread Starter

Roidsson

Joined Nov 17, 2020
7
Now I have mysterious idling adjustment problem in the right channel. Standard idling is 7mV.

- Idling adjust pot clock at 10-11 I can reach 8-9mV, on minimum it rises to 9-11mV

- If I put audio volume knob to 11, I can set it to 7mV and under, also If I put mute switch on -20dB.
So in practice power consumption is about 45W volume knob at 0, and at 11 consumption drops to 35W!

I swapped left and right channel power transistors, and problem still exist. Before repairing C356 was exploded and R355 burned. Amplifier sounds good, components looks good, no hiss or hum, but something is wrong. Resistor, capacitor, diode…?
 
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