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How important is transistor gain (HFE) in an amplifier?

CalvinBeck said:
I’d like to circle back to my original question. Suppose I pull the two differential pairs, which are Hitachi C1345s, hFE 600, test them with a Dca75 and find one or more of them to be out of spec—will replacing them with Ksc945CYTAs, hFE 240, affect the operation of the amp or will it just be a wash? Sorry for the run-on sentence and I realize this is a bit of a hypothetical stretch.
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Hypothetical indeed. Talking to myself, I would feel more comfortable having HFE 240 new ones in the main amps differential pair location, than HFE 600 well-known troublemakers in the HK330C.
I do not think HFE is very critical and 240 is not low.
Of course, just opinion, here.
 
In my personal experience it's important in differential pairs
In many of my amps there's known troublemakers which i used to swap out then discover all the issues were due to something else.
I really have mixed feelings re re transistorising/ re capping.

If it ain't broke
 
I have mixed feelings myself. But I also have a curiosity bug. This receiver I’m working on is not for a customer or a friend. I plan on making it my daily listener. I only do this as a hobby. I don’t consider anything I do or mistakes I make as a waste of time, just learning experiences. I sold my old DCA55 and upgraded to a 75. Just for fun I plan on making some graphs of a few of these “noisy” transistors. What’s the worst that can happen? A lifted trace? A massive random ESD? I’m having fun, that’s all.
 
I agree with the general sentiment of leaving working units alone if they’re within spec. But I have replaced noisy transistors (mostly 2SC458s) which brought the noise floor way down (i.e., no more hiss at any volume setting with no input playing). It’s hard to say if that change made an audible difference when listening to music at normal levels.
 
I have two 2sc458s on my amp board. I read in another post about them being repeat offenders, especially the LG variant. Mine are the “outhouse” style.
 
CalvinBeck said:
I have two 2sc458s on my amp board. I read in another post about them being repeat offenders, especially the LG variant. Mine are the “outhouse” style.
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The KSC1845 is one of the standard replacements for 2SC458s, except they won’t take as much current. So I tend to use KSC1815s in amp sections and 1845s in preamp and tuner sections. 1815s will take more current than 1845s at the expense of a little gain.
 
Gnesen said:
I agree with the general sentiment of leaving working units alone if they’re within spec. But I have replaced noisy transistors (mostly 2SC458s) which brought the noise floor way down (i.e., no more hiss at any volume setting with no input playing). It’s hard to say if that change made an audible difference when listening to music at normal levels.
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Yes, I should have been clearer in my post. By "working" I mean a piece of equipment still meets original specifications. Of course, if the equipment isn't available to test, with audio components, listening must be relied upon.

Doug
 
The 2sc458 was made untill recently by renesas and that part has nothing to do with the failure prone 2sc458LG. If you search, You may still get 2sc458 rank C which has a gain of about 300.
If that is not a good solution, then you could get the KEC KTC3198GR hfe of near 300 or KTC3198BL which has an hfe over 450. These also exist in the TO92S package KTC3199GR and BL.
They are KEC's version of the extinct toshiba 2sc1815GR and BL.
The 2SC945P or KSC945CG suggested above is another option.
The ksc1845 is a different part, better suited for differential pairs or where the voltage is larger than what the 2sc458 can take.
 
Thanks for the info JoseHH. I ordered a good quantity of KSC945s and KSC1815s. I was planning on replacing the 2SC458s with the KSC1815s. The differential pairs are far enough apart in hFE that I'd like to replace them with a more closely matched set. They are 2SC1345s. I don't know which would be a better substitute, the 945s or the 1815s. Any advice appreciated. I've attached a picture of the DCA75 analysis of the transistors in question.HK_330c_Main_Amp_Transistor_Data.JPG
 
If the parts are functioning correctly leave them alone, no need to change them. I am not familiar with the diagram of your unit, so I can not tell their function by the designators Q401, Q421 etc. It will help if you post the designators together with the part and rank of the parts. For example, Q401=2SC1345F , especially for the differential pair. Some of the parts that you measured have gains quite large. I can not tell if they are well matched because with the designatror I do not know which ones form part of a differential pair or a current mirror. The KSC1815 or the 2SC1815GR that you are going to find are not going to be good to substitute parts that have hfe larger than 600. Even the 2SC1815BL that i have measured have hfe of 500 at most and you are not going to find them easily. Be aware of many 2sc1815GR thar are counterfeit. They are not good, do not use them. They usually came with the marking perpendicular to the direction of the pins, typically they will read :
C1815 or C1815
GR331 GR133
Other fakes are marked with the letters in the same direction as the pins, and read
C1815
GR 2E
Fakes, do not use.
The original Toshiba parts are noticeably larger than the fakes and have always the markings in the direction of the pins, and the lettering is imprinted with a metalic or greyish epoxy paint. Not laser marked like the fakes. The On semi-Fairchild are laser marked, but their marking is very different from the fakes.

For large gains, there are few transistors that can be used. If you can locate some 2SC2240BL they woould have gains in the 500-550 range and can handle 100mA.
The 2sc1345 is a large gain transistor, I do not know if is a failure prone part. For this large gains, ther is the Toshiba 2SC3112A or KEC KTC3112A. Here they are 48 cents a piece as of today. The same company has the 2sc1345 for 98 cents a piece. They do not specify the gain rank. But the 2SC3112 are superbeta transistors so their Vbe may be too large or unmatched and i think that they may also oscillate if used as a differential pair.
You may also take a look at these: 2SC1571 (20 cents a piece) or 2SC1570 (1.49 a piece). I have no affiliation with that company but is known here to sell genuine parts. Another one is the NEC 2SC1222 (ECB) (35 cents part as of today) which are high gain, but they were also made by Samsung with pin out EBC which requiere twisting the pins a bit. With this info you should be able to fix you unit.
 
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Thank you, JoseHH, for taking the time to help me with my transistors. You provided me with a lot of information that I will have to look into. I will definitely take a look at those transistors that you mentioned. When I get a chance later I will update the screenshot I uploaded and label it better.
 
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