Transistor hfe ratings

[sneakers563]

I'm about to embark on building a bass amp for my friend based on Rod Elliott's P27. Up till now, I've only really built tube stuff, and I have to admit that transistors are a bit of a mystery to me.

I have a pretty basic question related to sourcing parts. On the power amp schematic, I see a few different transistors. For instance, the BC559. However, when I go to Mouser and check the datasheet, I see that there are actually 3 versions (A, B, and C) of the transistor, each with different hfe ratings.

Is there some guideline as to which to choose? Are these simply BC559's that have been presorted, or are some of them "improved" versions, with specs different from a "generic" BC559?

Hope this question makes sense. I see it's the case for most transistors, and I want to make sure that when I order them, I'm not inadvertently getting some that are spec'd outside of what the designer intended.

[davidsrsb]

The suffix versions are simply selections, for which you should pay a slightly higher price. The "C" version is the highest band. If the original circuit did not specify, you can use any of them.

[Mooly]

As above...

Most circuits (such as the ones in your link) will work OK with any appropriately specced part. It would generally be considered poor design if a circuit was critical to normal the normal spread of tolerances... that said however, there are benefits to using the higher gain groups (and particularly matching them too) in areas such as the long tailed input pair of the power amp.

[AndrewT]

I believe that all the BC parts are specced from when they used less accurate manufacturing methods. This forced the manufacturer to "select" different parameters to sell on in "matching" groups.
eg by Vce0, or by hFE, or by noise, etc.
the 559 and the 560 have the same noise spec.
the 560 has the higher Vce0, but the 556 has an even higher Vce0 but lower noise spec.
As stated above the a, b, c, or unmarked are the hFE spec.
I suspect originally all these devices came off the same production line.
Now, I suspect any manufacturer, and there are many, will be able to make any batch of devices to fit a particular set of parameters for each individual type number.

Can anyone in, or was in, the semiconductor industry give corroboration to my hypothesis? Or shoot me down in flames?

[Andy5112405]

Originally Posted by AndrewT

I believe that all the BC parts are specced from when they used less accurate manufacturing methods. This forced the manufacturer to "select" different parameters to sell on in "matching" groups.
eg by Vce0, or by hFE, or by noise, etc.
the 559 and the 560 have the same noise spec.
the 560 has the higher Vce0, but the 556 has an even higher Vce0 but lower noise spec.
As stated above the a, b, c, or unmarked are the hFE spec.
I suspect originally all these devices came off the same production line.
Now, I suspect any manufacturer, and there are many, will be able to make any batch of devices to fit a particular set of parameters for each individual type number.

Can anyone in, or was in, the semiconductor industry give corroboration to my hypothesis? Or shoot me down in flames?

These are mass produced items. Avoid A group as they are inferior specimens. B&C are selected components within certain gain criteria.Most of them will work in this circuit but I would go for B or C gain groups.

[sneakers563]

Thanks for all the help - exactly what I was looking for! I'll go for B or C versions.

[davidsrsb]

High breakdown voltage and gain are mutually exclusive. I would go for "B" devices as these are from the middle of the bell curves.