Hfe transistor question from new guy

[tonefiend]

Hello everyone,

Im going to be building a Dallas Rangemaster clone, and have been doing lots of research in preparation of
my build.   I have been looking at using the 2n388a npn germanium transistor but the only datasheet that I
could find, showed a min. Hfe of 40, but did not show a maximum Hfe on the data sheet..   I am reading
the most excellent article on the geofex site "Austin Treble Blaster" and in the article it talks about using
a transistor with and Hfe of between 75-100...    So I guess my question is that the datasheet for the 2n388a
only gives a minimum Hfe, so im not sure if I can actually use it for the rangemaster build.  If the data sheet
only gives a minimum hfe does that mean that maximum is infinity?    or do you have to calculate the Hfe yourself
somehow?

If I look at other datasheets for example
the NTE 101, it gives a minimum of 10 and a typical of 100. So based on this information I know that
the Hfe falls in the usable range based on the geofex article.

any help would be greatly appreciated,


thanks..  smitty

[peterg]

http://www.geofex.com/Article_Folders/ffselect.htm

Takw a look at the attached page for germanium transistor hfe and leakage.

[smallbearelec]

Where did you get, or plan to get, the transistor? If you mail-order an audited and tested device, it will hit the right gain bucket for the RM with little or no adjustment of the bias resistors. If you are using something unsorted, wherever it comes from, you will need to measure its leakage and gain to know whether it is suitable. The GEOFEX article on transistor selection explains how to do this. The article on building the RM covers the process of adjusting the bias resistors to accommodate a fairly wide range of gains.

Don't plan on using an NTE device. They can be OK for repairing an old radio, for example, but they are usually not predictable enough in gain and leakage to work well in a pedal. Invest in a breadboard and a multimeter if you don't already have, and let us know how things go.

[PRR]

> If the data sheet only gives a minimum hfe does that mean that maximum is infinity?

In our dreams.

If you bought a race-engine tested to give "at least 500 Horsepower", you would get 550HP, or 501HP, or if the maker was really generous, 600HP; but never 900HP much less infinite power.

Transistor hFE isn't as narrowly specified as race-engine power. However back in 1957 when they made those 2N388A parts, if they got some parts with hFE much over 100, they'd set them aside and sell them at a higher price under a different part number. If the maker didn't, someone else would. (There was a regular business buying bulk transistors and re-sorting them for particular specs.)

> talks about using a transistor with and Hfe of between 75-100...
> NTE 101, it gives a minimum of 10 and a typical of 100. So based on this information I know that the Hfe falls in the usable range

No, if you want 75-100, and the sales-spec is 10-100, you "could" get *all* parts with hFE of 11.

And 1957 was a very long time ago. We can't buy fresh-new 2N388A today, and the ones around may be prime spares or ancient rejects. For some work, it's not too important. A lot of radios were built with hFE near 10 or 20. And the original RM may have been built with loose-spec parts, and each one sounds a little different.

The word seems to be that 75-100 is a happy number. And classy pedal-parts vendors know that's what you want. They either test/sort their parts to that range, or they tell you the tricks to suit the RM for the parts they have.

[tonefiend]

> If the data sheet only gives a minimum hfe does that mean that maximum is infinity?

In our dreams.

If you bought a race-engine tested to give "at least 500 Horsepower", you would get 550HP, or 501HP, or if the maker was really generous, 600HP; but never 900HP much less infinite power.

Transistor hFE isn't as narrowly specified as race-engine power. However back in 1957 when they made those 2N388A parts, if they got some parts with hFE much over 100, they'd set them aside and sell them at a higher price under a different part number. If the maker didn't, someone else would. (There was a regular business buying bulk transistors and re-sorting them for particular specs.)

> talks about using a transistor with and Hfe of between 75-100...
> NTE 101, it gives a minimum of 10 and a typical of 100. So based on this information I know that the Hfe falls in the usable range

No, if you want 75-100, and the sales-spec is 10-100, you "could" get *all* parts with hFE of 11.

And 1957 was a very long time ago. We can't buy fresh-new 2N388A today, and the ones around may be prime spares or ancient rejects. For some work, it's not too important. A lot of radios were built with hFE near 10 or 20. And the original RM may have been built with loose-spec parts, and each one sounds a little different.

The word seems to be that 75-100 is a happy number. And classy pedal-parts vendors know that's what you want. They either test/sort their parts to that range, or they tell you the tricks to suit the RM for the parts they have.

ok thanks very much for the responese(s)  very helpful