OC71 Hfe Question

[MordechaiBenZev]

I discovered I had an old Rangemaster "kit" from SmallBear I forgot I had received several years ago, which included an auditioned OC71.  I assumed this meant that the transistor was in the right gain range for a Rangemaster, that is, somewhere between 70-100Hfe.  But when I tested it on my Peak Atlas meter...the Hfe was 19. 

I thought maybe I had done something wrong but I checked and I didn't.  I measured it two more times and that was the Hfe (at room temperature of about 70 F). 

I remember reading that some old Mullars could have the Collector and Emitter swapped, so that in one position it read one Hfe and in the other, a higher Hfe.  I don't know if this is the case with the OC71, but I gave it a shot and the meter was incapable of reading the leads any differently: the red dot lead was still identified as the collector, and the Hfe remained the same...far too low for a Rangemaster.

Can anybody clarify what the heck is going on here?  I know Steve isn't doing this stuff anymore but this was purchased several years ago when he was, and he had a dependable, solid reputation for auditioning and sending out transistors with the proper gains.  So I'm quite confused as to whether there's a different way to measure this that might reveal the *actual& Hfe of the device, or if for some reasons, I received a dud.

[antonis]

I measured it two more times and that was the Hfe (at room temperature of about 70 F).

Repeat measurement using a hair-dryer blowing on the BJT..

[mdcmdcmdc]

The peak meter should default to the higher of the two possible readings.
Could you breadboard a simple amplifier circuit and see what happens when you flip the transistor around?

[tonyharker]

The gain of a transistor will vary depending upon the collector current it is measured at.  The OC71 is specified to have a Hfe of 30-75 at 3mA Collector current. I don't know what Collector current the Peak Meter defaults to.

[mozz]

Just build it. I have heard/built some "low" gain transistors have some nice growl. There are 2 different schematics around for the rangemaster, 2 different emitter resistors, try both.

[mdcmdcmdc]

The gain of a transistor will vary depending upon the collector current it is measured at.  The OC71 is specified to have a Hfe of 30-75 at 3mA Collector current. I don't know what Collector current the Peak Meter defaults to.

A DCA55 should give a reading that's in line with normal DIYS measurements, a DCA75 will give a slightly higher hfe measurement.

[turbofeedus]

The gain of a transistor will vary depending upon the collector current it is measured at.  The OC71 is specified to have a Hfe of 30-75 at 3mA Collector current. I don't know what Collector current the Peak Meter defaults to.

Generally the DCA series testers test at Ic=5mA.

[Electric Warrior]

Just build it. I have heard/built some "low" gain transistors have some nice growl. There are 2 different schematics around for the rangemaster, 2 different emitter resistors, try both.

And some units with OC71s used a 20k or 22k pot..

[mozz]

/quote]

Thinking about it, I thought the OC44 was the sole transistor and never saw a schematic with a OC71. Chances are the few i built i didn't research much due to it being a simple circuit. I think the OC44 was a RF transistor so they probably got them cheap. I usually bias by the 68k. So now i have to try a 20k pot and see what it does.

I just blew the dust off a FZ-1 perf board i made years ago because i found some really leaky transistors, figured they would be perfect for that. Looking at my old work i see it's not even a FZ-1 anymore but has some mods and half FZ-1A. Use what you have on hand.

[Electric Warrior]

/quote]

Thinking about it, I thought the OC44 was the sole transistor and never saw a schematic with a OC71. Chances are the few i built i didn't research much due to it being a simple circuit. I think the OC44 was a RF transistor so they probably got them cheap. I usually bias by the 68k. So now i have to try a 20k pot and see what it does.

I just blew the dust off a FZ-1 perf board i made years ago because i found some really leaky transistors, figured they would be perfect for that. Looking at my old work i see it's not even a FZ-1 anymore but has some mods and half FZ-1A. Use what you have on hand.

No idea if there was any difference in price.
OC44s are usually very low leakage. Many of the black glass ones do not leak at all. The yellow jacket Mullards that Dallas used leak a little bit. Hfe can be all over the place (40 to 225 according to the datasheet).

Higher leakage types such as OC75s work fine when biased right, though - I've tried them in this circuit to see leakage really causes more hiss (and found no difference).