looking for an HG 2879 transistor with a gain of 67 or close to it

RF parts use to sell replacement transistors based on the gain rating written on the envelope the original transistors came in. I'm not sure if they still offer this but have you tried it since you know the gain of the part you want?

I'm kind of in the same boat as you... I'm after one that has a beta of 71. If I can locate one that's around 67, I'll ding you!

P.S. What's the lot number on the transistor?


~Cheers~

Another trick you might consider try is something I watched Blade Runner do a few times more than 30 years ago. With a transistor that had more gain than desired mounted to the heatsink, he applied just enough forward DC bias to begin scrubbing some gain off.

If you do it wrong, you run a high risk of burning the transistor open. I'm not even sure if a new HG can handle the abuse the old Toshiba did under these conditions. In any event, the key was to ride the current right on the edge of where you could begin to see gain reduce by retesting it again after a few seconds below 20 amps.

He did not increase the current to reduce the gain more. He just held the bias on longer. Don't pass the point you want because there is no way to reverse this process. As I recall, 12 volts was used and a resistor value starting with 150 ohms across the base / collector junction was used to start. It was reduced to 100 ohms for several seconds to "set" the gain where he wanted.

I never felt comfortable enough to subject a transistor to this myself but I have seen it done.

loosecannon said:

LOL that is crazy.

awesome, but i dont think ill be trying it this time.
thanks for the idea though, never would have thought of doing that.
LC

Click to expand...

Well, consider what happens to one of these transistors over time when it's either over driven or operated well above its rated collector voltage. It has long been understood that this abuse will reduce the gain of the transistors as compared to where they started life at. This procedure I watched done, accelerates that process... at your own risk.

LC,

I ordered a pair of HG2879's, if they are close in beta to my single 2879, I'll drop the new pair in and slide you the 71 beta 2879 I have, since it's much closer to 67 than your 78.

Might be a week or two as they are coming directly from HG Semiconductor.
I'll let you know as soon as I get them in.


~Cheers~

Shockwave said:

With a transistor that had more gain than desired mounted to the heatsink, he applied just enough forward DC bias to begin scrubbing some gain off.

Click to expand...

Sounds to me as if he was blowing out individual emitter-resistor wires with the overload current. That was the structural breakthrough that made high-power RF transistors practical. A single chunk of silicon large enough for high RF power won't heat evenly. The part that gets hot first now has a higher current gain than the cooler region next to it. The hotter region with higher gain will 'hog' circuit current away from the surronding less-hot area. Until a hole is melted in that region of the die. RIP transistor.

Fabbing 8 or a dozen parallel 1-Amp transistors in parallel on one silicon die gets you an 8 or 10 Amp equivalent transistor inside a single package. This allows the use of a thin nichrome (I think) wire in the emitter of each transistor. If you have ever looked inside a big audio amp or linear-regulated power supply with eight or more output transistors, there's a small power resistor alongside each one. Same principle. The transistor that heats up first draws the most current through that resistor. This reduces the base bias on that one, throwing more of the load onto the cooler transistors. Serves as negative thermal feedback, sorta.

When you put transistors in parallel this has the result of increasing current gain, or 'hfe', or 'beta' over what a single one shows.

Seems to reason that popping those tiny nichrome wires one or two at a time would reduce the measured current gain. I'm not a solid-state physics expert, but this is the only way I can explain his trick.

OTOH, crippling a transistor that used to have twelve parallel circuits down to ten or eleven of them can't help the RF-power gain. Even if it improved component matching at the low current of a tester, it should downgrade how the transistors share the load at peak currents. If one part has all twelve transistors going, and the one alongside has only ten remaining after it's "matched", I can't see what's good about that. Who cares if they match nicely at idle current with no drive, but won't share the load evenly at peak power?

I would rather have transistors balanced at the high-power end of the curve. That's when they get hot and the balance matters.

73

I wondered if there were little blue flashes under the transistor cap as junctions burned open? That would be easy to see if you only applied bias in the dark. In any event, the balancing wasn't only improved at DC. The goal was to stop the 2 watt balancing resistors from smoking and it did seem to do that too. No matter what, I still can't justify it as good practice.

Nomad, your idea would also explain why we don't see this gain reduction issue is most smaller transistors. It's all or nothing with them because they usually only have one set of wires and junction inside to burn out. While it would be nicer to think the gain was just being shaved off equally across the transistor die, sadly I think you're right and this will burn open wires inside.

Where at, LC?

~Cheers~