Lee, Your experience of the having audio that was partially working with evidence of low gain and distortion jives with a starved bias situation. Thanks for diving into this problem, Lee, the group has learned something useful from it.
Garey, Yes, I remember you suspecting low beta. It turned out to be right! As for the emitter resistor, R141, it really doesn't impact the biasing either at room or another temp. Whether R141's drop is small (20mV) or 10X larger because 820 ohm was used instead, the bias feed current through the 2.2M is virtually the same. That's because the base biasing scheme is current fed from a base resistor that has many volts across it. Now had Drake applied VOLTAGE bias on the base, then it would have made a difference. Regardless of bias the emitter resistor does contribute to greater uniformity of input resistance and voltage gain from device to device and over temp as you say. The amount of the voltage drop also contributes to greater linearity of gain (i.e lower distortion). To roughly estimate the degree of linearity improvement simply divide the emitter resistor voltage drop by KT/q, which is roughly 25mV. Thus the normal drop across R141 (20mV) improves linearity by approximately a factor of two. Doubling R141 would help linearity further but simultaneously reduces the votage gain of the stage by about the same factor. I would not be surprised to learn that the Drake circuit designer started out with no emitter resistance and then noticed his distorition was a little higher than he would prefer, yet he had some gain to spare.....thus R141 with only a tad of voltage across it was born. As for unit cells in power transistors blowing open, what happens is first is a melt down of the Silicon within the unit cell causing a short and then the fusing open of emitter or collector metalization feeding that cell. Depending upon how that metalization is routed a portion of that transistor may blow open or the entire current path may open should a main metal branch blow. SOA (Safe Operating Area) failure is rare in small signal transistors and common in power transistors. While the SOA mechanism in present in both types of devices, circuits tend to stress power transistors to the limits, thus exposing that failure mode. Dennis AE6C On Sun, Apr 17, 2011 at 12:25 PM, Garey Barrell <[email protected]>wrote: > Dennis - > > I think the saying is 'even a blind squirrel finds an acorn now and then, > if you put him under an oak tree!" :-) > > That's why I suggested Beta, a low B-E current just made it worse. I've > never seen a small signal BJT fail that way, but your explanation makes > sense. I have seen some of the early RF Power transistors that were > actually multiple transistors all collected in parallel and one or more > would blow open! > > I believe R141 is ALSO a form of temperature compensation, as the > transistor 'warms' up, the B-E drop lowers, increasing the bias current. > The drop across the E resistor also increases, decreasing the bias current. > Much more stable DC operating point. It DOES also add some negative > feedback for the signal, reducing distortion. It also stabilizes the > circuit with different transistors as without it the stage gain is limited > ONLY by device Beta and internal resistance. With the E resistor, the > output is the ratio of Er to Cr, swamping device variations, for a gain of > about 580. > > Very interesting exercise. Thanks very much for your help and expertise! > > 73, Garey - K4OAH > Glen Allen, VA > > > > > Dennis Monticelli wrote: > >> Lee, Garey, >> OK. Mystery solved. Curve tracers are wonderful forensic tools. >> Lee, when I first looked at the 2N3394 from your R-4B it's I-V curves >> looked fine except for a decided dropoff in low current beta. At first I >> didn't flag this as a serious issue. The beta of all transistors is a >> function of collector current, tending to peak right where the factory DC >> tests are run :-) The falloff rate at low collector current was greater for >> transistors built over 40 years ago vs a transistor made today. The 2N3394 >> in your radio still meets min spec of 55 (barely) at 2mA. However, Drake >> ran the collector current at 1/8 that value and used a base current biasing >> method (in common use at the time) that suffers from excessive dependency >> upon the beta value, not for voltage gain but for setting up the correct >> bias levels. Your particular 2N3394 has a beta of only 25 at 250uA which is >> why it set up too low. Once it set up too low the further declining beta >> below 250uA made things even worse, effectively negating the compensating >> extra current through the 2.2M base bias resistor (the collector voltage >> rises under the starved collector current condition and would normally help >> provide addtional base bias). When you placed a fresh 2N3393 in there (min >> beta of 90 at 2mA), the bias problem went away. Not only was its beta >> higher at 2mA, it probably fell off a lot less at 250uA also. >> So the only mystery left is why Q7 worked well enough to leave the factory >> but not at the present time. Low current beta is dependent upon "surface >> states" in the Silicon crystal in the region of the base structure. Early >> transistor manufacturing techniques left much to be desired in terms of >> surface state control and early plastic packaging (think leaching of ions) >> compounded the stituation. So I'm guessing the low current beta was >> marginal when it left the factory and slid down over time. Another >> posibility is a reverse application of BE voltage (even momentary) as that >> is known to degrade low current beta. Looking at the circuit the only way >> that's going to happen is an errant application of a test probe. So most >> likely it was slow degradation over time. >> Anyway, the data supports your 2N3393 fix. Given what I've learned I >> would recommned that concerned R-4B users make a simple DC measurement of >> the voltage across R141 and if excessively low (i.e. well below 20mV), then >> do exactly what you did: replace Q7 with a fresh 2N3393 and recheck bias >> levels. R-4B owners may want to do this because R141 is not there to manage >> bias, it is there for emitter degeneration, a form of negative feedback that >> reduces distortion under strong audio signals. If insufficient voltage is >> developed across R141 the distortion will be greater. >> Hope this explanation helps the list. I learned something here. >> Dennis AE6C >> >
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