TR7, considering TIP29 as a replacement for EP487
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Working on the PA brick of a project TR7, it was clear that someone had been concerned about the bias to the finals since Q2305, an EP487 was lifted. Expecting that we would need to replace that, I searched the forum and found from 2021 the thread "Re: Equivalent for EP-487 transistor?". Message /g/DRAKE-RADIO/message/57807 recommended a TIP29, so I went in search of those. Of course TIP29s are now obsolete, but Digikey has some NOS for a reasonable price, so I ordered a few.
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Having purchased a Peak Atlas DCA75 Pro semiconductor analyzer (which I like and will be discussed again in another thread about this brick), and having a couple removed EP487s to compare to the new TIP29s, I set about seeing how similar the two types of transistors were.?
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I will attach all the plots for both transistors, but of particular interest to me was the ones of Ic_Over_Vbe. The highest Ic that the tester used was 10 mA. Comparing those plots, it shows Vbe for the EP487 to be about 0.58V, and 0.64 for the TIP29, or 60 mV higher over most of the plotted current range. Even if 1980 TIP29s were similar to EP487s then, I was comparing transistors 45 yrs later, from a different manufacturer. And biasing the finals 60 mV higher I believe could result in a significantly different operating point, and resting power consumption.
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Since I had a couple other EP487s to use, and the ones I tested were similar in their characteristics and quite different from the TIP29s, I decided to use one of the removed ones. Note that of Q2305 and 2306, I believe that 2305 is the more critical one in setting the bias for the finals in that the emitter is at approximately 0.05V, and the bias of the finals is shown as 0.65V, the difference being determined by Vbe of Q2305.
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Where the EP487s are used simply as a pass transistor (e.g. Q2306 and other utility applications) perhaps a TIP29 is a suitable substitute. Note that the TIP29 has twice the gain as the EP487. But I did not want to raise the bias of the PA final transistors by 60 mV.
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Hopefully this may be useful to someone also dealing with considering replacement options for EP487s.
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I hope to continue the discussion of restoring operation of this PA brick (will start a new thread), and will continue that work when my friend returns from vacation (we took on this project for him to learn, and I am learning a great deal myself, having never had to delve into many of these areas of my own TR7).
-Glenn/WB0DKT
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