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Hello and I apologise if my first question is rather basic as I am new to electronics. I have started to repair vintage amplifiers and have found transistors that I need to replace are no longer available. I suspect in most cases modern equivalent transistors may have a better specification but what I need to find out please is the critical characteristics I must match or exceed in equivalent trs? I assume values for Pc, Vcb, Vce and Veb are critical but what about Ic, Tj, Ft, Cc and Hfe? Thank you for your help. Tim

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    \$\begingroup\$ the critical characteristics I must match or exceed in equivalents That depends on the circuit so basically how the transistors are used. A "better" specification isn't always "better". Suppose a circuit relies on a certain transistor to have a very low \$\beta\$, then a more modern transistor with a higher \$\beta\$ isn't what is needed. \$\endgroup\$ – Bimpelrekkie Aug 27 at 14:36
  • \$\begingroup\$ You might like to give us the part number so that everybody can make replacement suggestions. Vintage me often used 2N3055 and MJ2955. I forgot the specs. So I need to google and read the datasheet to refresh my memory. I only remember that hFE is important, and also "matched characteristics pair" is important. onsemi.com/pub/Collateral/2N3055-D.PDF. Cheers. \$\endgroup\$ – tlfong01 Aug 27 at 14:41
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    \$\begingroup\$ How vintage? 1950s and early 1960s, you'll be looking for germanium transistors which are quite different and considerably harder to find. \$\endgroup\$ – Brian Drummond Aug 27 at 14:50
  • \$\begingroup\$ The amplifiers are all 1970-1990 Bang & Olufsen only. The transistor I urgently need to find is a suitable equivalent to a Motorola MPS H54 PNP transistor. \$\endgroup\$ – Tim O Aug 27 at 15:24
  • \$\begingroup\$ Tim, the only remarkable specs for MPS H54 is its high voltage (80V), and its low noise figure at low impedances. Don't forget the case style - this one is standard TO-92 through-hole mounting. You may find similar transistors, but only surface-mount. \$\endgroup\$ – glen_geek Aug 27 at 15:48
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It's not that easy. Which characteristics of a transistor are important isn't an universal thing – it depends on what the transistor is used for.

So, you'll need to analyze what configuration the transistor is used for, and what the critical characteristic is. For example, when desiging a feedback amplifier, a high-as-possible amplification and a low-as-possible base-collector voltage might be desirable, but that doesn't help you if the external circuitry that you're stuck with is optimized for something else. And it doesn't help at all because not every transistor is used as that – often, limited gain is factored into the design e.g. to ensure stability.

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For quality amplifiers, the ability to reject power supply hum and spikes is important for musicality.

After reading thru the book by Doug Self on Audio Power Amplifier Design, I realized the Vearly was important.

If you operate the transistor at 10 amps and 50 volts, with a Vearly of 100 volts, the straight_line model of the Early Effect becomes (100 + 50)/10 or a simple 15 ohm resistor connected (in the model) from VDD to the output load.

Thus with 8 ohm load and 15 ohm resistor, the attenuation of VDD hum and trash is 8/(8 + 15) = not very good attenuation.

We, of course, depend on the large open_loop gain of the negative feedback to push down on energy (hum, trash) not related to the music.

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