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
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.
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.