Transistor Substitutions
There are a great many alternative transistors that could be used in this design (I have never understood why there are so many diffent transistors produced that perform the same or a similar function). The ones listed below are just a very small sample and are those that I know have worked well in this and similar amps and that should also be readily available. The case styles and pinouts are not necessarily the same as the original device.
BC212 - BC212L, BC556, BC560, 2SA872
2N1711 - BD139 (preferably selected for high hFE)
MJE371 - BD140
2N3055 - 2SC5200, 2SC3281, MJL3281A, TIP3055 (parallel pair)
Another suitable output device would be the MJL21194 or, if you prefer the TO-3 style case, the MJ21194.
Because to the high dissipation in the output devices (about 45W each), I suggest that any proposed output transistor should have a thermal resistance (junction to case) of less than 1 degC/W, or alternatively a parallel pair should be used (with 0R1 emitter resistors).
The BD140 will need a larger heatsink than the MJE371 since its thermal resistance (j-c) is much higher than the MJE371 and variations in the temperature of this device will cause the quiescent current to drift.
For a similar reason, R10 should be three 1ohm 3W resistors in parallel (or a single 0R33 7W or 10W) which should be stood-off from the pcb for good ventilation since temperature induced changes in the resistance value will also affect the quiescent current stability.
If alternative transistors are used, it may be necessary to alter the value of R1 (22k) to enable the output dc offset to be set to zero. Similarly, you may need to change D1 (4V7) or RV2 (4k7) to achieve the desired quiescent current. Any changes in required component value should be small. If you need a large change then it is likely that something is wrong elsewhere.
The component identification above refers to the circuit diagram (Fig 3) in the 1996 Electronics World article. Please note that there is an error in this diagram. The negative end of C4 should go to 0V (earth) not the -ve supply rail, otherwise supply rail ripple will be injected into the feedback path (TR4 emitter) causing excess hum. Also, there is an updated, but unpublished, circuit for the power supply using 5A ic voltage regulators. If anyone would like a copy then send me a note.
If anyone wishes to query alternative transistor substitutions either post a message or email me (the same goes for any other questions on this amp).
Geoff
[Edited by Geoff on 04-19-2001 at 06:16 AM]