Updating old power transistors

[hifileslie]

My old Naim 160 power amp uses BDY56 (TO3) output transistors. Can I update them with better modern ones such as the Toshiba 2SC5200( I can fit (TO3-P in place of TO3) . Would they be a direct swop or is there anything else to consider?

[janneman]

One thing to look out for is the bandwidth of the replacement transistors. *** far as I recollect the BDY's are not very wideband, so replacing them with the Toshiba's may lead to oscillations and you would have to adjust the compensation - not for the faint at heart!

Jan Didden

[capslock]

A faster output transistor may give you local oscillations, especially with complex loads. Cure: base series resistors (about 10 R) or ferrite beads on the base leads, as close to the transistor case as possible.

Loop oscillations occur if the output stage is too slow, i.e. introduces too much phase lag. Faster output transistors will reduce the phase lag, so there should be no problem. At the same time, they might increase the gain bandwidth, so at the now higher frequency there may be an acerbacated phase lag problem. You should try this also with capacitive loading etc.

Essential for this kind of work are an oscilloscope and a power supply with current limiting (electronic, resistors or some very fast blowing fuses).

Eric

[hifileslie]

Thank you. Without an oscilloscope how can I identify any problems? Will I be able to hear them?

[hugobross]

I hope this helps

[Elso Kwak]

Hi Hifileslie,
MJ21194 would be a excellent choice, from www.onsemi.com They are TO-3.