15V or 12V zeners is OK, but your rail voltage is low and you must decrease 2k7 resistors value to feed zeners with proper curent.
Sorry, I don't understand what you mean.
I put the scope probe on the the line out jack and I see the oscillation once it reaches enough output. I think the output section is just amplifying that.
What is preamp clip level on line out? You need only 1V from line out without clip to run amp to the clip level.
Hi Mile,
I finally found all the broken parts so now I have the voltages back to where they should be. Now for the oscillation. I measured the output voltage on the output of U2 and at the point of oscillation I have .585vac and I see 14.85vac on the output into an 8ohm load. One other thing I'm not sure if I mentioned is that I used TIP142/147 for the outputs. I'm not sure if those need different compensation.
Thanks, Terry
PS; I still only have 1.8mV across each 0R22 emitter resistor. Is it possible that the low bias is responsible for some of this? There is no bias adjustment on this amp.
I finally found all the broken parts so now I have the voltages back to where they should be. Now for the oscillation. I measured the output voltage on the output of U2 and at the point of oscillation I have .585vac and I see 14.85vac on the output into an 8ohm load. One other thing I'm not sure if I mentioned is that I used TIP142/147 for the outputs. I'm not sure if those need different compensation.
Thanks, Terry
PS; I still only have 1.8mV across each 0R22 emitter resistor. Is it possible that the low bias is responsible for some of this? There is no bias adjustment on this amp.
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Hi Mile,
I saw that after I had posted last. I pulled R23 and R24. I still have only 4.2mV across the pair of 0R22 resistors. Next I soldered a 100k trimmer across R21 and was able to dial more bias in but couldn't go any higher than about 7.5mV or it would jump very high. Still, no matter what the bias, the oscillation remains. I tried doubling C19 but still no change. Suggestions?
Thanks, Terry
I saw that after I had posted last. I pulled R23 and R24. I still have only 4.2mV across the pair of 0R22 resistors. Next I soldered a 100k trimmer across R21 and was able to dial more bias in but couldn't go any higher than about 7.5mV or it would jump very high. Still, no matter what the bias, the oscillation remains. I tried doubling C19 but still no change. Suggestions?
Thanks, Terry
I worried that the TIP142/147 may have been the problem so I ordered MJH11019/20 since I couldn't find the spec'd parts. They came in today and I installed them. No Change. I don't know why I didn't think to try before but I changed out the BD139 and now the amp works perfectly. Just thought I'd report in so others who may be interested in this amp will know it is fine.
One more thing, I used a 22-0-22vac 100vA transformer. The rails are +-32Vdc unloaded but drop to +-28vdc at clipping which is 20vac at the output for only about 50 watts RMS. I will hook up a bass and see how it sounds. I'll report back.
Blessings, Terry
One more thing, I used a 22-0-22vac 100vA transformer. The rails are +-32Vdc unloaded but drop to +-28vdc at clipping which is 20vac at the output for only about 50 watts RMS. I will hook up a bass and see how it sounds. I'll report back.
Blessings, Terry
One cannot get 20Vac (28.28Vpk) when the supply is at 28Vdc.
There is a loss through the amplifier that ensures the maximum output is less than the sagged supply voltage.
The supply voltage will have ripple.
The low point in the ripple waveform determines the maximum voltage from which the amplifier loss must then be subtracted.