UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Another DC coupled amp (Ampeg bass combo)

dave cox · 12th Jan 2018, 5:22 pm

Amp won't work without U5, as half of it is in the feedback loop. Actually, I don't think it would blow up unless you had a load connected as it might just swing to one rail or the other! If one of the 0R33's is toast then I would be looking for another dead one ...

Is U5 getting very hot? If you can keep your finger on it it's OK. An NE5532 is the kind of thing you might have lying around, so try substitution but usually they're quite reliable. I would check the voltages around R42/R50 and Q3. Q3 c-e should be between 3V5 and 8V5 depending on the pot and the temperature with the remainder of the 32V split near equally between R42/R50.Q1 and Q5 are operated as cascodes (some will point it out if I'm wrong) so the base voltage will be pretty static but tracking heatsing temperature.

dc

12th Jan 2018, 5:22 pm	#10
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,062	Re: Another DC coupled amp Amp won't work without U5, as half of it is in the feedback loop. Actually, I don't think it would blow up unless you had a load connected as it might just swing to one rail or the other! If one of the 0R33's is toast then I would be looking for another dead one ... Is U5 getting very hot? If you can keep your finger on it it's OK. An NE5532 is the kind of thing you might have lying around, so try substitution but usually they're quite reliable. I would check the voltages around R42/R50 and Q3. Q3 c-e should be between 3V5 and 8V5 depending on the pot and the temperature with the remainder of the 32V split near equally between R42/R50.Q1 and Q5 are operated as cascodes (some will point it out if I'm wrong) so the base voltage will be pretty static but tracking heatsing temperature. dc Last edited by dave cox; 12th Jan 2018 at 5:47 pm.