Hi folks, need some words of wisdom -
I have just entered the testing phase for a stereo version of the dynaco MK III that I have made.
OK, heres the thing. Connected the first channel up using EL34 (as I have a couple of old ones hanging around and didn't want to smoke my new KT88's). Cautious power up measuring the bias voltage across a 10 R resistor (adjusting for 0.7v = 70 ma). All the tubes light up, no smoke, no funny smells etc, all looks good. Put in an input signal from CD player through a 100 K pot and fantastic clear sound - a great victory!! All looks good, listing to the music and very pleased with myself. I decide to crank up the volume, goes OK but as I am cranking up the volume, the bias voltage shoot up to about 3v and at 'midrange' volume, I get a sudden abrupt, loud and disturbing buzzing through the speaker and about 2 seconds later, bang, the 3A fuse blows.
Hmm, so I let everything cool down, replace the fuse and give it another quick go. Hey presto, all seems well at low volume - amp working fine. I then shut it down, not game to crank it up for fear of damaging something!
I have attached a circuit diagram. All is the same except I was using EL34's instead of the KT88's. Should also mention I was using a 6R test speaker on 8R tap.
Any ideas?? Maybe just too much current for the EL34's? As I mentioned, sounds great at lower (not quiet but not loud) volumes but the response to cranking the volume was quite spectacular.
Cheers,
Rob
I have just entered the testing phase for a stereo version of the dynaco MK III that I have made.
OK, heres the thing. Connected the first channel up using EL34 (as I have a couple of old ones hanging around and didn't want to smoke my new KT88's). Cautious power up measuring the bias voltage across a 10 R resistor (adjusting for 0.7v = 70 ma). All the tubes light up, no smoke, no funny smells etc, all looks good. Put in an input signal from CD player through a 100 K pot and fantastic clear sound - a great victory!! All looks good, listing to the music and very pleased with myself. I decide to crank up the volume, goes OK but as I am cranking up the volume, the bias voltage shoot up to about 3v and at 'midrange' volume, I get a sudden abrupt, loud and disturbing buzzing through the speaker and about 2 seconds later, bang, the 3A fuse blows.
Hmm, so I let everything cool down, replace the fuse and give it another quick go. Hey presto, all seems well at low volume - amp working fine. I then shut it down, not game to crank it up for fear of damaging something!
I have attached a circuit diagram. All is the same except I was using EL34's instead of the KT88's. Should also mention I was using a 6R test speaker on 8R tap.
Any ideas?? Maybe just too much current for the EL34's? As I mentioned, sounds great at lower (not quiet but not loud) volumes but the response to cranking the volume was quite spectacular.
Cheers,
Rob