Hej Koonw,
I am not going to change a thing, it is not the purpose of the exercise, it is to understand what has gone wrong, that is all.
R14 is black, it is rated at 3W and clearly exceeded that, I do not know by how much, enough to burn the coating off the thing.
Thanks for looking btw, it is appreciated
I am not going to change a thing, it is not the purpose of the exercise, it is to understand what has gone wrong, that is all.
R14 is black, it is rated at 3W and clearly exceeded that, I do not know by how much, enough to burn the coating off the thing.
Thanks for looking btw, it is appreciated
You keep up the good work while we see what else to comment:
Assume that the circuit as such, what do you think of NFB level to input LTP stage? 2k/2k is 50% ratio (usually 10%), assume the output is 25V max, it's 12.5V on the LTP, that is not right is it?
For R14 you can disconnect it, I don't know it's which is which, NFB or PFB but I presume it's NFB (with simulation), and drawn at least 3W, like you found either the feedback level is too high (and maybe due to oscillation) or resistor/cap themselves.
Assume that the circuit as such, what do you think of NFB level to input LTP stage? 2k/2k is 50% ratio (usually 10%), assume the output is 25V max, it's 12.5V on the LTP, that is not right is it?
For R14 you can disconnect it, I don't know it's which is which, NFB or PFB but I presume it's NFB (with simulation), and drawn at least 3W, like you found either the feedback level is too high (and maybe due to oscillation) or resistor/cap themselves.
Thanks,
I am going to put a fuse on the B+, the ONLY fuse on the thing is a 10A fast action fuse, so that is as much use as a fondue maker on a desert island, I am going to start at 1.5A.
Also find attached a revised schematic, it looks much better thanks to Nick and Koonw, I think I have it, the NFB to LTP measured 2k on the fluke, but then I did some mythamatics on it as there are 3 resistors in parallel and Requiv is 27k, so that is what I put in, and zoom zoom, thanks to Koonw on this, as he spotted the issue, and when he pointed out the ratio it made some sense.
I have procured from RS some 5w 22k resistors, and some fuse holders, plus I have got some 470nF and 100nF caps from Hi Fi collective, going to replace the lot see.
I am trying to simulate the error, I have shorted some caps etc, but still only get small amounts of current over R14, as R14 has failed and is now not resisting as it should (at ambient it checks out fine but I am unsure what happens live) so I will simulate this and revert back to y'all.
And just to say a big thank you to all and any that helped here, I know we have not fixed it yet but I feel we are some ways toward it!
A big thanks
Björn
I am going to put a fuse on the B+, the ONLY fuse on the thing is a 10A fast action fuse, so that is as much use as a fondue maker on a desert island, I am going to start at 1.5A.
Also find attached a revised schematic, it looks much better thanks to Nick and Koonw, I think I have it, the NFB to LTP measured 2k on the fluke, but then I did some mythamatics on it as there are 3 resistors in parallel and Requiv is 27k, so that is what I put in, and zoom zoom, thanks to Koonw on this, as he spotted the issue, and when he pointed out the ratio it made some sense.
I have procured from RS some 5w 22k resistors, and some fuse holders, plus I have got some 470nF and 100nF caps from Hi Fi collective, going to replace the lot see.
I am trying to simulate the error, I have shorted some caps etc, but still only get small amounts of current over R14, as R14 has failed and is now not resisting as it should (at ambient it checks out fine but I am unsure what happens live) so I will simulate this and revert back to y'all.
And just to say a big thank you to all and any that helped here, I know we have not fixed it yet but I feel we are some ways toward it!
A big thanks
Björn
So just some update news.
Amp is now fully functional, I traced the problem to a 460uF strapped across the 1000uF on the output.
I it was rated at 200v and some no name horse apple, So replaced the output caps, added some fuses, changed all the coupling caps and WOW, it sounds awesome.
Plate current is about 105mA per 6080, dissipation is well within limits and could probably suffer a bit more but I think I will leave it, bias seems fine, swings well, I must stop myself!
I am using my 46 at the minute, and you forget how much punch the OTL has.
Thanks for all the help guys, I probably would have bailed on it, if you are ever in Devon, let me know I owe y'all a pint.
Thanks
Björn
Amp is now fully functional, I traced the problem to a 460uF strapped across the 1000uF on the output.
I it was rated at 200v and some no name horse apple, So replaced the output caps, added some fuses, changed all the coupling caps and WOW, it sounds awesome.
Plate current is about 105mA per 6080, dissipation is well within limits and could probably suffer a bit more but I think I will leave it, bias seems fine, swings well, I must stop myself!
I am using my 46 at the minute, and you forget how much punch the OTL has.
Thanks for all the help guys, I probably would have bailed on it, if you are ever in Devon, let me know I owe y'all a pint.
Thanks
Björn
Postscript on OTL amps
A quick postscript to this thread. A few weeks ago, I bought the OTL from Bjorn. I met him and collected it from Devon – a lovely man who was very helpful. It’s been playing perfectly for the last three weeks (not continuously!) and I thought I should give an update in case it is helpful for anyone brave enough to join the wonderful world of OTL amps.
First, the sound. It exceeded all my expectations The sound is incredibly pure, detailed and clear. Every voice and instrument sounds just as it would if you were in the same room. With a live recording, you can hear every hand clapping, not just a clatter of sound. My wife was really impressed and she normally frowns on anything to do with my audio system.
But OTLs are powerful beasts and have to be handled with respect. This one has a huge power transformer. The voltage spike generated when it was turned off would sometimes blow the 1A fuse installed by Bjorn on B+. Simple fix: I cut the B+ wire just before the fuse and ran it through an external switch. Switch closed when switching on (no problem with transformer surge as the heaters are cold) and opened before switching off to isolate the valves from the spike.
It is also essential to switch on all the ancillaries (preamp, etc) BEFORE switching on the OTL and to switch them off AFTER shutting down the OTL. Don’t ask me why, all I know is that if you don’t do this you can blow the B+ fuse when switching the ancillaries.
Finally, heat. It gets very hot. Attilio builds beautiful amplifiers but he disregards heat. When installed normally, the copper plate holding the valves gets too hot to touch and there is no ventilation for the components underneath. They cook in an unventilated wooden box.
My wife found the cure! She forbade me from having the OTL on display and it was relegated to a corner of the room, behind an armchair. This meant it had to be mounted vertically and I could safety remove the back cover. Everything is now in free air and the vertical position encourages convection. It still gets hot, but not too hot to touch. If I had kept it horizontal, it would have needed a cooling fan.
I hope this is useful. The OTL has been one of the best upgrades to my system that I have ever installed. I am very happy and would like to thank Bjorn for all his work on the amp and those on the thread for your advice and encouragement.
A quick postscript to this thread. A few weeks ago, I bought the OTL from Bjorn. I met him and collected it from Devon – a lovely man who was very helpful. It’s been playing perfectly for the last three weeks (not continuously!) and I thought I should give an update in case it is helpful for anyone brave enough to join the wonderful world of OTL amps.
First, the sound. It exceeded all my expectations The sound is incredibly pure, detailed and clear. Every voice and instrument sounds just as it would if you were in the same room. With a live recording, you can hear every hand clapping, not just a clatter of sound. My wife was really impressed and she normally frowns on anything to do with my audio system.
But OTLs are powerful beasts and have to be handled with respect. This one has a huge power transformer. The voltage spike generated when it was turned off would sometimes blow the 1A fuse installed by Bjorn on B+. Simple fix: I cut the B+ wire just before the fuse and ran it through an external switch. Switch closed when switching on (no problem with transformer surge as the heaters are cold) and opened before switching off to isolate the valves from the spike.
It is also essential to switch on all the ancillaries (preamp, etc) BEFORE switching on the OTL and to switch them off AFTER shutting down the OTL. Don’t ask me why, all I know is that if you don’t do this you can blow the B+ fuse when switching the ancillaries.
Finally, heat. It gets very hot. Attilio builds beautiful amplifiers but he disregards heat. When installed normally, the copper plate holding the valves gets too hot to touch and there is no ventilation for the components underneath. They cook in an unventilated wooden box.
My wife found the cure! She forbade me from having the OTL on display and it was relegated to a corner of the room, behind an armchair. This meant it had to be mounted vertically and I could safety remove the back cover. Everything is now in free air and the vertical position encourages convection. It still gets hot, but not too hot to touch. If I had kept it horizontal, it would have needed a cooling fan.
I hope this is useful. The OTL has been one of the best upgrades to my system that I have ever installed. I am very happy and would like to thank Bjorn for all his work on the amp and those on the thread for your advice and encouragement.