I have a music angel tube amp model xd-se with kt88's. It is still under warrenty but emailing for the last week,I got no reply.I get hum on one channel only(seperate power supplys )so It looks like I will have to repair it myself.The hum stays the same no matter what the volume is so I'm thinking power supply(filter cap).Any ideas. swapped all tubes from other side and still hums(not a tube problem). I need a schmetic of mine to test further.Would a filter capacitor go so soon?The amp is about 2 months old with only about 10 to 20 hrs on it.
Here's it is!
Hi
Look at the link below for schematic as well mods for this amp.
http://www.lampizator.eu/AMPLIFIERS/CHINA/KT88/Music%20Angel%20KT88%20reworking.html
Regards
Fib
Hi
Look at the link below for schematic as well mods for this amp.
http://www.lampizator.eu/AMPLIFIERS/CHINA/KT88/Music%20Angel%20KT88%20reworking.html
Regards
Fib
Well it's a crude and simple circuit (and poorly made), but it should be simple to repair.
First off, try removing the preamp valves from the faulty channel, and see what effect either of those has on the hum? - to locate which stage contains the fault.
Although with an amp that new, I would suspect a manufacturing defect (not soldered something properly) rather than a faulty component. But isolating the stage the problem is in, will allow you to find the fault easier.
First off, try removing the preamp valves from the faulty channel, and see what effect either of those has on the hum? - to locate which stage contains the fault.
Although with an amp that new, I would suspect a manufacturing defect (not soldered something properly) rather than a faulty component. But isolating the stage the problem is in, will allow you to find the fault easier.
UPDATE:Since last time I got ahold of the seller.He was away and said it was under warrenty.I asked for a schmetic from him and got the same one that was on this thread.He said it was an old schmetic and that mine may have some improvements.None of the values for res or cap were the same.Here is some info.The triode switch operates 2 relays(j1-j2)When I last repaired electronics,relays were designated by k1 and k2.In the schmetic k2 is the switch for triode operation(we used to call switches s1 ect)This schmetic is almost useless but I found a bad filter cap(250v 180fd). It now works great. Thanks for the help.I asked the seller if he gets the new schmetic,to send me a copy and I will post it. Keith
First mod I made to mine was to fit an earth. The earth point on the incoming mains plug was not connected.
I earthed the incoming mains plug to the chassis via one of the transformer legs with a biting washer.
Apart from re-biasing I have made no other mods to mine and it sounds very good indeed. I paid £90 + postage for mine about 18 months ago, originally intending to use it for parts...
absolute bargain!
Ed
I earthed the incoming mains plug to the chassis via one of the transformer legs with a biting washer.
Apart from re-biasing I have made no other mods to mine and it sounds very good indeed. I paid £90 + postage for mine about 18 months ago, originally intending to use it for parts...
absolute bargain!
Ed
Hi All,
last week I bought MA xd800mk3 amp. I knew that it comes with different parts values compared to original schematic posted on web. I've already write down the schematic which I found inside and it's very similar to goldline's version. But still I found some serious errors in mine amp. For example the negative feedback resistor is 25 Ohm! G2 tetrode resistor is 240k! I only turned it on for a few minutes to check is it working and check voltages on capacitors to prevent eventually capacitors overvoltage explode. Ofcourse the voltage on almost every capacitor was too high. 435v on 400v rated cap. 485v on 450v rated one and so on. Some of these caps are already bulged. So I ordered new ones. I want to make this amp my main amp, so I will make it as good as I can. New rectifier diodes, chokes in power supply, every electrolytic bypassed with MKP, jantzen superior caps, 6n1p and ecc82 tubes with rechecked and re-set work points and potentiometer in 12AU7/ECC82 cathodes to set the gain equally. I will post some pictures and corrected schematic. I would be VERY THANKFUL for Your sugestions
last week I bought MA xd800mk3 amp. I knew that it comes with different parts values compared to original schematic posted on web. I've already write down the schematic which I found inside and it's very similar to goldline's version. But still I found some serious errors in mine amp. For example the negative feedback resistor is 25 Ohm! G2 tetrode resistor is 240k! I only turned it on for a few minutes to check is it working and check voltages on capacitors to prevent eventually capacitors overvoltage explode. Ofcourse the voltage on almost every capacitor was too high. 435v on 400v rated cap. 485v on 450v rated one and so on. Some of these caps are already bulged. So I ordered new ones. I want to make this amp my main amp, so I will make it as good as I can. New rectifier diodes, chokes in power supply, every electrolytic bypassed with MKP, jantzen superior caps, 6n1p and ecc82 tubes with rechecked and re-set work points and potentiometer in 12AU7/ECC82 cathodes to set the gain equally. I will post some pictures and corrected schematic. I would be VERY THANKFUL for Your sugestions
That's hard to believe... What is the feedback resistor that you are referring to? The one connecting the output to the cathode of the first stage? Did you actually measure the screen grid resistor or just read the resistor code?
I also couldn't believe what I see on my multimeter. 25 ohm feedback resistor is branded as 'dale' and other 'dale' branded resistors on pcb are 7k8 and I think someone put 25 ohm instead 7k8 because they looks the same. I measured the G2 resistor. On schematic there is 240 ohm and in mine amp is 240 Kohm, so this could be also a simply mistake. I've found another difference in circuit. I have 470k ohm and 6800p from kt88 anode to 12au7 anode instead 2x150k ohm resistors. Maybe someone know what is this for? Is this some kind of feedback?
Wow, that's un-belieable, looks like someone really messed up... As for the 470k and 6800pF, they are for the plate-to-plate feedback. If you operate the KT88's in pentode mode, then they might be helpful, but for triode operation, not so much, therefore should be switched out of the circuit.
So it appears that you do not have the original factory built unit, but one that has been worked on by someone. So I think you should restore it back to the stock condition first, and modify it only if you do not like the sound.
So it appears that you do not have the original factory built unit, but one that has been worked on by someone. So I think you should restore it back to the stock condition first, and modify it only if you do not like the sound.
I'm the first one, I did remove the original protect stickers from the enclosure while opening it. This would be unbelievable until it was not from china 
Wrong resistor values only infect the sound, but overvoltage capacitors can provide to explosion. In my opinion Music Angel amps are only for hobbysts who can operate multimeter and really like to solder something from time to time I was expecting this when I was buying this amp
Wrong resistor values only infect the sound, but overvoltage capacitors can provide to explosion. In my opinion Music Angel amps are only for hobbysts who can operate multimeter and really like to solder something from time to time I was expecting this when I was buying this amp
Many of the Chinese amps are notorious for this.
UK voltage often runs higher and the low specification rectifiers and caps do not have enough lee way to cope.
Instead of sound you get the magic smoke coming out of your amp!
Been there, done that!
This is the circuit which is working in my MA. I've written resistor values which I found in my stock amp. I changed ecc82 (driver) cathode bias circuit with 100r potentiometer to even the gain of them, I throw out input tube (6n1p) cathode capacitor, change input potentiometer for blue alps, which I found in my stuff, replace coupling capacitors for Jantzen Superior, replace buffering capacitors with higer voltages (not necessary) and put cypass mkp caps for every electolytic. I also modified power supply with chokes (1,2H for all tubes and also 10H for input/driver), larger caps in fixed bias circuit, higher voltage caps in main voltage doubler, uf4007 diodes bypassed with 10n caps instead slow ones mounted earlier and bypass most of electrolytics.
I plan to make this amp fully balanced, cause I have balanced es9018 dac, so I think it will be better to omitt phase splitter. I want also to use 6n6p tube as driver. I'm not sure to change also 6n1p for 6n6p. The main problem is the needed current. 6n6p is better on higher currents.
For now I will play above circuit for some time to hear how it sounds.
http://images77.fotosik.pl/249/3bb6ef7aa3bacecf.jpg
This photo shows the interior after described modifications. I spend about hour searching the cause of hiss. The problem was original (brown) resistors in cathode of the first stage (1,1Kohm). I changed both for metal one and the hiss has gone. I've got some brum in both channels which came from 6n1p tubes. I think it is from heaters, because they are AC. Right channel about 5mV, left 1mV, but for now I will leave as it is. Maybe in future I will DC all input/driver tubes.
Overall the sound is good, I have to wait to burn in all components (new tubes and capacitors). I'm not satisfied with the power, I think these tubes can play louder. Next goal is to check the sound without NFB and feedback to driver anodes and check voltages stability under full load.
Maybe someone have made some further mods with success and would like to advise something?
Last edited:
Good work Dankal6,
Now you have to deal with the (to) small power transformer and the voltage doubler circuut .for more outputpower with this powersupply you have to move more to class B
removing NFB only give more gain and more distortion. In me opinion the only way to get your wanted result is to change the power transformer or use a externel one .
please look for the Indel TSL400/001 made in Poland......the perfect transformer for this job.
Now you have to deal with the (to) small power transformer and the voltage doubler circuut .for more outputpower with this powersupply you have to move more to class B
removing NFB only give more gain and more distortion. In me opinion the only way to get your wanted result is to change the power transformer or use a externel one .
please look for the Indel TSL400/001 made in Poland......the perfect transformer for this job.
Got the same model, bought it almost 4 years ago, been running it hard almost all day every day on the original tubes before I changed them out last summer, the chinchin88s lasted a really long time oddly enough, I wouldn't be surprised if I pushed 15k hours on them though they were really weak at that point.
Anyhow, these music angel amps are notorious for changes in the circuit as what they have in house changes, mine only has 1 power transformer for the B+ and one for the 6.3v heaters,
which I changed for a self wound toroid because it was running damn hot.
It didn't have any chokes either, though I couldn't see any ripple on the b+ I still threw 1000uf worth of 450v caps in there salvaged from PC PSUs, still works fine to this day.
One quirk mine had was that with the stock 12at7s and 12au7 drivers one channel wanted to oscillate horribly after heating up, solved by lifting the front end circuit off the PCB with point to point wiring.
Right now I have been using 6CG7s in place of the 12at7/au7s and it seems really happy with that, 6CG7 is just a noval 6SN7gt.
Anyhow, these music angel amps are notorious for changes in the circuit as what they have in house changes, mine only has 1 power transformer for the B+ and one for the 6.3v heaters,
which I changed for a self wound toroid because it was running damn hot.
It didn't have any chokes either, though I couldn't see any ripple on the b+ I still threw 1000uf worth of 450v caps in there salvaged from PC PSUs, still works fine to this day.
One quirk mine had was that with the stock 12at7s and 12au7 drivers one channel wanted to oscillate horribly after heating up, solved by lifting the front end circuit off the PCB with point to point wiring.
Right now I have been using 6CG7s in place of the 12at7/au7s and it seems really happy with that, 6CG7 is just a noval 6SN7gt.