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Old 21st April 2008, 04:43 AM   #1
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Default music angel kt88 tube amp schmetic needed

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.
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Old 21st April 2008, 01:14 PM   #2
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Default Here's it is!

Hi

Look at the link below for schematic as well mods for this amp.

http://www.lampizator.eu/AMPLIFIERS/...reworking.html

Regards

Fib
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Old 21st April 2008, 02:25 PM   #3
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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.
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Old 1st May 2008, 07:14 AM   #4
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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
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Old 1st May 2008, 09:44 AM   #5
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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
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Old 24th September 2015, 04:25 PM   #6
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Default schematic

Here the schematic i found in my xd-800 mk3
good luck with your modifications !
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File Type: jpg music angel kt88.JPG (227.1 KB, 394 views)
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Old 23rd December 2015, 09:35 PM   #7
dankal6 is offline dankal6  Poland
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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
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Old 24th December 2015, 05:50 AM   #8
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Quote:
Originally Posted by dankal6 View Post
For example the negative feedback resistor is 25 Ohm! G2 tetrode resistor is 240k!
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?
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Old 24th December 2015, 10:26 AM   #9
dankal6 is offline dankal6  Poland
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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?
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Old 24th December 2015, 10:36 AM   #10
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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.
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