Music Angel XD-800MKIII

Since my Audioromy 383 FU13 died almost a year ago I have been researching numerous amplifiers capable of driving the MagnepanSMGa ´s 90db/W. I don´t want to repair the Audioromy and continue using it because its B+ voltage of 870V is to high for comfort. After some research and posting on DIY audio I decided I needed at least 20W. Ideally an amplifier with B+ of approximately 300v would be nice. My first though was the 6C33C by  Dmitry Nizhegorodov, but in single ended it needs some huge and expensive transformers and does not really reach 20 Watts without parallel single ended or push pull, I considered an OTL, output transformer-less version like Tim Mellows which is popular on DIY audio now. But the Magnepans low 4ohm resistance is not really suited for OTL tube amps. So my attention turned to Push Pull amplifiers, classic Mullard 5-20 circuits, with output tubes like 6L6, 807, EL34, KT88. The problem was that B+ needed to be higher than 300V to get enough power. I was pretty interested in the EL84 for a while as it is cheap and has a nice operating point. But I would probably need Paralell pushpull for power reserves which means cost and complexity rises.
My studies resulted in slowly loosing interest in building from scratch. I looked at the Engineers Amp/DCPP by Pete Millet which was very well suited, and I looked into building a Dynaco S70 based on PCBs from Triode electronics, another great amplifier. Both would drive the Magnepans well and have a lot of support for sorting trouble or upgrading. But at this point I had moved into territory I wasn’t really feeling engaged about. I need a sense of exploration and wonder in my projects, not just walking beaten paths. So if I was going to work on a typical push-pull I might as well buy a finished amp. I decided to buy a Music Angel XD800 MkIII. This is a Chinese amplifier based on the classic Williamson circuit of 1947. it has a B+ of 450V, a 100V higher than I wanted but still half of the Audioromy. This particular Music Angel has a strong following in Norway, where I live, and is a cheap basis for experimenting. DO NOT confuse it with the YC808 – which is all about trouble. This amplifier I consider sort of a kit. There are several versions and a lot of experience to be found on “improving” or changing the amp for personal preferences. I looked long and hard at this amplifer when I bought the Audioromy a few years ago, and now I feel I should have gone this rute from the beginning.

Anyway, a Music Angel XD800 mkIII has been bought, used, and is in the mail to me now. It is the version built until July2008, the last version based on 12AT7 as preamp tube.
This may be the amplifier for transplanting into the Empire Chassis
Here is an older circuit diagram:

 
Variations to the build. (this is just a sketch  for now – more details will follow):

 
Preamp tubes. Both versions use 12AU7 (ECC82) as the driver tube. For several years the MA KT88 came with 12AT7 (ECC81) preamp tube – this is the original version. In autumn 2008 there was a change the amplifier was modified to use the ECC85 in the input – typically it is now the Chinese 6N1 sitting there (because it is cheap)
Grid stoppers. Early versions had grid stoppers, then Lampizator interfered and made them produce without gridstoppers introducing instability. Later they reemerged.

Chokes. Although all MA XD800´s have the pots from chokes, not all models actually have chokes.

Straight/slanted cabinet. From 2010 to 2011 the cabinets were built with slanting fronts, although some were made using left overs of the opposite type.

I got my Music Angel in the mail yesterday. It looks and works well, but today I decided to open it up to take a look around and adjust bias, if necessary. I found that the cathode resistors to the KT88s were wired unusually. both were connected to the same cathode resistor. I reconnected them, properly, and readjusted bias, discovering that one tube was not conducting current properly. it cut out after about two minutes. So now a new set of power tubes need to be bought. See picture:

Both cathode wires go to V7 instead of one to V7 and one to V8

Same with V5 and V6

I also found there was no safety earth, So I soldered one to the mains socket and connected it to the choke-pot bolt with a crimped ring and lock washer. There are no chokes in this amplifier, just their holes and cover.

Follow this thread for updates

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5 thoughts on “Music Angel XD-800MKIII”

  1. Hello TAO!

    Thanks for sharing this great post.
    I own a Music Angel XD-800MKIII
    recently biased the tubes. But there are 6 trimpots I turned 2 of them to find out that these were not the one to bias the tubes.
    Than I found the corresponding tube for the other 4. However I turned these 2 and don't know what they, where to check the value and which value it should be 😦
    Could you help me?
    Many thanks in advance!
    Regards,
    Dieter.

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  2. Hi Dieter,
    The outer blue potensiometers are for the power tubes, as you found out. To measure bias current you actually measure the voltage over t red resistors. Applying ohms law you should find a current of about 40 mA. With a 10 ohm resesistor you should read 0,4V over the resistor. In triode mode with 450 V on the tube and 0,04A you get 18watt per tube. The two tubes in paralell will influence each others readins a bit so you have to go back and forth a few times. Also, some people like to have the amps at a lower operating point I have seen 35mA or lower, since this is dodgy chinese engineering 🙂

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  3. Hi Tao,

    I already biased tubes 1 to 4. The problem was when I was searching for the correspondent trimpots, I turned 2 trimpots that were not related to any tube.
    Now, I don't know what these trimpots do, where can I check there value and which value it should be… 😦

    I uploaded took a photo of the PCB and highlighted the trimpots I am searching for answers 🙂

    http://img3.fotos-hochladen.net/uploads/pcb5jfigr63bq.jpg

    Thank you!
    Regards,
    Dieter

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  4. Sorry I misunderstood you.
    I am afraid I have not studied those pots. From your photo it looks like they may be connected to the near by tube sockets. If you find the data sheet for the tube you might learn something.
    Again, sorry, I cant healp you right now.

    Good luck

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  5. Hi Tao,

    no problem!

    After some analysis in the circuit, I found out that these trimpots connets to the KT88s pins 2 and 7 which are the heaters one for each pair of KT88 and then go the 12AU7 and ECC85 one pot per channel. The voltage summs up 6,1Vac. I ajusted them to have around 3Vac to the ground on each terminal. I have the feeling that 1 of the power supply transformer is getting much warmer than the other.
    I will further analyse the power supply PCB to confirm that these are only for the heaters.
    I like the SNR already, but I think that there might be room for improvement on running the heaters with DC. I will probably get 8 LM317 and rewire the heaters.

    Do you have any experience with the mods proposed by lampizator?

    I am considering some mods before going for better tubes.

    Regards,
    Dieter.

    Like

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