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 July 2008, 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):
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.
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