My tube interest started from buying a set of Magnepan SMGa speakers. Now these aren´t very efficient providing about 89dBl/watt. I had been surfing the net for amplifiers and found tube amps. They looked really cool and promised a completely different sound to ordinary amps. They were not as low distortion but the distortion was second order harmonics and sounded good.
Although it might seem contrary, I´m not an audiophile. I want engaging music not absolute purity of reproduction. So, anyway, I ended up importing an Audioromy from On9Mart.com via an Ebay auction. It wasn't the prettiest amplifier around, but it had the omph to drive the Magnepans, (30W×2 RMS Class A) and those FU13 tubes really looked cool. It has a lot of amplification so when I tried it on a pair of efficient 96dB speakers, there was no room on the volume to dial in a precise volume. Don't use this amp for speakers over 91dB sensitivity.
Now the Audioromy, I soon learnt, is a dangerous beast. It carries about 870V Direct Current in the high tension supply to the power tubes, and outputs a whopping 350W of heat (which is actually low for a FU13 tube!). There is no user manual and you have to turn it over and poke around 5 cm from the high tension leads to adjust bias.
There was nothing else to do than to get rid of it or learn about tube amps and live with the risk. It´s worth living with though! Initial listening impressions proved that it is smoother then a typical mid range consumer amp - by far. One week after getting it, my old Cambridge Audio 540A was sold. An initial feeling that the Audioromy was to bright subsided after a month, as did the smell. Run these things with the windows open, the first week. there is a loot of gassing from the transformers. It stinks! The Magnepan SMGa´s I use it with are not especially bright speakers, so they work well together. I have tried it with several other speakers with 90-95dB /W efficiency, and they are not such a good match. Appart from the volume issue, i think they sound a bit bright. Audioromy FU13 and Magnepan are keepers. I will have trouble finding a better combination at this price point.
Biasing the Audioromy FU13 amp:
There is no instruction book with the amp.
The bias procedure is straight forward, but very high risk. I found it on the WWW.
Thanks to http://www.audiostereo.pl/bias-w-audioromy-fu13_45138.html
Turn the amp upside down. Unscrew the bottom. If you are not scared when you open it up you should read up on electrical safety first .1000V safety PDF download
Along the front of the PCB there are two gray resistors. On each side of the blue wires there are two trim pots. On the closeup picture you can see that the inner most resistor, marked with a red dot, corresponds to the potentiometer at the corner. The other two are marked with blue. On your amp the type of resistor may be different. You will need to check in practice whether the potentiometers correspond. They are not color marked from the factory.
Use alligator clips or multimeter probe clips. Connect your multimeter across one of the resistors. Stand well back, left hand in back pocket, make sure you are not grounded. Turn on the amp give it a few seconds to heat up. There is 870V on the tube sockets!!!!! It should measure 0,7V / 10 ohm = 70mA, that's the factory bias setting. If its not correct, use a 1000V isolated screwdriver and adjust the corresponding potentiometer. Turn of the amp, wait for capacitors to drain, and hook up the other resistor. Repeat.
You will find that one drifts when you adjust the other. You will need to go back and adjust once more.
Some German owners have found that reducing the bias to 55mA increases the lifetime of the tubes without ill effect. So when in doubt go for a bit lower value than 0,7V.
If you whant to see what people are writing on the forums about this amplifier take a look at:
And finally the circuit diagram: