Jadis biasing

Dear Doc,

I though biasing of tube amp is only needed when one replace the tube. Recently I read from sources that tube amp need to have its bias adjusted once every 6 months, failure of which may result in damage to the amp.

I wasn't told about this when I bought my Jadis Orchestra Reference a year ago. I cannot find any pot at the exterior of the amp for such adjustment, may be it is internal, can I do it myself?

Thanks

Norman

Hi Norman!

I can understand your concern. Your Jadis Orchestra uses the EL34 power tubes. They need to be biased at about 100mA of standing current per tube to have good linear operation. In the case of over bias, your Jadis has a 300mA fuse for protection. So your amplifier has good design and good parts, which does not call for worry.

Setting of the bias requires some technical competence. Usually, the bias is set carefully when the amplifier has reached thermal stability. A voltage is measured across the cathode resistor, to determine the amount of current (voltage = current X resistance) and set for 100mA. For tube amp, usually there is no need to adjust the bias within months. The Jadis, even though runs a bit warmer than the Dynaco ST70 or Sonic Frontier similar model, the bias should stay and not drift. However, the bias may be different if the domestic supply in your house fluctuates from the usual 230V (I came across 205V or 250V in private residential places, call Singapore Power if that happens.)

If your amplifier sounds great and the tube does not shows abnormality (the colour turns orange red or the sounds distorts), then I would suggest you leave things as it is. If you are really concerned, why not give the local agent a call or get the amp checked? Tube amps, unfortunately, need more care and pampering than transistor types.

If you are game enough, why not invest in some up rated power tubes such as the Sevtlana, Sovetek or the Golden Dragon? Please do consult the local agent for warrantee issues. My suggestion is you have the bias checked in you home, where the amp draws powers. It is because the bias may be affected if your home has a slightly different voltage (+-10% from the 230V statutory value) from the place the amp was checked.

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