Hughes & Kettner DUAL EL84, QUAD EL84 Standard Set Up / Cabling, Tubes, Servicing & Maintenance

Statesman Series

2. Standard Set Up / Cabling

•Did the Mains voltage fluctuate or spike while the amp was on? In all-tube amps, over-voltage surges in the Mains net can certainly cause drop-outs. Over-voltages are often caused by generators and faulty high-current power circuits.

•Perhaps a fuse blew even though none of the tubes is actually defective? An old fuse, tube de-ionization or Mains voltage power surges may have triggered the fuse.

Things to Bear in Mind When Replacing Tubes

Replacing tubes is a job best left to qualified professionals! Accordingly, the following guidelines are addressed and apply to qualified service technicians only:

•Pull Statesman’s Mains plug and allow for a discharge time of at least two minutes before removing the chassis from the rear of the amp.

•The upright mounted preamp and power amp tubes of all Statesman combos are seated in a self-contained chamber that is easily accessible from outside. And so they are optimally protected against vibrations through the loudspeaker’s vibrations.

3. Tubes, Servicing & Maintenance

STATESMAN is factory-loaded with EL84 and 12AX7 tubes. Once they’ve been burned in – that is, operated continuously under a load –they are subjected to a rigorous selection process. Their electrical specs and mechanical status (microphonics) are checked, and then they are installed in an amp and their sonic performance is auditioned. One of the most important steps in this process is tube matching, whereby tubes with the same characteristics are teamed up in matched sets of power tubes.

When to Replace Tubes

The tubes in STATESMAN are exemplary in terms of quality, workman- ship and long service life. Nonetheless, tubes show definite signs

of wear when their service life is nearing its end. Telltale signs are increased microphonics, noise and hiss, muddier tone through loss of high-end frequencies, degraded performance, etc. Take these indications seriously and replace old tubes. Not only do these side effects take their toll on sound quality, they also indicate the aging tube will soon fail!

Note: Replacing tubes for experimentation purposes is not recommended. Installing the wrong tubes will damage the amp and cost you a lot more than you bargained for in repair costs. Before you start swapping tubes, ask yourself these questions:

•Was the fault or failure of the tube caused by the tube itself or by a flawed peripheral device or component, perhaps a defective speaker cable? If you don’t get to the bottom of the problem and remedy it, it may crop up again even after you replace the tubes.

Illustration: QUAD EL 84, 2x 12AX7, 4x EL84

•After removal of the protective grid, the measuring points and controller for setting the correct bias current and the heating symmetry (“hum balance”) are also directly accessible.

Illustration: QUAD EL 84.

•The illustration shows (from left to right) the aperture for the bias trimmer, the jumpers to the measuring points, and the aperture for the trimmer to the hum balance. The two trimmers may not be mixed up, since otherwise there is a danger of output stage damage through excessively high current in the tube!

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