Hughes & Kettner Statesman QUAD EL84 - Manual (Page 11)

Statesman Series
11
english
2.
Standard Set Up / Cabling
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.
• 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.
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!