Hughes & Kettner Puretone manual Standard SETUP/CABLE Connections

2.0 STANDARD SETUP/CABLE CONNECTIONS

3.0 REPLACING TUBES, MAINTENANCE AND CARE

The PURETONE ships with EL34 and 12AX7. Once they've been burned in (initial continuous operation under a load), these undergo an in-depth selection process where their electrical values and mechanical status (microphonics) are checked. Beyond that, they're installed in a finished amp and their sound is auditioned in a listening benchtest. One of the most important steps is power tube matching, a process whereby tubes with the same characteristics are selected and combined to make matched sets.

WHEN IS IT A GOOD IDEA TO REPLACE TUBES?

The tubes in the PURETONE are exemplary in terms of quality, workmanship and long service life. If you nevertheless come across a problem, please be sure to run down the following checklist before you swap out your old tubes for new ones:

Was the fault or failure of the power tube caused by the tube itself or 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 blow fuses or even the amps. Over-voltages are often caused by generators or improperly installed high-current power circuits.

Did a fuse blow even though none of the tubes is actually defective? An old fuse, tube de-ionization or mains voltage power surges could conceivably have caused it to blow. In this case, replacing tubes of course is a waste of money and time.

Tubes definitely show signs of wear when their service life is coming to an end, such as increased microphonics, noise, muddier tone through loss of high-end frequencies, degraded performance, etc. You should take these indications seriously and replace old tubes because not only do these side effects take their toll on sound quality, they also indicate the affected tube is about to fail.

WHAT SHOULD YOU KEEP IN MIND WHEN

YOU'RE REPLACING TUBES?

The golden rule: Replacing tubes is a job best left to qualified professionals. Accordingly, these guidelines are addressed to reputable technicians who earn their living with a screwdriver: Pull the mains plug of the PURETONE and allow for a discharge time of at least two minutes before removing the sheet metal panel on the rear of the amp. Once the panel is off, you can cautiously ease the tubes out of their sockets. A single power tube may only be replaced if the replacement tube is precisely matched to the original, in other words, it has the same characteristics.

As a rule, if you are replacing power tubes, use matched sets only.

NOTE: You may install new, matched power amp tubes into the PURETONE without having to bias the amp anew. The new set may even have characteristics that differ from those of the old set, just make absolutely sure that the tubes are matched.

The Class A circuit ensures that the amp is biased automatically, so to speak.

HOW CAN YOU EXTEND THE SERVICE LIFE OF TUBES?

Never operate the PURETONE without connecting a load (loudspeaker)! With this in mind, you should always use high-quality, heavy-duty speaker cables that won't crimp or snap.

Use the STAND BY switch! Warming tubes up unnecessarily wears them out that much quicker. Make a habit of shutting down the anode voltage via the Standby function and you'll enjoy many more hours of operation.

Avoid exposing the amp to vibrations, especially when it's powered up. Switch the amp off a good while before moving the amp so that the tubes can cool off completely first.