Replacing Tubes - Koch STUDIOTONE Operating Instructions Manual

while using the outputs [5] [6] [7] for recording or other
purposes.
WARNING: Never play the STUDIOTONE without a
speaker connected if the Speaker On/Off switch [9] is
in the "On" position. This may cause serious damage
to either the power tubes and/or the output
transformer.

REPLACING TUBES

•
SAFETY FIRST: DISCONNECT THE POWER CORD
FROM ITS POWER SOURCE AND LET HOT TUBES
COOL.
•
CHECK THE 'TUBE LOCATION DIAGRAM' ON THE
CHASSIS FOR THE CORRECT TYPE NUMBERS
AND LOCATIONS.
•
REPLACE TUBES ONLY WITH ORIGINAL KOCH
HIGH QUALITY TUBES. (IF OTHER TUBES ARE
USED THE WARRANTEE OBLIGATION EXPIRES)
•
BEFORE REPLACING THE POWER TUBES,
REMOVE THE CE SAFETY CAP FIRST.
•
AFTER REPLACING THE POWER TUBES, PUT THE
CE SAFETY CAP BACK IN PLACE.
NOTE: The STUDIOTONE has an auto-bias circuit so it
does not have to be rebiased after replacing the EL84
power tubes.
If you have any doubts, please take no risks and let a
qualified technician do the job.
Preamp and Power tubes wear out and have to be changed
from time to time to maintain the amps best performance.
Tubes behave like strings, they lose highs, lows and
dynamics and after a period of time they have to be
changed. Exactly when is hard to say but this is an
indication: if you play almost every day change tubes each
year, if you play once or twice a week change tubes every
2-3 years.
Tubes rarely fail. If they are bad, these might be the
symptoms:
PREAMP TUBES (ECC83, 12AX7, 7025):
- Microphonic whistling or squealing on one or both
channels.
- No or low amp volume on one or both channels.
- Excessive noise on one or both channels.
POWER TUBES (EL84):
- Loud crackling that is not affected by front panel controls.
- Intermittent or regular blowing fuses.
- Weird amp 'distorting'.
- Hum.
Changing a tube is a simple and quick fix for most
problems in your amp.
In case of failure, just one or both POWER TUBES can be
replaced without rebiasing.
Please always use two matched EL84 (= one duet)
replacement tubes to obtain the amps best
performance.
Again, if you have any doubts, please take no risks and
consult your dealer and/or a qualified technician.