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How do I know when to replace valves?
Over several thousand hours the emission will drop to
the point where the amplifier can no longer compensate
then the power and performance will deteriorate. Valves
may also fail in other ways such as becoming
excessively microphonic or "noisy". The heater may fail
which will stop the valve working. There several other
ways in which a valve may fail.
Valve life will depend upon such things as hours of use
and number of on/off cycles. It is not good practice to
remove the valves unnecessarily as this can strain the
pins and cause tiny air leaks.
In our experience the 845 tube is very reliable and can
last for several years of regular use. Similarly with the
6SL7/6SN7. If the sound deteriorates or you suffer
background noise (other than hum), one or more of the
valves require replacement. All the valves will need
replacing after 3000 to 5000 hours use.
Generally speaking valve failure will fall into
one of these categories:
1, The valve continues to work but the emission gets
low. In the case of 845 output valves this will result in
not being able to set the bias properly.
2, The valve gets noisy/microphonic. Usually happens
to the small valves, can be confirmed by tapping gently
with a plastic pen.
3, Heater fails. No glow in centre of valve. Valve is cold.
A valve that is lit up is not a guarantee that it is working
properly; conversely a valve that is not lit up will not be
working at all. 6SN7/6SL7 double triodes have two
heaters. It is possible that only one half can fail.
4, Dramatic Failure. Occasionally the demise of a
power valve may be obvious with internal sparks and
noise through the speakers. In that case, switch off and
do not use until a replacement is available. Before re-
commencing use see chapter 6 "Bias Adjustment". If
there was any associated burning smell etc, we would
recommend a qualified person examines the inside of
the amplifier first. This could also blow an internal fuse.
If the amplifier sounds OK the valves are probably fine. If
the emission drops you will have difficulty setting the
bias for the output valves.
5, Changing valves:
Before changing the 845 valves ensure that it safe to do
so by switching off and removing the mains plug at least
20 minutes before hand. This will enable dangerous
voltages to dissipate, and the valves to cool down.
If possible check the bias setting before you attempt to
change the valve(s), in order to familiarise yourself with
the procedure.
If changing both the 845s be ready to adjust the bias in
order not to overload the power supply. Don't worry how
low the reading goes this will not cause damage. Do final
adjustment when the amplifier is fully warmed up.
If all is well there should be no more than a barely
detectable hum from the speakers, and the amplifier
should sound OK when tested. Note that only 845 valves
are compatible with the amplifier, 805, 211 and similar
looking tubes cannot be biased and may damage the
amplifier, and could be dangerous.
*See engineers notes at rear
6, If you cannot set the pointer in the black section,
then the valve is probably faulty or is unsuitable. (If
both valves read zero the HT fuse may have blown).
If the valves are brand new, you will need to check
again after approximately 10 & 100 hours, after that
only occasionally or if you suspect a problem.
7, To avoid damage to the amplifier and electric
shock hazard you must use only valves marked
845, 6SL7, 6SN7 or equivalent. Use only valves
which you know to be new or good condition and test
the amplifier thoroughly before resuming normal use.
8, Replacing the small valves:
6SL7 and 6SN7. Neither of these requires any set up
procedure. It's just 'plug and play'. (These valves are
similar with the same pin connection; accidentally
interchanging these two valves would not cause
damage, buy will reduce the amplifier performance).
6SN7 should only be replaced with modern types.
Icon Audio is happy to check the valves/amp or re-
bias your amp free of charge.
GZ34/5AR4 Generally these valves are very reliable
and only need replacing together with the 845s
unless a problem is suspected. If the GZ34 fails the
6SL7 and 6SN7 cannot function.
8 Specification & Features
(Typical conditions @ 240v 50Hz)
•
845 directly heated triode output valves
•
6SL7 double triodes for first stage
•
6SN7 double triodes for 2
•
Hand wired point to point components
•
No printed circuit board
•
PTFE valve bases for low noise/leakage
•
42w RMS at clipping (18.5v @8 Ohms)
•
48w RMS at clipping (17.0v @6 Ohms)
•
39w RMS at clipping (12.5v @4 Ohms)
•
Signal to noise level -85db
•
Freq response 15Hz-22kHz +0 - 0.1db @36w
•
Freq response -2db=12Hz-43kHz @36w
•
THD 0.15% (0.25% high gain) 1w 1kHz
•
THD 0.36% (0.5% high gain) 8w 1kHz
•
THD 0.5% (0.7% high gain) 36w 1kHz
•
Sensitivity 380mv x50(H) 1v x 18.5(L)
•
Feedback 6.3db (high gain) 14db (low gain)
•
Custom hand wound transformers using grain
oriented iron
•
Supplied with attractive safety guard
•
Audiophile oversized metal film resistors
•
Audiophile High quality polypropylene or Jensen
audio capacitors (optional upgrade)
•
Internal wiring using silver audio cable
•
Valves carefully matched for best performance
•
Gold plated Input & speaker terminals
•
5 amp (10 amp 115V)T mains IEC fuse (with spare)
•
220/240volts, 320watts (0) 450w full power
•
250 ma T 845 anode fuses (with spares)
•
Size W: 44cm. D: 40.5cm. H: 26cm. 35kg net
•
IEC mains lead, (5 or 13amp fused UK only)
•
CE certified. ROHS & WEEE compliant
Specification subject to change without notice.
nd
stage
Page 8
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