© IRVING amplification 2013
Operation
Please ensure that a loudspeaker, or loudspeakers, of suitable impedance (8 Ohms or 16 Ohms) are
connected before attempting to play through the amplifier. Attempted operation without a
loudspeaker may damage the output valves. The output of the amplifier is 35W continuous sine-
wave. Power output can reach higher levels in overdrive conditions. We would therefore
recommend a loudspeaker system rated at 50W or higher.
The amplifier is fitted with a standby-switch. There is still controversy among experts about whether
these are necessary to increasing valve longevity. We would recommend switching mains power on
and wait for about 15 seconds before switching standby on. For any intermission in playing switch
standby off but leave power on (ready for a quick restart). At final shut down switch standby off and
then switch power off (no delay required).
Valve Replacement
The lifetime of the valves depends on how often the amplifier is used and for how long. Musicians
performing frequently, may feel the need to change at least the output valves every year or two. The
output valves (EL84) usually need to be replaced more often than the pre-amp and phase-inverter
valves (ECC83).
Typical symptoms that one or more valves need replacing are: loss of volume, unpleasant tone,
crackling sounds, discolouration inside the valve, visible arcing (flashes) inside the valves, and
excessive microphony (tapping an ECC83 may cause a ‘ping’ in the speaker – which is OK, but any
loud ringing or acoustic feedback via the valve are problematic).
The USA designation 12AX7 is an exact equivalent of an ECC83 and can always be used. If less
overdrive capability is required, it is fine to replace ECC83 valves with a lower-gain equivalent (e.g.
5751; ECC81, 12AT7; ECC82, 12AU7; etc.). Higher grade ECC83 equivalents, such as E83CC, may also
be used. However, as the B & J 35 has relatively low overall gain, too much further gain reduction
may result in not being able to fully drive the output stage. This would not lead to any technical
problem but would mean that the full 35 Watt output might not be achieved.
The amplifier has a ‘cathode-biased’ output stage. This means that the amplifier establishes the bias
voltage adaptively and the valves can be changed without any need for re-biasing. This means you
can change the valves yourself (if you want to). Once the valve is cool, rock the valve gently in a
circular motion while pulling it out of the valve socket (or when pushing the new one in). It is
unlikely that the glass would break, but to be safe use a suitable protective glove.
Expert opinion varies on how important it is to use a selected (balanced) double triode (ECC83 or
compatible) for the phase inverter (the ECC83 which is closest to the EL84s) and to use a matched
output quartet of EL84 pentodes. Generally, some unbalance in either of these stages will result in
slightly more distortion (increased even harmonics in particular) which some guitarists may actually
prefer. (We are talking about the very subtle distortion here, not the big fuzzy kind.) Nevertheless,
for the highest possible clean headroom, good balance is desirable. We would recommend obtaining
