Orange Graphic Amp
The Orange Graphic amplifier is a traditional all valve design capable of driving 120W RMS into 4, 8 or 16 ohm speakers. It has a single input channel with two input sockets; one high and one low sensitivity. A total of four tone controls are provided. These are: standard bass and treble controls which both provide boost or cut relative to level response; 'boost' which lifts the whole of the treble range evenly without providing excessive high treble; and something called 'F.A.L.' which is a switched bass cut control that gives 6dB/octave bass cut from six selected frequencies. A mains selector allows the amplifier to be used on all normal AC mains supplies including 110 volts AC. The unit is said to be fully protected by fuses and the fuse rating, mains voltage selection and speaker impedance selection are all clearly marked on the metalwork.
The construction is traditional in almost every way with all the valves and transformers etc. mounted on a steel chassis which is fitted into a (leatherette) covered, plywood box. The only deviations from tradition are the bright orange paint on the metalwork. The components used are all of the very highest quality, e.g. Parmeco transformers, Mullard and Brimar valves, and Plessey electrolytics. No components of 'second quality' or 'unidentified' make have been used at all. Most small components are mounted on a single glass fibre printed circuit board. Soldering and wiring is good and plastic ties have been used to keep the wiring tidy.
The non-removable mains cable is retained by a special rubber grommet. This is a good idea but it was found not to be gripping the wire tightly.
The following table gives the performance of the unit as measured in our laboratory.
|Power Output||148W RMS||@10% total harmonic distortion|
|Distortion (total harmonic)||4.6% |
|@120W RMS 8 ohm load |
@ 60W RMS Hi input
@ 1W RMS @ 1 KHz
|Distortion levels are high but reasonable for a valve amplifier|
|Sensitivity||12.5m VRMS |
|Hi. I.P. for 120W output @ 1 KHz. |
Lo. I.P. Boostcontrol min.
Bass and Treble set central.
|Plenty of sensitivity for guitars.|
|Treble range||16.8dB||@10KHz||-fair range|
|Bass range||26dB||@ 50Hz||-good range|
|"Boost" range||11.3dB||@ 3KHz||-a useful control which is not usually provided|
|"F.A.L." range||27dB bass cut||@ 100Hz (6 switched positions)|
|Slave OP||-1.5dBm||for 120WRMS out of main amplifier||fractionally low for many slave amplifier as 0dBm is normal input sensitivity.|
|Output Protection||Open Circuit||O. K.|
|Short Circuit||The amplifier worked correctly after 2mins short circuit but the fuse did not blow, as it should have done. The amplifier became very hot and there was a smell of hot varnish.|
|Noise||-94dBm referred to I.P.||tone controls set to best square wave response i.e. flat frequency response.||Reasonable.|
|Capacitive||O.K.||1uF and 8u load||good stability margin.|
As an engineer I must admit that I tend to see valve amplifiers in the same light as steam locomotives! The technology is old but they can work perfectly well and they are easy to maintain. As a musician I am also aware that there is a "valve" sound which is different from that of transistor amplifiers and is preferred by many musicians to transistor amps. The difference is mainly in the different nature of the distortion. Total harmonic distortion of 4.6% at rated output looks bad compared with the 0.1% or so which is typical with many transistor designs, but in this case, I am tempted to remark that this is part of "the valve sound". Certainly, most valve amplifiers will give a similar distortion figure.
Tone controls work well but more meaningful names and a better description of their operation on the instruction card would be useful. The "boost" control is described as "a secret design" but the circuit used is not original. Orange set a high standard of engineering and do not need to sell "magic".
The short circuit test of the output protection gives some cause for concern even though the amplifier survived the two minutes of short circuit while being fully driven. The fuse, which should have blown, did not; and the amplifier became very hot indeed, it is only fair to point out that designing output protection for valve amplifiers is very difficult and few amplifiers of the valve era would have survived the short circuit test. The temperature rise under normal conditions is perfectly all right.
A circuit diagram is provided on the instruction card, and this together with the basic nature of the circuit should make maintenance quite easy. This is a feature which will be attractive to many professional bands.
The word "Graphic" in the amplifiers' title is a little misleading — perhaps refers to the front panel decoration, which includes an attractive coat of arms. The graphic does not have as many facilities as most transistor amplifiers of a comparable price but it is very well made and should give good service to anyone who is looking for "the valve sound". One would have to took a long way to find another valve amplifier of comparable quality.
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