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Valve Biasing

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Valve Biasing VALVE AMP BIASING Biased information How have valve amps survived over 30 years of change? Derek Rocco explains why they are still a vital ingredient in music making, and talks you through the mysteries of biasing N THE LAST DECADE WE HAVE a signal to the grid it causes a water as an electrical current, you alter the negative grid voltage by seen huge advances in current to flow from the cathode to will never be confused again. When replacing the resistor I technology which have the plate. The grid is also known as your tap is turned off you get no to gain the current draw required. profoundly changed the way we the control grid, as by varying the water flowing through. With your Cathode bias amplifiers have work. Despite the rise in voltage on the grid you can control amp if you have too much negative become very sought after. They solid-state and digital modelling how much current is passed from voltage on the grid you will stop have a sweet organic sound that technology, virtually every high- the cathode to the plate. This is the electrical current from flowing. has a rich harmonic sustain and profile guitarist and even recording known as the grid bias of your amp This is known as they produce a powerful studios still rely on good ol’ – the correct bias level is vital to the ’over-biased’ soundstage. Examples of these fashioned valves. operation and tone of the amplifier. and the amp are most of the original 1950’s By varying the negative grid will produce Fender tweed amps such as the What is a valve? bias the technician can correctly an unbearable Deluxe and, of course, the Hopefully, a brief explanation will set up your amp for maximum distortion at all legendary Vox AC30. give you a full understanding of performance, thus ensuring that volume levels. In a ’fixed bias’ amplifier what is happening inside your amp the valve is operating correctly. The If you turn the cathode is and the enigma that engulfs the valve therefore operates literally as your tap full on grounded and a term known as biasing. A valve is a ’valve’ regulating the current flow you get a huge rush separate negative made up of a minimum of four from the cathode to the plate – of water. If your voltage is component parts: the heater, that’s why in England we call amplifier does not have applied cathode, grid and plate. They are it a valve. enough negative voltage at to the grid of all housed in a sealed, airless the grid then you will find you the output vacuum and this is why the What is bias? have an under-biased amplifier. valve. Some Americans call them vacuum tubes. The amplifier bias, and the This allows too much current to amplifier The heater warms the cathode, problems associated with it, pass through the valve and will manufactures, such which when heated allows the confuses many musicians If you cause it to burn out quickly and as Marshall, fit electrons to flow from the cathode think of a kitchen tap as a you will lose punch and clarity in a little trim-pot on the negative (negatively charged) to the plate valve and the your sound. power supply. This allows for easy (which is positively charged). The resetting of bias when the output grid is situated between the Types of bias valves are changed. cathode and the plate. You can now see that In Mesa/Boogie amps no such By applying the bias of the trim-pots are fitted but, contrary to amplifier affects popular belief, by changing a few its tone. More resistors, bias on these amps can be interestingly, adjusted. I would always fit a trim the type of bias pot where possible on these arrangement amplifiers as it will give you more that the amplifier valve choice in the longer term and manufacturer chooses also make it easier for a tech to set also affects the sound. the bias. These amps generally have Generally there are two high outputs and more headroom types of bias arrangement than cathode bias amplifiers; used in guitar amplifiers. A examples being Marshall ’cathode bias’ amplifier has a and ’Boogie. resistor going from the cathode to ground. This resistor is set to Matching and provide the correct current draw of dual matching the valve by setting the negative Now we need to understand the voltage. This enables the tech to terms ’matched’ and ’dual VALVE AMP BIASING Preamp valves (front) power output valves (rear) next to a rectifier valve matched’ (which are significantly under working conditions and load This simply means that the stock is the same amplifiers with only slight different) when replacing output to measure the output of the valve. new but is not of current modifications or simple bias valves. Output valves will normally This is measured in milliamps per production. These valves are alterations. The classic Marshall last around two years, but higher volt and ensures that the valves also generally made by British, German JTM 45 with KT66s, and quality British and American valves have the same output. This and American factories such as Malcolm Young’s Marshalls can last a least twice as long, procedure enables us to identify Mullard, Philips, GE and RCA. fitted with 6550A are just a due to better components and and supply valves with very These factories, sadly, no longer couple of examples. build quality. different characteristics. For produce valves. But these valves I believe the main reason so We are often bombarded with example, valves with a higher are the highest quality available as many players still chose valve- science and weird and wonderful output will generally distort sooner, they were made with very strict driven product is down to tone. methods of how to match valves. and valves with a lower output will production control and high A valve has a clipping point that Simply, the term ’matched’ means take longer to distort. Valves quality components. They are provides that rich distortion and that when a fixed amount of matched on current draw and well worth seeking out. They have sound which can’t be replicated negative voltage is applied to the output levels are termed as dual a longer life, better tone and by a transistor. Undoubtedly some grid, the valve will allow a certain matched. They will give you a richer superb reliability. manufacturers are now very close amount of current to flow through harmonic sound, as they all In recent years we’ve seen the to replicating the tone of a valve it. The amount of current allowed working evenly, and they’ll prevent Americans take over the control amp with alternate technology by each individual valve can vary premature component wear. and distribution of a number of but with minimum impact on the significantly, and therefore by Russian factories, giving rise to the valve market. matching, or grouping those with When do I need to names Svetlana and Sovtek. The The great manufactures of the same current draw readings we re-bias? Tesla factory has also reformed as yesteryear, like Mullard, Philips, can ensure that the amplifier is Having the bias checked should JJ. Their valves are very good. The GE and RCA all used different drawing evenly with a similar always happen when you change Chinese-made ones, which plagued materials and techniques to current draw from each valve. your output valves. Valves that are musicians for many years, now produce valves. In the same So, having the valves matched (all made today are not as consistent as seem in decline. way that a great guitar shapes drawing the same current), gives those produced in the early days of the tone and characteristics of the amp technician the opportunity valve production. We quite Why are valve amps your amplifier, so does a valve, to correctly set the negative grid commonly see great variance so popular? so a lot of thought must be put voltage so the amplifier functions between valves from the same A valve amp’s sound is into buying valves and the correctly. If the valves were not batch. So to simply buy a matched tremendously flexible and can be characteristics desired. matched, as the technician sets the pair of EL34s, for example, even by tailored to the individual’s taste. Musicians have kept valves alive negative grid voltage, they would the same maker, is no guarantee Many valves like the EL34, 6L6GC, over the last 30 years and with the all draw different amounts of that they will work correctly as they 5881, KT66, KT77, KT88 and current Russian and Eastern Block current therefore making it may draw more or less current and 6550A all share the same base valves continuing to improve, the impossible for the amplifier to be differ from the way in which the configuration, yet they all have future looks good. Maybe in these set-up correctly. amp has been set-up. different sounds and can be used in days of samples and MP3 files we It is very important to The cost of using an amp understand, however, that this Useful numbers technician varies; some offer a full- method of matching, based on the service, others will just check the Here are some useful numbers Eire current draw, is only half the task. bias setting. The thing to remember for technicians who undertake P Keenaham amp repairs Because valves also have different amplifier biasing is that these guys see amps day-in Peter Keenahan, output levels, when you fire up 00353 1 671 8886 and day-out and they can prevent your guitar through the amplifier, Bristol London trouble happening – vital for all valves with different outputs will Soundburn Audio T A Services gigging musicians.
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