A moderate amount of hot biasing produces a smooth, warm and ‘bluesy’ drive, too much and the sound becomes more aggressive and ‘fuzzy’ This shifts the bias point to the right-hand end of the load line and the output signal will be clipped on the positive cycle of the output signal-this is called grid current limiting and is shown in Figure 5. Reducing the value of the 1K8 cathode resistor causes more anode current to flow so that the tube runs physically hot. Because the bias point is offset from the centre of the load line, the output signal clips to a greater extent on one half of the cycle than the other-this is asymmetric clipping. Additionally, by manipulating the bias point on the load line they could exercise a degree of control over the input sensitivity in each stage of the amp so that the signal clipped in a more controlled manner. Their amps contained several tube stages in series (typically four stages) to massively boost the input signal until it went outside the linear region of the tube. It wasn’t long before designers began experimenting with multi-stage tube preamplifier circuits-pioneers of the art include Alexander Dumble, Randall Smith and Mike Soldano. Consequently, this is a very clean sounding amplifier and introduces relatively little distortion into the signal-early guitar amp manufacturers weren’t striving to generate huge amounts of overdrive and sustain with their circuits, just amplify the signal so the guitar could be heard above the drums. Another way of putting it, is to say the input sensitivity of the stage is at its lowest (input sensitivity describes the input voltage required to drive the stage into the non-linear region on the curve and cause clipping). This centre biased amplifier section has the maximum threshold of clipping possible (highest headroom), that is where the grid can be fed with the largest possible signal before clipping. The small input voltage (x-axis) controls anode current (y-axis) flowing through the tube to give amplification. Figure 2 shows the ‘transfer function’ or ‘load line’ for this amplifier. The preamp and power amp sections both have an influence on the overall tone and feel of the amp.ĭistortion is low in this type of amplifier circuit as the 1K8 cathode resistor sets the bias point right in the middle of the tube’s linear region as shown in Figure 2. Together, voltage and current amplification give power. In a typical vintage amp this gain stage is followed by passive tonestack and then the power amp section, which amplifies current. With a 12AX7 tube in there this circuit gives about 30dB of voltage gain, with a 12AU7 expect about 15dB of gain. The grounded cathode amplifier turns a small voltage from your guitar pickup into a large voltage-it’s a voltage amplifier. This circuit is also a standard building block for amplifier circuits and is used by many other amp manufacturers including Soldano, Boogie and the British amp manufacturer, Marshall. This type of circuit is called a ‘ grounded cathode‘ gain stage and is found in many classic Fender amps, including their Deluxe and Twin Reverb ‘Blackface’ and ‘Silverface’ amps manufactured in the 1960s and 70s. Figure 1 on the right shows a typical tube preamp stage. The console was upgraded with the following for better stability and performance in 2012.Īll of the electrolytic capacitors were replacedĪll the molex style connections were replaced with gold plated versions.Ĭhannel path pushbutton switches replaced with high quality gold-contact equivalents.Your tube amp is made up of a preamp and power amp section. The mute groups, solo-in-place, and the muting relays themselves can be defeated on a per-channel basis or globally. The relays also function on solo button logic, which allows for solo-in-place, which didn't exist on the original console. The other modification was the insertion of muting relays, which allows up to 10 mute groups to be assigned using thumbwheels on the channels. This means that the mix can be left undisturbed from the start of the session to the end, and a session can include tracking, overdubbing and mixing at any time, without changing the status of the channels. We have changed this configuration in two ways: The channel audio now goes to the output buses from the small pot labelled Monitor, while the tape return goes to the fader. This necessitates a complete changeover of the board before you can enter mix mode in a session. The fader is normally routed to the output buses for recording. In a standard Series II, the monitor path from the tape machine is through the small pots labelled Monitor, unless the Mixdown button is pressed, which routes the channel audio to the 2-Mix. If you are familiar with Neotek Series II desks, the modifications we have made to ours may interest you.
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