Volt Amp Co

Introducing TubeSync ® The Output Valve Biasing System

TubeSync® is a new component that overcomes many of the problems associated with conventional tube amplification. It addresses these problems without changing the fundamental tube amplification process, hence the resulting sound output still exhibits that pure analogue magic. TubeSync can be easily integrated into any guitar tube amplifier. It can also be retrofitted or incorporated into the circuitry during the design stage.

Eliminates tube matching

Eliminates the need for bias current matching of amplifier output tubes,this increases manufacturing efficiencies, due to reduced testing.

Reduced tube cost

Eliminates the need to purchase tubes in ‘matched pairs' and replaces conventional biasing components.

Increased tube life

Automatically micro-adjusts the bias on each tube to ensure the full potential of each tube in the system is realised throughout its working life.

Reliability assurance

Replaces conventional tube testing methods by performing an ‘in circuit' test every time the amplifier is powered up.

Half power back-up

If TubeSync® detects a faulty tube, it can automatically 'switch out' the offending tube and run the amp at half power, until the defective tube can be replaced.

Maintains optimum performance

Dynamically measures the amplitude of the drive signals supplied to the grids of each output tube and optimises performance accordingly.

Simultaneous distortion

Maintains the classic warm sound of the amplifier by ensuring tubes distort symmetrically.

Reduced power consumption

Reduces quiescent power consumption when compared to conventional amplifier biasing techniques by an average of 20%.

NEW ' Hot Anode'

TubeSync® knows the optimum operating temperature of your tubes, so there is no need to warm up your amp. TubeSync® will get you to where you need to be just as soon as you start playing.

Half Power

Capability to switch out a pair of tubes to run the amp at half power (100 w heads). Each time half power is selected each pair of tubes will be alternated to ' share the wear'


Unique algorithms can be incorporated to tailor optional performance characteristics and features to customer requirements.

How It Works


At Power-On the TubeSync module performs a self calibration routine (indicated by the fault indicator LED's flashing 1&3, 2&4 alternately). To prevent damage to the cathode emissive material, which can occur if an attempt is made to draw electrons from it, before it reaches working temperature (a process known as ‘cathode stripping'), during this period, the cathodes of the valves remain disconnected from 0V and maximum grid bias is applies to each of the control grids.

TubeSync also monitors the current for any leakage during this ‘warm-up' period, so if the HT is applied to the anodes and screen grids of the output valves, any current detected will indicate a faulty valve which will very likely ‘flash over' when the cathode is connected. Under these circumstances, the fault indication and handling system will be instigated.

After the initial warm-up and calibration period there are two things which the module needs to know about the amplifier before connecting the cathodes to 0V.

The first is that if the anode high voltage (HT) supply is present, i.e., whether the ‘standby' switch is open or closed and it does this by checking to see if all the cathode currents are below the ‘minimum' threshold value (default 10mA, but adjustable using Bias Runner software). In the event that the switch is open, the Bias Engine module does not attempt to re-bias the amp and will restore the last known working value of grid bias voltage for the amplifier. TubeSync will then monitor the cathode current until it becomes greater than the minimum threshold current limit, when a bias correction will be made as necessary in order to bring all the valve cathode currents within +/- 0.5mA of the set value.

The module will also restore the last known working value of bias voltage if there is an audio signal present at power-up and will not re-bias the amplifier until the first ‘silent' period, when corrective biasing will take place.

TubeSync has two biasing modes during normal operation, namely, a current value when audio is present, i.e., when the amp is being ‘played'; the second is a current value when no audio is present, for example, during an interval period. Performers are often advised to use the ‘standby' switch when the amp is not being played. Depending on the amp manufacturer, the effect of this switch may be to turn off the high voltage to the output valves completely, or, in some cases, to leave the anode supply on the output valves and remove the high voltage supply to the preamp and phase splitter valves.

In the former case, it is a known fact that valves left for long periods with their heaters running, but with no appreciable cathode current draw, will suffer a failure mode known as ‘cathode poisoning'. This has the effect of reducing the amount of current which can be drawn from the valve when it is being driven normally and gives a similar effect to a ‘worn out' valve. In the latter case, the output valves are left to run at their full quiescent current value continuously, thus causing needles heat generation, power consumption and detracting from their useful working life.

When TubeSync detects that the amp has not been played for a preset time, it automatically switches between the two biasing modes, the current settings may be adjusted using the bias runner software, for example, to run at the ‘normal' current setting, when the amp is being played, but drop back to a lower current setting when it is not, thereby reducing power consumption, heat generation and extending valve life, whilst eliminating the possibility of cathode poisoning. Other modes of operation are, of course possible, for example, actually keeping the valves up to their ‘played in' temperature, even during interval periods by slightly increasing the quiescent current during non-audio periods or by making both the audio and non-audio conditions equal, so that no effective adjustment takes place.

The V4 model allows the user to switch between a ‘standard' default bias setting of 30mA and a ‘custom' value which is factory set to 35mA with audio present and 20mA with no audio present. These custom values may be changed by a TubeSync appointed amp technician using the Bias Runner software.

The audio threshold value may be increased using the bias runner software; in the unlikely event that the internal noise in the amplifier is high enough to make the software think that audio is present. This is the threshold between what is considered to be signal and no signal conditions. The 1 to 100 numbers in the software are arbitrary; a value of 100 corresponds to approximately 4 watts of audio power required before a ‘signal' is detected and a value of 1 represents approximately 100mW. The default value of 50 is suitable for most applications and corresponds to an audio signal of approximately 2Watts.

The currents are continuously measured and ‘micro' adjustments made if necessary to this value, every time there is no audio signal present for longer than approximately 2 seconds. This ensures an almost perfect balance even when tubes are hot / leaky etc.

During non-audio periods, if a current higher than the maximum threshold value (default 100mA, but adjustable using Bias Runner software) is detected in any valve, it will be switched out of circuit and the appropriate LED will be lit to indicate the faulty valve; a fault condition will also be logged in software against the appropriate valve. This can later be interrogated by a TubeSync appointed amp technician using the ‘Bias Runner' software. If the amp has 4 valves, one of the other valves on ‘the other side' of the push-pull quartet will also be switched out automatically in order to balance the amplifier. Automatic re-biasing will take place on the remaining pair of valves in order to compensate for the change in HT voltage when two valves are switched out. The amp will now run at approximately half-power so that the performance can be continued without interruption (albeit at a lower power). If a V4 model is being used, with a footswitch connected, the on-board LED will indicate that a fault condition has occurred and the amp is running in ‘limp-home' mode. The fault LED's should then be consulted to determine the faulty valve.

If an over-current condition is detected when audio is present, TubeSync uses a complex algorithm to determine whether the high current is normal and due to the amplifier being driven hard with audio or whether a real fault condition is present. In this way, protection is given without the risk of false tripping. This dynamic fault detection operates on sustained currents in excess of 250mA.

The power to the amplifier must be re-cycled in order to reset the fault condition. TubeSync will then treat the valve as normal again and monitor it for a fault condition once again; however, it is strongly advised that the faulty valve is replaced before recycling the power to the amplifier for several reasons. A faulty valve sometimes does not exhibit any obvious fault until it is hot or under stress (when driven hard during a performance), so allowing it to cool can result in it apparently operating normally, however it is likely to fail at some point in the near future, probably during a performance. A second reason is that a valve which is ‘leaky' or ‘gassy' may exhibit a slow ‘thermal runaway' condition which will be detected and isolated by TubeSync. However, if ignored, this can lead to the development of other much more severe faults such as internal arcing to the heater (which is earthy, so by-passes the cathode current detection circuitry), resulting in damage to other components including the output transformer.

One major benefit of TubeSync is that it allows the use of unmatched valves (including even mixed valves types such as EL34, 6L6, KT66, etc.), therefore only the faulty valve need be replaced instead of the usual need to change a quartet when one valve becomes faulty.

If a valve is replaced but the new valve still indicates a fault, then the amp should be immediately returned to a qualified amp technician for investigation as this indicates that another serious and potentially damaging fault could be present with other components in the amp.

If a valve is completely ‘worn out', missing or the heater is open circuit, this will also be indicated as a fault condition on the LED panel. Note the open circuit detection feature is disabled for approximately 1 minute after power on calibration routine, to prevent false tripping due to slow warm up times of certain types of valve.

The automatic switching principle is taken further in the V4 model, which has a provision to connect a footswitch in order to deliberately switch between ‘pairs' of valves in a 4 valve amp. This facility gives the user the ability to, say use a ‘pair' of 6L6s' and a ‘pair' of EL34's and switch between them using the footswitch. The valves could even be biased to different currents, if desired' by an appointed TubeSync' amp technician using the Bias Runner software, giving the ultimate in customization!

A switch on the V4 gives the possibility of running the amp permanently in ‘two valve' mode, reducing its power output for smaller halls etc. When this mode is selected, a ‘share the wear' feature is instigated which ensured that each time the amp is powered up, or indeed, each time the two valve mode is selected, the opposite pair of valves will be used, thus ‘sharing the wear' between the valves. A second switch on the V4 disables this feature to allow specific valves to be switched in as described in the previous paragraph.

Please contact Geoff Pugh for more information

Tel 01785 812071

Tubesync ® Bias Engine ®

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