Automated bias

If you’ve ever owned, regularly used or just love tube amps, then you’re probably familiar with the bias setting. It impacts the tone, proper operation and  lifespan of a tube amp’s output section. An engineer will manually set the bias voltage when replacing the power tubes. But regular checkups in between swaps are strongly advised to avoid various issues and maintain a consistent sound. What if the amplifier itself would automatically take care of setting the optimum bias and keep it that way no matter what? A deBont bass amp will do just that. No more manual adjustments… ever!

What is ‘bias’

A tube will amplify the signal presented on its input, called the (control)grid. More precise, it will only do so for a signal that’s ‘more negative’ relative to the tube’s cathode voltage. An audio waveform however consists of both a positive and negative part. To amplify the entire signal, there are two main options:
– make the cathode more positive than the grid (grid stays at 0 volts, called cathode bias or auto bias)
– make the grid more negative than the cathode (cathode stays at 0 volts, called fixed bias)

Triode

This predefined voltage difference between the grid and cathode is called the bias. There are several ways to do this. Cathode bias and fixed bias being the most common. This article focuses on the power section where the fixed version reigns supreme; the grid of the tube is pulled negative, while the cathode is at (or close to) 0 volts. Fixed bias in the power section of a push-pull amplifier, as seen in most if not all bass amps, is used in its own special way.

Push-Pull amplifiers

A push-pull output stage, also known as class-B or class-AB, is mandatory if a tube power amp is expected to deliver some serious power. This specific output stage will use one tube (or several in parallel) for the up-going part, and another for the down-going part of the original waveform. Much like a seesaw, pushing and pulling current through the output transformer. To achieve maximum power efficiency, the fixed bias voltage is made extremely negative, down to the point where the tubes can only amplify a positive going signal, also known as biasing near cut-off. I mentioned earlier that an audio waveform consists of both a positive and negative part. To amplify the entire signal in this case, the negative part of the original signal is inverted (flipped to positive) before its presented to the power tube(s) responsible for the negative part of the original waveform.

Looking at pure class-B amplifiers, problems start where Cross-over distortion due to wrong biasthe ‘top-half-tubes’ take over from the ‘bottom-half-tubes’ or vice versa. Tubes are inherently non-linear in their amplification and this is especially true for the take-over-part. Something called crossover distortion comes into play and the effect can be seen in the image to the right. The yellow line should be a clean sine wave just like the blue one, but the crossover distortion is clearly visible. You can probably imagine how horrible this sounds!

The best way to battle crossover distortion is by having both tubes amplify the area close to the take-over-point simultaneously. In other words, the ‘down-going-tube’ will start to amplify while the signal is still in the ‘up’ region. Same but reversed goes for the ‘up-going-tube’. This is called class-AB and will cancel crossover distortion. Only a small overlapping region is needed and should be used to maintain efficiency. The amount of overlap is determined by the bias voltage. This is one of the main reasons why proper bias is so important.

Setting the power tube bias

How to determine if a power tube is biased correctly? You measure the amount of current a tube is conducting when there’s no signal present; the quiescent- or idle state. A small trimmer or similar device can be adjusted to change the negative bias voltage, and with it the quiescent current flow. The exact value depends on the amp and its design choices.

If the bias is set too high (not negative enough, also known as under-biased), the amp will run hot, tube life is decreased and more power is required from the power supply. It could even severely damage the amp and power tubes.

If the bias is set too low, also known as over-biased, crossover distortion becomes an issue again, it won’t sound right and it can also damage the tubes for the same reason standby switches can cause issues if used for extended periods of time. More can be found in a previous article about standby switches.

It’s also important that all tubes conduct the same amount of quiescent current. If not, there will be a constant DC current imbalance across the output transformer. This imbalance can result in a weak sound.

Dusty tubes

Tubes will age, bias will drift

All tubes will age, including the ones used in the power section. The rate of deterioration is dependent on how they’re used, how hard they’re pushed and of course, the type of tube. One property subject to change in the aging process is the tube’s transconductance. It dictates the amount of current a tube will conduct at a given input voltage on the grid(!) The older the tube, the less current it will conduct for the same input voltage. It’s also important to note that not all tubes age at the same rate. As mentioned before, the bias setting is all about the amount of current a tube conducts if no signal is present. Less current due to ageing means the setting is no longer in spec.

Bias settings can drift due to mechanical issues as well. The vibrations caused by transport or the bass frequency rumblings themselves can make the trimmers used to set the bias wander a bit. Quality of the used components is of course a factor, but it’s a real issue nonetheless.

This is why tube amps need to have their bias settings checked every once in a while and not just when new tubes are installed. Failing to do so will compromise proper bias setting and will eventually impair the sound quality long before the power tubes themselves are due for replacement.

Automated bias

All deBont bass power amps will feature automated bias. In essence, it will spare you all the hassle spread out above. This in-house developed technology is seamlessly integrated into the amp. It will measure the current flowing through each power tube several times a second and will automatically make any necessary bias adjustment . This means the power tubes will always have an optimum bias over their entire lifespan . It will assure a consistent sound quality, trouble free usage and no need for any manual adjustments ever again.

Replacing the power tubes is a breeze as well. Plug in a new batch of tubes, disconnect the input, turn the amp on and let it sit for 15 minutes. That’s it. No special tools or devices required. Spot on bias setting every time.

Besides the obvious benefits of never having to worry about the bias, the same technology will also notify you if a tube is at the end of its lifespan and needs to be replaced. It will detect faults like a defective tube or problems with the bias voltage itself. If any of these errors threaten proper operation, the amplifier will go into protection mode to prevent any possible catastrophic damage.

Using tube amps just got a whole lot simpler.

2017-11-12T20:07:19+00:00 May 17th, 2015|0 Comments

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