PAB Autobias & Dual Class mode
In 2011 we designed an exclusive system called PAB Autobias ( for Precision Auto-Bias). It is a patented innovation from Doge for a Bias friendly setting and more accurate than manual bias setting which is the reason its name includes the word “Precision”. This system is managed by a microprocessor.
Later in 2015 we improved the microprocessor management and allowed it to control relays to possibly modify the voltage and current of the tubes. This innovation allowed us to offer on the same amp, either class A or class AB.
So we renamed this global system as Doge PADC (Precision Autobias with Dual Class mode)
Tubes amplifiers have this different with solid state amps that they need some setting for each tube. That’s what make the charm of tube world and that’s also the constraint which makes that some tube amp owners move
back sometimes to solid state world.
The reason of this “bias setting” is not so difficult to understand. While transistors are manufactured by robots, tubes are handcraft made by human beings and each one is a piece of art as “grid” (the piece between
anode and cathode) wires are as thin as hairs. As a result each tube manufactured has a difference resistance.
Normally the anode current in all tubes should be the same. But because each tube is different as explained just above, there is a “bias” that makes the anode current is not always the same in each tube. So a setting
is required whereas in solid state world not any setting is needed as we are not in handcraft word but mass production.
So what is called “Bias setting” is a setting to get the same current passing through all tubes of the amplifier. Doing this setting requires at least a voltmeter and a screw driver, but also experience, DIY knowledge.
As worst not only you need the same tools but in addition to open the amplifier and put it on the back to do it, operation which should never been done by the user itself as very high voltages are Inside the amplifiers. On the two photos below, we
show you a Mingda amplifier internal view (a quite popular Chinese brand ), with the trimmers inside the chassis and on the second view a technician setting the bias with the amplifier turned-on, opened with its 800V!
The advantage of our PAB autobias, is that you don’t need any tools to set the bias but your fingers ! (As said Steve Jobs when he introduced the iPhone
is 2007 Keynote). It just uses two buttons on front panel or remote controller. Just below you can see a screenshot of DOGE 9 PAB front panel.
All the job to make this friendly user interface is done by a sophisticated STT-Thomson microprocessor. To make this simple it required us huge job of research and a partnership with the Digital
department of the South-West Central University of China. It is really a sophisticated technology that we are proud of and that we have patented.
Because our PAB autobias uses a sophisticated micro-processor, it also allow us to offer many additional features, like:
■ DCA : Dual Class A
■ APM : Advanced Protection Mode
■ DTI : Tube Dead Indicator
Some of you with technical knowledge may argue us that there are already autobiases on the market, like the “Adaptive AutoBias”. Yes. However, even this bias is also friendly to use, our own opinion at Doge is that it is not a proper technology to set the bias, because “Adaptive Autobias” works in real time and all tube experts will tell you that the bias has to be set without signal and should not move depending the signal. Doing a real-time bias creates more distortion and we disagree that decrease it as the inventors claim. We could have used it as it is not patented, no invented by them but used in early 2000’s but we wanted to go to what we think to be the proper way. And we are happy that the respectable Austrian firm AYON created a system similar than us called “fix autobias”. We are not rich enough to have bought a Ayon amp to see if it is a clone of ours and we have patented if only in China. If you want to have deeper explanations about the advantage of our PAB autobias versus the “Adaptive Autobias” just click here.
Why is it important to have an autobias ?
The “bias” has to be set each time you change of tube. This need can be simply because you want to try another brand of tubes, or if one tube crash before its expected life span. But it is also important to know that
because the metal parts inside the tubes are extremely hot, the resistance value of each tube change year after year and the settings that were goods at date of purchase might not be accurate two years after.
HOW our Precision Auto Bias does operate ?
Our PAB is quite simple. It uses only two buttons and nothing else (no tools, neither meters…). There is one button for Bias checking and one button for Bias reset. Some of you may wonder why we didn’t we made only one button ?
The reason is simple. The Bias reset is a process that takes around 45 seconds because our precision is of +/- 0.0001 Ampers and it takes a few dozen of seconds to reach that precision. On the other hand, the bias
checking function requires only 5 seconds. Because both Bias checking or Bias reset requires to put the amplifier in mute mode, the purpose of the bias checking button is to put your amplifier in mute mode the shortest time possible is not any as
most of the time, you won’t need to pass through the step of the Bias reset. So, with the Bias checking function, you can check it as frequently as you wish, without the need to stop your music for too long. However we recommend our customer not to
focus on this too often. One check twice per year is enough. Our wish is that you listen to music first not focus on settings 🙂
The process itself of our PAB can’t be more simple :
Each tube has a dedicated LED. If the Bias is set at the good value, the LED will light in blue color. If the Bias value has changed and is not anymore accurate, then the same LED will light this time in red color. If
you realize that at least one tube has its LED lighting in red color, then you will have to push the Bias reset button and wait 45 second to have your bias to be accurate again. When all LED are blue, the process is finished, and you can go
on to play music again.
During he was engineer for JOLIDA, our manager Mr Zey LIU already invented a way to make the bias job more easy for the final user. This technique was called “EZ-biasing” or “Easy Bias”. The purpose was to use only a screwdriver and not anymore a meter, as the good setting was indicated by a green LED. But Mr Liu was not fully satisfied by this system which had a weakness, the LED stay lightened even if the bias value is too high and still requires a screwdriver. As a result of his quest to perfection, in 2011, Mr LIU wanted to go one step further and created a full automatic bias, using a microprocessor who would do automatically the job instead of setting instead of the final user. Thus, was created our “Precision Auto-Bias” (PAB).
You might read is some reviews of the products we design for LUA in german magazine STEREO that the PAB autobias, called by our customer LUA GS-Autobias was invented by them. We want to clarify we have installed under licence our autobias in their products, but PAB autobias was designed in January 2011, released on June 2011 on Doge 5 and for the first time in November 2012 on a LUA product.
PLEASE NOTE THAT BIAS SETTING ARE ONLY FOR POWER TUBES, like KT88, EL34,… and not for input tubes likes 12AX7 or 12AT7 inside our preamplifier or CD player. All these small tubes have already traditional autobias with RC at the cathode.
Note for users who only want a deeper knowledge :
In the 60’s already appeared some auto-bias (using a resistor and capacitor below the cathode). This auto-bias was more friendly to use for the average user than manual bias, but had in one hand the requirement of using absolutely perfect matched tubes, and on the other hand this “old” autobias decreases of around 30% the potential max power of any tube used. As for example, an EL34 push-pull of 50W, will deliver only 30W with the auto-bias technique of the 60’s.
And at last, while aging, it doesn’t remains as accurate as manual bias, because the electrolytic capacitors used have a limited life span. This technique is still used today by a few brands because it is the most simple way to manufacture a
tube amp, but not the most efficient way.