2011 Auto-bias module improved
 

The 2011 version of the Vanderveen Tentlabs Auto-bias module has drastically been improved. The new application note will be soon on our websites. This article shortly describes the improvements and how to use them.

1. Switch for half quiescent current

With trim pot R14 the reference voltage V-ref can be set, which determines the quiescent current per power valve. For instance: V-ref = 0.6 V, then Io = 0.6/10 = 0.06 A = 60 mA per valve. V-ref can be measured between point J3 and ground. See figure 1 for more details.

New is the jumper J1 with its two contact points. When these points are connected together by means of a switch, then automatically the quiescent current halves.

Why did we design this improvement? The larger the quiescent current through the power valves, the shorter their life span is. So, halving the quiescent current makes the life span of the valves much longer. Suppose you listen to the amp, while doing a lot of other things at the same time. You are not fully concentrated to the music, it is in the background. Under this condition you don't need the full quiescent current through the amp, you will be satisfied with half the current, and the life span of the valves will be long. Imagine, late in the evening. The high-end hour! Now you need the full quiescent current through the valves to hear all the beauty and details. The tubes age, but it is not for such a long time, only during this beautiful hour where you forget the world and deeply delve into the listening pleasure. With the new switch you can adapt the amp to your concentration on the music, the life span of the valves will be much larger. This new switch can be placed on the front or back side of your amp. Not connecting this switch makes the quiescent current equal to the standard condition: Io = V-ref / 10.


2. Quick start up by trimming V-bias

The original version of the auto-bias module had two jumpers to select the negative bias voltage range, selectable between 0-90 V and 90-180 V. The new module still contains these two jumpers, but now we have created the possibility to trim V-bias close to the desired value.

Before implementing the new auto-bias unit into your amp, you now first need to measure the negative voltage at the control grids of the power valves, or to measure the voltage over the cathode resistor (with its capacitor in parallel). Suppose, as an example, you measure 45 V over this resistor, or -45 V at the control grids.

These measurements make clear that you do not need the full V-bias of -90 V, but 45 + 10 = 55 V surely would be enough. Why is this important?

The auto-bias module needs time to bring the bias voltage to its desired value. From 90 V to the 45 V of the example, takes much more time (some minutes) than going from 55 V to 45 V. From 55 to 45 takes say 15 seconds and you therefore don't have to wait that long before the amp is in range. Figure 2 shows where and how to measure V-bias and how to trim it to the desired value.

The general rule is: trim V-bias 10 V more than the voltage at the control grids without the auto-bias module. You can trim this with R2 and measure at J4.


3. In-range reaction time is larger

We have enlarged the in-range reaction time of the module with a factor of five.

The reason why is that under certain conditions the overall negative feedback time behavior of the amp plus the reaction time of the auto-bias module could create a very low frequency oscillation, like a kind of motor boating. You shall not hear this effect, but it causes the quiescent currents to swing around their desired value, and this is something we don't appreciate at all. By making the in-range reaction time larger, we have prevented this situation.

4. Step by step implementation

1) Before applying the new module, first measure the negative voltage at the control grids of the existing amplifier, or the voltage over the cathode resistor with its capacitor in parallel. We for now call this voltage V-neg. (for instance V-neg = 38 V). On the power section of the auto-bias PCB, set the jumpers JP1B+JP2B to the V-bias voltage range 0-90 V. Suppose you have a V-neg larger than 90 V, then place the jumpers JP1A+JP2A in the 90-180 V range.

2) Implement the auto-bias module as described in the application manual.

3) Connect mains only to the auto-bias module and trim V-bias to the measured V-neg plus 10 V (for instance at 38 V + 10 V = 48 V). Be sure that the actual amplifier is NOT switched on. In this step we only trim the auto-bias module to its desired optimal range.

4) Switch the amp at "on", the high voltage should also be switched on. Next trim V-ref to the desired quiescent current level. For instance: Io = 65 mA, then V-ref = 650 mV. 5) Now the auto-bias module totally takes over and automatically brings the amp to its desired quiescent current level and keeps it constant, during silence and music.

5) Switch the amp at "on", the high voltage should also be switched on. Now the auto-bias module totally takes over and automatically brings the amp to its desired quiescent current level and keeps it constant, during silence and music.

Click here to go to the original autobias page.