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Constant power operation

Posted by 6AS7_6SN7 on April 10, 2018 at 10:28:43

About two years ago I was intrigued by this very interesting article of John Broskie:

https://www.tubecad.com/2016/06/blog0347.htm

The article refers to SE design but it suits equally fine for PP class A operation (as I will show in the following).

"What if we are willing to accept the price of Class A operation, but we are not willing to have the output tubes plate dissipation exceeded? In other words, how do we achieve a constant power dissipation of the output tubes regardless of the wall voltage's wandering? The answer lies in a slight modification to the auto-bias circuit so that it adjusts the idle current to match the changes in B+ voltage."

Starting from the original Broskie schematic I moved to LT Spice world in order to check if constant power operation of tetrodes (in the following I have used the 829B) works fine (it does) and if there may be any other criticality (at least on paper).

Referring to the LT Spice schematic (where there is the LT1431, that is the LT equivalent of TL431 of Broskie circuit), the 829B sees a cathode-to-plate voltage of about 560V and -17 V of grid(23V)-to-cathode(40V) voltage. The bottom 50 Ohm resistor, R13, sees 2.5V of voltage drop, so the resistor experiences a current flow of 2.5V/50-ohms or 50mA. So does the 829B idle at 50mA? No, not exactly, as the LT1431 draws 17V/18k or 0.945 mA; thus, the 829B idles at almost 51 mA of current flow. If the LT1431's reference pin sees more than 2.5V, it will pull down its cathode K voltage, which will decrease the 829B's grid voltage, which in turn will decrease the output tube's conduction. Conversely, if the reference pin sees less than 2.5V, it will let go of its cathode voltage, which will increase the 829B's grid voltage, causing an increase in the output tube's conduction. Auto-bias in a nutshell.

The following graphs show the 829B cathode voltage (green), the LT1431 K terminal voltage (blu) and the output audio signal (a 100 msec 330 Hz burst, that is 33 cycles, whose amplitude spans the whole class A range of this circuit) after initial 'hot' power up.

The difference between the two couple of graphs (42 seconds of time history or the zoom centered on the audio burst occurrence after 38 seconds) lies in the value of the 829B cathode capacitor (47 microF or 470 microF).

It is amazing that the 47 microF capacitor results in much better audio waveform w.r.t. the larger one.

I recommend to play with simulations of this circuit because you can learn a lot about the way tubes behave under sudden load variations.

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