6H13S lamp fixed bias options. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers

 Comments on the article

Cathode bias is not suitable for this lamp (and even in transformerless amplifier circuits it is generally impossible). Therefore, a fixed external offset from a separate source. What?

Further analysis is given for two output stage OTL modes (1) Ua=100V, Ug=-33V, Ia=100mA (2) Ua=160V, Ug=-70V, Ia=65mA. Here and below, the numbers are given for one triode. In both cases, the anode power at idle is 10W at the nominal mains voltage (and, therefore, at the anode). But this is the denomination. But in practice, the mains voltage walks, and the anode voltage will float along with it. This is where the scheme can go wrong. Further, we assume network fluctuations within 175-260V. These are really extreme conditions, but what the hell is not joking ...

1. Rigidly fixed battery or stabilized bias. Absolutely unacceptable! With an increase in the mains and anode voltages, the anode current goes up according to the 3/2 law and the lamp dies. Soon enough if the power margin is small. In my case, when plugged into an electric kettle, the mains voltage drops by 6V, and the anode current drops by 20% (from 48 to 40mA) - what will happen if the mains voltage rises by 20V ?!

2. Fixed ratio Ua/Uc. This will be the case if the sources are fed from the same network, and their (anodic primarily) output resistance is small enough. An increase in the anode voltage is partially offset by an increase in a decrease in the grid potential. With mu=2, if Ua=2Ug, the current will be constant, and the power will be proportional to the mains voltage. In the selected modes, however, full compensation does not occur. It is easier to represent this graphically by replacing the marking of the Ua axis with the mains voltage on the I–V curve:

The most attractive bias is in the constant power mode, where the overcompensation of the anode shift by the grid CURRENT software leads to an almost unchanged anode power.

A linear approximation for any mode of this lamp is Uc = -0.75*Ua + dU, dU=+37..+60V (excluding the output resistance of the anode source and the output resistance of the common wiring). It is clear from the equation how to obtain such a bias: we take a source that gives an output of -0.75 from the anode one, and remove the excess of 37..60V with a zener diode. In practice, it is more convenient to put the zener diode at a voltage of about 10V, less than the required shift, and adjust the actual shift with a potentiometer.

Option 1 (for Ua=100V) (without adjusting potentiometer).

The CSS soft start capacitance at start locks the lamp for 2-3 seconds. When the power is turned off, the 27V zener diode fixes -27V on the grid for about 10 seconds.

Option 2 (for Ua=160V, implemented in hardware in the Mammoth I project).

For the convenience of adjustment (yes, simply because the secondary winding was only at 72V, and it was recruited from two incandescent and two "anode") - the zener diode was taken at 24V, and the adjustment in the left shoulder is carried out by potentiometers R4 (common) and R5 (balance) . Unlike the previous scheme, here the compensation actually occurs not in terms of power, but rather in terms of current. Because the power ceiling was not enough to embed a 50V zener diode. The resistance of the voltmeter is about 100 kOhm (not fundamental, the main thing is not to be too small).

And now I warn you ... If the time constant of the grid power filter is significantly greater than that of the anode one (and it cannot be otherwise) - the compensation is slow, and immediately after the change in the mains voltage, the lamp behaves as in battery bias. That is, it goes to pieces. You still need to have a margin for anode power and current. And if there is time, select the minimum acceptable capacitances in the bias filter in order to speed up its response. Somehow, out of habit, I did not regret the farad ...

Acknowledgments, links, notes

• Website of Atma-Sphere and its founder Ralph Carsten, atma-sphere.com
• Site of Atma-Sphere amplifier owners, otlamp.com

Publication: klausmobile.narod.ru

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