Ballast triode

Line output stage with ballast triode, tube in the picture above (Philips Goya 90, approx. 1969)

Ballast triode in function: the anode plate, seen through a small hole in the metal cylinder, turns red-hot when the screen is dark.

A ballast triode is a tube triode ( electron tube ) that serves as a parallel regulator for voltage stabilization. Designed as a beam triode , it was used specifically to stabilize the high voltage of the picture tube in early color television sets .

The ballast triode is connected in parallel to the high voltage (anode voltage) of the color picture tube and has the task of "taking over" the power that is currently not required by the picture tube and converting it into heat. If the screen is dark, the ballast triode is under full load, which means that its anode is glowing red. This circuit principle was necessary because the high voltage generation in the color television was too high impedance at that time to stabilize the high voltage via the regulation of the line end tube (series stabilization). Changes in the image brightness would otherwise have led to strong fluctuations in the image size (“pumps”, “magnifying glass effect”).

In the early American and Japanese color televisions from the mid-1950s, the type 6BK4 was used as ballast triode, in German color televisions from 1967 to around 1972 the PD500 and the improved PD510.

In early American color television sets with 21-inch picture tubes, a high voltage of 20 kV was used with a maximum current (full brightness) of approx. 1 mA. The first German color television sets with the A63-11X (63 cm color picture tube) worked with 25 kV at a maximum of approx. 1.2 mA. These services were made available by the high-voltage winding of the flyback transformer and rectified with a tube diode (one-way rectification).

A particular problem with the first ballast triodes (6BK4, PD500) was the X-ray radiation generated at full load , which was often not adequately shielded, especially in German color television sets of the first generation in 1967. Later ballast triodes (e.g. PD510) had a higher proportion of lead in the glass, which reduced the escape of X-rays.

When, from 1968 onwards, increasingly low-resistance circuit variants with high-voltage cascades were used for high-voltage generation, stabilization was also possible via the control of the line end tube , which managed without the notorious ballast triode and completely replaced it by around 1973.