6AX4 datasheet question

[Bas Horneman]

In the attached datasheet it says max DC 165mA. Is that the maximum continuous current that my be drawn? I'm confused because later on it says Volgate drop at 250mA is 32V, implying that it would be normal to draw 250mA from it.

[smoking-amp]

The 165 mA rating is for DC on that one diode. For a full wave rectifier, one could be drawing 2X that 50% of the time. So the other rating is useful for full wave applications.

[Bas Horneman]

Thanks smoking-amp.

[kavermei]

Perhaps it is meant to be a worst-case DC voltage drop?

On a related note, I often see triode or pentode data sheets where the absolute rating of Va,max is blatantly exceeded in the curves. What's up with that?

[kavermei]

smoking-amp,

does a typical horizontal deflection circuit in a TV use full-wave rectification?

[tubelab.com]

does a typical horizontal deflection circuit in a TV use full-wave rectification?

No, the horizontal deflection circuit uses a pentode that is switched from non - conducting to conducting a fixed current by a nearly square wave drive signal. The fixed current is applied to the primary of the flyback transformer which has a nearly fixed inductance. This generates a linear ramp voltage that goes to the magnetic deflection yoke to "sweep" the electron beam accross the CRT screen. The beam is moved from left to right as the voltage ramps up. Once the beam reaches the right side of the screen the current is abruptly shut off. The abruptly collapsing magnetic field generates a high voltage spike in the secondary of the flyback transformer in a manner similar to the ignition coil in a car. This is applied to the high voltage rectifier tube (1B3 or 3A3) which rectifies the pulse and generates the high voltage needed to run the CRT. The CRT has conductive coatings on the inside and outside of the glass creating a high voltage filter cap.

The pulse created by this action would ring for several cycles at the resonant frequency of the flyback circuit (usually 150 to 300 KHz) if the ringing was not DAMPED. This was the job of the DAMPER tube (hence the name). It captured the unused energy that would otherwise cause trouble and used it to elevate the entire primary circuit of the flyback transformer to a higher voltage than the B+ voltage. This new supply voltage ran the entire sweep circuit, and even supplied a few other circuits. It was often called "boosted B+" or "boost voltage" or just "boost". Think of it as a self bootstrapped power supply. The cathode of the damper tube operates at the boost voltage, hence the outrageous H-K voltage ratings in these tubes.

So, enough of the history. The damper tube is a half wave rectifier that is driven with a ramp current. This current ramps from near zero to a peak value at a 15.734KHz rate. The average DC value is the DC current delivered to run the sweep (and other) circuits and the peak value is the current just before the horizontal (line) output tube is switched off. These were never designed to operate as rectifiers from a 60Hz (or 50Hz) line although many people have used them for that purpose.

The 165 mA rating is the maximum safe DC current that should be drawn from the tube. This is governed by two things the maximum plate dissipation and the maximum cathode current. A new tube in good condition can support currents in excess of the 165 mA rating, but as the tube ages the voltage rdrop across it will increase which increases the plate dissipation. Eventually a runaway condition will occur leading to a tube arc. The peak current in a rectifier tube producing a 165 mA DC current will be well over 250 mA. In spite of these warnings, I have cranked 200 mA through 4 6AX4's in a full wave bridge configuration without failure. YMMV

I often see triode or pentode data sheets where the absolute rating of Va,max is blatantly exceeded in the curves. What's up with that?

A tube which is intended for operation into an inductive load (like an OPT) will experience an inductive storage effect. If the DC plate voltage on a hypothetical tube is 500 volts, and the tube can pull this down to 100 volts when driven to full conduction, where do you think the voltage will go as the tube transitions to a non conducting state? The energy stored in the primary inductance of the OPT will pull the plate up roughly an equal amount in the opposite direction. The plate of that hypothetical output tube with 500 volts of B+ can (and does) swing from 100 volts to 900 volts. The curves in the "over spec" region are needed to draw an accurate load line.

[kavermei]

George, thanks!, very insightful.

I always thought of the "absolute maximum rating" for plate voltage in the data sheets as the maximum *instantaneous* voltage which should never be exceeded (not even by voltage induced by reactive loads). So am I correct in saying that what is in the data sheets is really the absolute maximum *DC* plate voltage, whereas during signal peaks it is permissible to be almost double?

Oh and I know, tubes have guidelines rather than ratings