Va,Ia exeed limiting values in datasheeds ?

[karsten21]

Dear folks,

I wonder how the graphs in most datasheeds exeeds the limiting values. As example the ECC82 from JJ show a graph for Ug1 = -20V which ends at Va = 400V. The limiting Value for Va is max 300V. Same situation for Ia which reach 23mA and the limit is 20mA.
Does this means, that the limiting values given by the manufactor should not be reach in run mode but can exceed for a few seconds without to break down the device? For "sand" devices means exceeding the limits the absolute death of them...
So if I want to measure Ia/Va/Vg curves I can do this by exceeding the limitings by say ~ 30% for Va and ~5% for Ia ? But how long is this allowed without to destroy the tube?

Thanks for your help and sorry for this real basic question..

Regards
Karsten

[tubelab.com]

For "sand" devices means exceeding the limits the absolute death of them...

Sand based life forms are typically loaded with another sand based life form and the linits are indeed absolute plus acceptable manufacturing tolerances.

I wonder how the graphs in most datasheeds exeeds the limiting values.

In the early days of vacuum tubes it was common for even signal tubes to be loaded with an inductor. The expected signal swing at the plate of a tube can be as high as twice the B+ voltage in an ideal situation. This is expected and needed when plotting a load line into a transformer.

Often the plate voltage can exceed twice the B+ in a tube stage that is misloaded and ran into hard clipping. This can be the case in a guitar amp, and it does lead to fried parts.

[Alain Poitras]

Originally Posted by karsten21

Dear folks,

I wonder how the graphs in most datasheeds exeeds the limiting values. As example the ECC82 from JJ show a graph for Ug1 = -20V which ends at Va = 400V. The limiting Value for Va is max 300V. Same situation for Ia which reach 23mA and the limit is 20mA.
Does this means, that the limiting values given by the manufactor should not be reach in run mode but can exceed for a few seconds without to break down the device? For "sand" devices means exceeding the limits the absolute death of them...
So if I want to measure Ia/Va/Vg curves I can do this by exceeding the limitings by say ~ 30% for Va and ~5% for Ia ? But how long is this allowed without to destroy the tube?

Thanks for your help and sorry for this real basic question..

Regards
Karsten

All the datasheet curves graphs exceed the limit values for convenient use, you have to choice an operating point inside those limits but the instantaneous voltage, current and power with signal swing can exceed those limits, specially in the case of power output stages.

Some curves graphs already include a maximum plate dissipation line but you can draw it yourself easely. For each plate voltages on the "x" axis, you calculate the plate current ( Ia = Dissipation / Va ) and mark it on the grid, then, you just have to reliate those points with a line to get the dissipation limit curve.

In a class "A" amplifier stage, the current don't vary much at maximum power but in a class "AB" or "B" push-pull power amplifier, it is much greater at maximum power than with no signal and you have to consider that for the operating point choice.

It is alway better to stay under the limits for longer tube life, specially the maximum plate dissipation over which the tube can fried, I consider about 10% less than datasheet maximums is safe. The maximum plate voltage is to avoid arcing between electrodes and is less critical.

Alain.

[Enzo]

And keep in mind that data sheets are intended for engineers to use during design. Within those specs, the manufacturer expects the tube to have a reasonable lifetime of reliable service. The specs are not indicators of just where a tube will fail. They don;t recomend a tube be run over 300v, but that doesn't mean that the tube will fail at 301 volts.