Part of what happens when a tube amp is run without a load connected to the output depends on the type of the amp.
For a Single Ended amp that has a driver capable of turning the output tube completely off, then the output tube plate may go to much more than 2X B+.
But for Push Pull amp the plates may, or may not go to more than 2X B+.
Example, for a triode push pull output stage, one tube may cut off, but the other
tube plate will have a very low plate impedance, rp.
Depending on the leakage reactance between the 2 halves of the primary, and the distributed capacitance between the 2 halves of the primary, the low rp of one tube may, or may not prevent the other half of the primary from rising to more than 2X B+.
But a Pentode push pull output stage has an extremely high plate impedances, rp. As one tube cuts off, the other tube may not prevent the other plate voltage from rising to more than 2X B+. Again, it also depends on the primary halves leakage reactance to each other, and the capacitance from one half of the primary to the other half of the primary.
And then there is what can happen if the amp negative feedback and lack of a load on the output causes the amp to oscillate.
As always, there may be exceptions to the above observations.
(Your mileage may vary).
Just to be safe, always have a load connected.
Not all circuits are created equal.
For a Single Ended amp that has a driver capable of turning the output tube completely off, then the output tube plate may go to much more than 2X B+.
But for Push Pull amp the plates may, or may not go to more than 2X B+.
Example, for a triode push pull output stage, one tube may cut off, but the other
tube plate will have a very low plate impedance, rp.
Depending on the leakage reactance between the 2 halves of the primary, and the distributed capacitance between the 2 halves of the primary, the low rp of one tube may, or may not prevent the other half of the primary from rising to more than 2X B+.
But a Pentode push pull output stage has an extremely high plate impedances, rp. As one tube cuts off, the other tube may not prevent the other plate voltage from rising to more than 2X B+. Again, it also depends on the primary halves leakage reactance to each other, and the capacitance from one half of the primary to the other half of the primary.
And then there is what can happen if the amp negative feedback and lack of a load on the output causes the amp to oscillate.
As always, there may be exceptions to the above observations.
(Your mileage may vary).
Just to be safe, always have a load connected.
Not all circuits are created equal.
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More about plate cap tubes, and where they arc . . .
The 4-65A transmitter tube has a maximum DC plate voltage rating of 3kV.
It is made for class C service, where the plate may swing 2X or more than B+.
Then there is a special version of the 4-65A. I can not remember the tube type number,
But it is rated for maximum DC plate voltage of 6kV.
The difference? The special tube has had absolutely all sharp edges and burrs removed.
Sharp edges and burrs will arc over at lower voltages, but the labor involved to remove all of them is expensive.
The 4-65A transmitter tube has a maximum DC plate voltage rating of 3kV.
It is made for class C service, where the plate may swing 2X or more than B+.
Then there is a special version of the 4-65A. I can not remember the tube type number,
But it is rated for maximum DC plate voltage of 6kV.
The difference? The special tube has had absolutely all sharp edges and burrs removed.
Sharp edges and burrs will arc over at lower voltages, but the labor involved to remove all of them is expensive.
4PR65A? IIRC, the "PR" in 4PR1000A stands for "pulse rated". I suspect these tubes are rated to higher plate voltages so they could be used in high-voltage pulsers for scientific and radar applications.
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