Hartke 3500 Bad Tube Preamp Output

[x-pro]

The main question is what filter?
after the bridge (without capacitors), there is neither direct nor alternating current.

[Helmholtz]

The main question is what filter?
after the bridge (without capacitors), there is neither direct nor alternating current.

Not sure I understand your question.
Don't mix up voltage and current.

Without filtering the unipolar (= single polarity) voltage after the rectifier consists of DC and AC.components.
A power supply filter (being a low pass filter with a very low corner frequency) reduces the AC ripple content, consisting of a fundamental of twice the mains frequency + harmonics.

[x-pro]

It is clear that we are talking about the form of tension. Otherwise it is a kind of current.
Correctly indicated, smoothing out pulsations. This means that the voltage there is pulsating, with the frequency that you announced. But it is neither constant nor variable.
I raised this question so that the author of the topic understands that there are pulsations and where they come from.
In this case, more precisely, the voltage waveform is not sinusoidal.

[Helmholtz]

I Otherwise it is a kind of current.

That doesn't make sense to me. Power supply filters filter voltage - not current.

In this case, more precisely, the voltage waveform is not sinusoidal.

Any waveform consists of a number of sinusoidal harmonics + a DC component.