Plate Voltage vs Frequency Response

[nickb]

To my simple mind, lowering the plate voltage in a typical PP tube amp will slightly improve the HF response as the lower volts =>lowered gains => reduced miller capacitance => reduced HF loss. Of course Rp goes up too and that reduces the HF. Net effect pretty much a wash. Checked with simulation.

Yet, I see it stated again and again that lower plate V=> reduced HF, most recently here 5R4 Rectifier Swapping | 5R4G/GB/GYB | kcanostubes.com that was mentioned in another (recent, not relevant) thread.

So,

Q1: Huh?
Q2: Why do I never seem to have a complete amp lying around to test?

Afterthought: I can see that if you lower the plate voltage on a PP output stage that the load line will cross the Vg=0 curve higher i.e. saturation and that will give you more higher order harmonics when cranked. This makes we wonder if people have been confusing harmonic content with frequency response.

[pdf64]

My hunch is that it may not be possible to demonstrate an effect one way or the other, probably because when responding in the broadly linear operating area, there may not be a measurable difference big enough to be perceived, eg >+1dB over the audio range, when comparing a 'higher' and 'lower' VB+.
Rather it may be more to do with the overdriven response.
Certainly I perceive much more 'sizzle' when overdriving a regular guitar amp using a GZ34 than a 5R4, probably due to the tubes being run so much harder as VB+ sag is reduced / power output increased.
Whereas if a comparison was made at lower power level, thereby eliminating overdrive in either mode, I doubt that I could perceive a difference.

[nickb]

I agree. I tried looking at the ratios of the amplitudes of odd harmonics. The big difference is in the ratio of the 3rd to all the higher ones. In the high voltage case the 3rd is the largest. In the low voltage case the fifth is the highest. Conclusion is that it's the third harmonic that's giving the subjectively increased HF.

[J M Fahey]

If frequency response changed significantly with plate voltage that fact would be mentioned in datasheets.

It is not.

Same tube can be used from DC to , at least, TV frequencies, meaning tens and even a hundred MHz .

So any variation within the audio range will be negligible.

As nick b says, any variation in one parameter evens out with another.

[Chuck H]

I've observed that tubes can "feel" and even sound tighter and more stiff at higher plate volts. I can easily see how this could be perceived as more top end by a human, even if not by a machine.