Leo and his 420V 6V6

[loudthud]

When I postulated that a 10% change in B+ made about a 1dB change in the gain of a triode, Steve Conner poo-pooed it. When I pointed out that each stage does it and it adds up to 3 to 6dB over a whole amp, he didn't respond. Most people perceive this as compression.

BTW, most solid state amps have stable gain that doesn't change.

[Chuck H]

I think a lot of it has to do with power supplies and the nature of tubes being higher voltage devices. Differences in operating parameters are measured in tens (or a hundred) volts in circumstance instead of the volt or two you get with a 15V supply that is typically regulated to some degree. Tubes are "elastic" where transistors are not. Or they are but have never been implemented that way and could never experience the same operational swings as tubes. It could make a tonal difference (compression) and certainly a "feel" difference.

[mhuss]

Many moons ago I was had occasion to use an early solid state Vox 2x12 combo, and that would get really "squishy" when you cranked it.

[Helmholtz]

The (subjective) relevance of a 3dB increase in SPL can easily be tested with a "clean" amp.
Feed a 1kHz signal and vary signal level by +3dB, i.e. increase input voltage by 41%.

[Tom Phillips]

Many moons ago I was had occasion to use an early solid state Vox 2x12 combo, and that would get really "squishy" when you cranked it.

That brings up memories of the early generation of Kustom solid state amps. When pushed hard and holding a sustained note, the power supply sag was so great that you could see the front panel pilot light go dim.

[g1]

The (subjective) relevance of a 3dB increase in SPL can easily be tested with a "clean" amp.
Feed a 1kHz signal and vary signal level by +3dB, i.e. increase input voltage by 41%.

Don't we get into the different formulas for voltage and power gain to db here?

[Helmholtz]

Don't we get into the different formulas for voltage and power gain to db here?

That's something I considered when specifing the voltage difference.
A 3dB change in voltage causes a 3dB change in power and SPL.
3dB in voltage means a factor of square root of 2, 3dB in power means a factor of 2.
Power is proportional to voltage squared, so everything is in line.

[uneumann]

3dB in voltage means a factor of square of 2, 3dB in power means a factor of 2.

In case people are head-scratching... I think you mean this...

3dB in voltage means a factor of square-root of 2, 3dB in power means a factor of 2.

[Helmholtz]

3dB in voltage means a factor of square-root of 2, 3dB in power means a factor of 2.

Yes, that's what it should read.Thanks.
Edited my post above.

[Helmholtz]

Anyone going to try the experiment?

I think it's best to compare the different power levels with the same amp.
This avoids possible sources of error with comparing 2 amps and provides best repeatability.

It is essential that the amp is adjusted for clean sound with the higher level, as amp distortion would cause signal compression.
Also added harmonics will influence perceived loudness.

Ear sensistivity to SPL differences increases with sound level (also depends on frequency).
So the test should not be done at very low output.

In controlled lab experiments (Zollner/Zwicker) an SPL difference as low as 0.2dB could be detected at 100dB SPL using a 4Hz-AM modulated 1kHz test signal.
At much lower SPL the threshold increases to around 1dB.

[Tom Phillips]

An interesting test could be to set up a clean tremolo signal with various power levels between the peaks and the valleys and then determine when the test subject can no longer distinguish the difference in the sound between the tremolo ON vs. OFF conditions.

[Helmholtz]

An interesting test could be to set up a clean tremolo signal with various power levels between the peaks and the valleys and then determine when the test subject can no longer distinguish the difference in the sound between the tremolo ON vs. OFF conditions.

That's essentially how the Zollner/Zwicker test was done.

[Tom Phillips]

That's essentially how the Zollner/Zwicker test was done.

I'd like to set up the test but, for now, I need to stay focused on other tasks. However, at the moment, this task is stuck in my head like a song that you can't stop hearing. Shazbot!

[35L6]

Another interesting effect is that a very small increase in volume is perceived as an improvement in tone rather than volume . I've fooled myself before that way prototyping pedals . Thought I'd hit on something only to find it was a slight increase in gain .

[Leo_Gnardo]

In controlled lab experiments (Zollner/Zwicker) an SPL difference as low as 0.2dB could be detected at 100dB SPL using a 4Hz-AM modulated 1kHz test signal.

I can imagine the poor test subjects, listening to what sounds like a cheesy burglar alarm at annoying volume. "Zero point two decibel down, whew what a RELIEF!!! Yes that is quieter by golly. Can you turn it completely off now?" The things people go through for science.