Presence control

[pdf64]

My understanding was that the role of the engineer was to implement a solution to reach the desired result, generally by using previoulsy proven solutions to similar situations.
And it's the scientist's role to find out why a particular implementation works.
When that is known, engineers can then use this knowledge to inform as to why one solution might be preferable to another etc.
Obviously there's a lot of crossover, but still a basic distinction.
Therefore Enzo and MWJB have engineering solutions to the OPs problem, and Merlin, Steve et al are taking a scientist's perspective by wanting to know how/why MWJB's proposal works.
I feel that Enzo's solution has the best chance to solving the OPs problem, but if we could get something quantifiable on the pre-amp voltage / brightness thing, it would be a big step forward. Peter.

[Steve Conner]

I always believed that it worked as follows:

Tubes intended for audio have a portion of their characteristic over which they can be assumed linear. Outside of that, they have "cutoff" where the plate current stops. Cutoff is not abrupt, the characteristic starts to curve as it approaches zero, and the curved part is called the knee.

When the tube runs on higher voltage, the linear part of the characteristic gets bigger, but the knee stays the same size in absolute terms. This means that it's now smaller compared to the linear part. This means the tube has more headroom, and when it finally does distort, you can see from Fourier analysis that the harmonics generated will be higher order than if the voltage was low, and the knee took up a larger part of the loadline. The higher order harmonics lead to a brighter, more aggressive overdriven tone.

Therefore, you won't notice any difference in tone from supply voltage, until you actually start overdriving things. Loudthud's results agree with this (thanks Loudthud!)

[guitarmike2107]

My understanding was that the role of the engineer was to implement a solution to reach the desired result, generally by using previoulsy proven solutions to similar situations.
And it's the scientist's role to find out why a particular implementation works.
When that is known, engineers can then use this knowledge to inform as to why one solution might be preferable to another etc.

Not that his matters to this thread, unfortunately the definition of Engineer differs from country to country, for me an Engineer is “degree qualified” Applied Scientist, some would take that further and say that chartered engineers are the only true engineers, but whatever, And what many people here in the UK call Engineers is actually what I would call a technician. Hopefully that makes my thread make more sense then.

[MWJB]

"Therefore, you won't notice any difference in tone from supply voltage, until you actually start overdriving things." Maybe not at small intervals (20-30v), but if you say halved the plate voltage on V1 of a Super Reverb you will hear the difference at pretty well all useable volumes...likewise if you increased plate voltage from <150vdc to 250vdc or more. Agreed my examples are on the extreme side, but if there wasn't "any" difference, then there still wouldn't be "any" difference whatever the degree of rise/drop.

[Steve Conner]

OK, I should have perhaps qualified that by saying "you won't notice any difference provided all other factors are held constant." As Loudthud showed, decreasing the plate voltage decreases the gain of the tube. So, even though V1 doesn't distort, it'll not drive the following tubes as hard. And you will notice that. Even a "clean" channel is probably clipping on pick attacks, and the amount of clipping will change.

As Douglas Self observed (yes, I'm a hopeless Douglas Self fanboy) changing any one component in an amplifier usually has several effects. To hear just one of those effects in isolation, you might have to change several components in the right proportion. Or, as some ecologist said whose name I forget, "you can never do just one thing."

This is where Enzo and MWJB's engineering approach starts to show its value, since their wisdom consists of things that they've already tried out.

[loudthud]

There is something I have previously observed when driving a volume pot with a bright cap. There is an increased load at high frequencies above where the bright cap has done all it can do. In this region the gain of the tube is a little less than expected. You can see it as a dip in frequency response right on the plate. This makes sense because an increased load is in parallel with the plate resistor. This not withstanding, an increase in gain means you can run the volume control lower and that increases the effect of the bright cap.

Another thing that I think goes into effect is that cathode bright caps have more affect when B+ is higher. This would be a simple experiment.

One thing I did observe when conducting the post 26 experiment was that even with relatively low signal levels, you could see the distortion at the 150V B+.

Edit:

I took some distortion readings on the prior setup with the high heater voltage and then some more readings with the heater voltage at 6.33V. The distortion meter I have loads the output of the 12AX7 quite a bit. The output signal decreases to about 2/3 so the input impedance of the meter must be about 100K. I used a .022uF cap to couple the meter to the tube. The input signal from a low distortion oscillator set to 100mVp-p at 1kHz. On the 6.33V setup additional readings were taken at 40mVp-p and the 400Hz high pass was engaged because there was some 60Hz on the distortion meter's output. The distortion products look like mostly 2nd harmonic with some 3rd creeping in at lower B+.
　
Heater voltage 7.01VAC
　
B+....V(k).......V(p).....Gain.....F(3db)......Dist@.1V

350...1.538.....245.1......58......102kHz....3.32%
300...1.296.....211.5......56.8...102kHz....4.01%
250...1.064.....177.8......55.2...101kHz....5.04%
200...0.824.....143.3......52.8...100kHz....6.80%
150...0.582.....109.3......46.6...98kHz.....10.95%
　
Heater voltage 6.33VAC
B+....V(k).......V(p).....Gain.....Dist@.1V....Dist@.04V

350...1.540.....245.3......61.........3.34%......1.25%
300...1.300.....211.6......59.2......4.05%......1.50%
250...1.058.....177.3......57.4......5.12%......1.87%
200...0.827.....142.9......54.4......6.99%......2.46%
150...0.596.....108.9......49.0.....11.11%......3.58%

I didn't expect the gain to go up when the heater voltage was decreased. I don't know if this is an actual property of vacuum tubes or an error in my test from one day to the next.

[Old Tele man]

...filament voltages do affect tube operation, take a look at John Harper's website: Tubes 201 - How Vacuum Tubes Really Work

...especially, his comments about "Filamentary Tubes" and how similar effects occur in all tubes, but to a lesser degree.

[balijukka]

It may be a trick in the peanut between the ears.

Could it be, that just having more headroom preserves the original brightness of the signal. The low order harmonics coming fom the guitar.
These are riding on top of the fundamental (or the second),
With less headroom, we come close to the clipping level and it first cuts these harmonics riding on top of the fundamental, while the fundamental is still preserved. So the relation of the fundamental and the harmonics is changed. This of course creates more higher order harmonics but - Here I'm shooting in the dark - we dont hear them as brightness, but some sort of buzz.

Also with more output, you'll get more brights. You'll get more of the rest too, but we react on the increaset top. Loudness effect ??

I remember noticing similar effect with the marshallesque 47p cap between the PI outputs. Adding that treble cut cap actually makes the thing brighter.

jukka

[Gaz]

this is awesome.

[melvin]

So, Lowell, what was the original amp in question? And I don't know about you, but I don't like raising the voltages because it definitely sounds brighter, but not my kind of bright. Actually, I think Steve Conner was onto something when he mentioned the affects of NFB and speakers ages ago. Perhaps your speakers are too mellow?

[BackwardsBoB]

Presence control? Can I get one for my wife?

[melvin]

need less negative feedback?