Presence control

[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?