Presence control

[lowell]

does a presence control w/ feedback ADD presence or only taketh away? I have an amp with no feedback that could use some brightness. I don't wanna try presence if it doesn't ADD highs.

[MWJB]

The cap to ground, in parallel with the NFB load resistor, adds presence to a circuit with a negative feedback loop, the presence control turns the cap off.

However, a lack of a negative feedback loop should sound tighter & brighter than the same amp with a negative feedback loop, presence control or not.

[Steve Conner]

A presence control shunts away some of the negative feedback at high frequencies. This gives the amp's output a high frequency boost. The frequency response is almost flat with the presence knob at 0, and considerably treble boosted with it at 10. I measured about 6-8dB boost on the last amp I worked on.

As you can guess from the above, if you have no NFB, a presence control wouldn't do anything. In fact there's nothing to connect it to.

But as MWJB says, the tone of a NFB-less power amp is usually pretty bright and rich (wouldn't say "tight"), because of speaker interaction. Without NFB, the speaker gets a current drive that overcomes voice coil inductance, giving considerably more treble. It also lets the speaker resonate more at its bass resonance frequency, so you get a bass boost too. You really hear the character of the speaker, more so than if the amp had NFB. The presence knob also releases some of this character, because it boosts treble by taking NFB away.

Probably the best thing to do in this case is reduce the bass somehow. You can't add highs, so try taking lows away, and the amp will seem brighter. You could rig up a NFB loop that only functions at bass frequencies, just like the normal circuit but with a really big presence cap and the presence control turned to 10.

[Enzo]

Well, you can add a brightness cap across the volume control, that is what the bright switch on a fender amp does. Unless the control is maxed, you get relatively more highs. And in places where there is voltage division of the signal, a small value cap parallel the upper leg resistor will also increase relative brightness.

[MWJB]

Also worth having a look at the preamp voltages, raising these will brighten the amp. What do you have on the preamp plates now? (In fact knowing what amp we're discussing, or seeing a schm might help).

[Merlinb]

Also worth having a look at the preamp voltages, raising these will brighten the amp.

Why's that then?

[MWJB]

Higher preamp voltages give better high end (assuming all other factors remain the same), lower preamp voltages will give a darker, browner tone. Of course, it's useful to know what voltages someone has to start with, as well as cathode & plate resistor values, but if you're generally happy with the tone of the amp & want a very similar sound, just a bit brighter, then bumping up the preamp plate volts might do the trick. You can still apply Enzo's bright cap suggestions.

You might have to see 20 or more volts difference to hear an audible change. Preamp tubes don't draw much current so you might need to make relatively aggressive changes to the preamp dropping resistor in the power supply. Be aware of any other parts of the amp that are fed from common nodes, that might not benefit from having their voltages raised.

[Merlinb]

Higher preamp voltages give better high end

But why? The theory seems to disagree with that observation. I'm not saying you're wrong, I just wondered if you could explain why it happens.

[MWJB]

"The theory seems to disagree with that observation." Which theory is that?

The more voltage you run through a tube, the less it distorts & the tighter & brighter it sounds. You could up the value of the cathode resistor but this will have a more pronounced effect on the character of the tone, tweaking the voltage (for a given plate/cathode combination) will give a more consistent change in the tone...IF that is what you are looking for.

[Merlinb]

"The theory seems to disagree with that observation." Which theory is that?

Well I was trying to think what factors might cause bass cut/treble boost when you increase the supply voltage.

Increasing the voltages will increase the headroom, so the amp will distort less for a given input, meaning less generation of harmonics.

Each stage becomes more linear before clipping, so again, less generation of harmonics.

Surely fewer harmonics can only mean a less bright sound?

The internal anode resistance will decrease slightly, so the output coupling cap's pole -and the cathode bypass cap's pole- will rise slightly, but the change is so microscopic that I suspect it is too small to be significant.

The gain and output signal swing will increase, however, so maybe the PA gets overdriven more which causes more harmonics? But then we wouldn't expect to hear any difference when playing clean.

Higher voltages increases susceptibility to microphonics (again, a microscopic amount), which might increase intermodulation distortion. I'm clutching at straws here...

The more voltage you run through a tube, the less it distorts & the tighter & brighter it sounds.

I don't think we can take that as a fact,

[MWJB]

"Surely fewer harmonics can only mean a less bright sound?"

Why? What if the harmonics you are losing are part of the initial problem?

"The internal anode resistance will decrease slightly, so the output coupling cap's pole -and the cathode bypass cap's pole- will rise slightly, but the change is so microscopic that I suspect it is too small to be significant."

Are you suggesting that a triode with a given cathode & plate resistor combination will sound the same at 150v on the plate as at 250v on the plate? I hope not. This is one of those scenarios where it is quicker to change a resistor & see than it is to speculate. You don't give any basic perameters for what you might consider "high/low", or what kind of difference might be "significant". A 12AX7 triode running at 150vdc with a 100K/1.5K combination will run at approx 0.7mA, with 250vdc on the plate you'll see more like 1.2mA, or nearly twice the current, strikes me as significant.

"Higher voltages increases susceptibility to microphonics (again, a microscopic amount), which might increase intermodulation distortion. I'm clutching at straws here..."

I'm only suggesting running at voltages that are already in use in other amps, say mid/late 200's tops.

"Quote:
The more voltage you run through a tube, the less it distorts & the tighter & brighter it sounds. - I don't think we can take that as a fact" No really, please do.

[Merlinb]

"Surely fewer harmonics can only mean a less bright sound?" Why? What if the harmonics you are losing are part of the initial problem?

Because nearly all harmonics are integer multiples of the fundamental. i.e., they are always higher in frequency than your audio signal. So by adding more harmonics you're adding more high frequencies.

Are you suggesting that a triode with a given cathode & plate resistor combination will sound the same at 150v on the plate as at 250v on the plate?

No. Rather I'm saying that at lower voltages I would expect it to sound brighter, due to an increase in THD.

You don't give any basic perameters for what you might consider "high/low", or what kind of difference might be "significant".

I suppose a relative increase of, say, 0.5dB over the mid/treble range, relative to the bass range, would look reasonable. But in this case even going from 150V to 400V will only move the poles up by a factor of less than 1.1

A 12AX7 triode running at 150vdc with a 100K/1.5K combination will run at approx 0.7mA, with 250vdc on the plate you'll see more like 1.2mA, or nearly twice the current, strikes me as significant.

Yeah, but you can't hear current.

The more voltage you run through a tube, the less it distorts & the tighter & brighter it sounds. - I don't think we can take that as a fact" No really, please do.

Well, um, facts, kinda, need evidence.

[Steve Conner]

I believe the reason is that when the voltage is increased, the harmonics generated are higher in order, though they may be less in amount. And human hearing weights the audibility of harmonics according to the square of their order.

My own (patchy) experiments seem to vaguely support this observation.

[Merlinb]

I believe the reason is that when the voltage is increased, the harmonics generated are higher in order, though they may be less in amount.

Hmm, that's plausible, I hadn't though of that- thanks Steve, I'll look into it.

[Steve Conner]

I should probably look into it too. A while back I promised to measure the distortion characteristics of the Techtube ECC83 alongside an original Mullard, and I could get some results on distortion spectrum vs. voltage as part of the same experiment. Any ideas on how to do it appreciated :-)