Presence control

[MWJB]

"Well, um, facts, kinda, need evidence."

Then rig up a SPST in say a Super Reverb, one way it feeds the stock 10K preamp dropper, the other feeds a 100K preamp dropper instead, tell me what you hear...you'll hear it at all frequencies I promise you.

I don't know what kind of evidence you expect from me, other than testimony. This is a web forum, all people see here are words/opinions. What "evidence have you put forward that is in any way more tangible than what I have. We're looking at the same scenario from different perspectives, you from the theoretical, me from the practical, as result of personal experience.

Again, it takes longer to read this thread than it does to change a power supply resistor, then folks can decide for themselves.

[Merlinb]

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 :-)

Got a spectrum analyser? Surely that's all you need, and a variable power supply of course.

I think Right Mark's free audio analyzer could work: Latest News. Audio Rightmark

[Merlinb]

"Well, um, facts, kinda, need evidence." Then rig up a SPST in say a Super Reverb, one way it feeds the stock 10K preamp dropper, the other feeds a 100K preamp dropper instead, tell me what you hear...you'll hear it at all frequencies I promise you.

That's observation, granted, but I think it takes a bit more than someone's own fallible hearing for something to become a fact. It needs a theory to explain why it happens, that what I was doing, throwing around some possibilities. I mean, can we be sure that raising the voltages always has this effect, every single time? And how much? What other factors come into play?
I mean, raising the PSU voltages affects the whole amp, and that's a big change. Can we be sure that it is simply the higher voltages which cause this, or is it some knock-on effect which could be attacked more directly next time we want more brightness, without having to mess with the PSU.

As you can see, I don't like sweeping statements about audio, mainly because they're nearly always wrong. They belong to the subjectivists, not us audio engineers.

[Steve Conner]

Got a spectrum analyser? Surely that's all you need, and a variable power supply of course.

Yes, I have an audio spectrum analyser and a variable power supply. The question is, what exactly should I do with them? Of course I have some ideas of my own, but a second opinion would be nice.

MWJB: A while back I had to turn down the HT regulator on my old Ninja Toaster amp. I noticed it didn't have enough headroom and was hardly regulating at all. To restore the headroom I had to drop the main HT from 475V to 400, which would be the equivalent of EVH dropping hs legendary variac from 115V to 97. When I did this, it seemed to me that the dirty channel got darker and mushier. I ended up fitting a presence boost to restore the lost high end.

Of course I could have imagined all of this by Douglas Self's experimenter expectancy effect

[Merlinb]

Yes, I have an audio spectrum analyser and a variable power supply. The question is, what exactly should I do with them? Of course I have some ideas of my own, but a second opinion would be nice.

Hmm, well I guess you would need to try out a number of topologies, to see what results are consistent. These might include:

1. Standard stage w. resistor load and Rk. Nothing changed but the supply voltage.

2. Repeat but with large bypass caps added.

(Maybe repeat the two above, but with different Rk values?)

3. Resistor loaded but with constant voltage biasing, such as an LED.

4. CCS loaded, constant voltage bias.

5. CCS loaded, resistor biased.

6. Same as above but using some other triode types, other than ECC83 / 12AX7, to see if it is common to all triodes or just that one.

I guess you'd want to observe the harmonics at low level, high level, and also when clipping.

[MWJB]

Hearing is how amps are evaluated. People play & buy amps that they like the sound of. Of course, you have no way of knowing how fallible my hearing is, or isn't (so equally you have no reason not to trust my judgement - you could consider that I am actually credible & offer my suggestions through goodwill), but again why would anyone reading this, looking for tips to brighten their amp, believe your theorising over my practical observation, or the other way around? A fact is a fact, whether you know the theory or not...if it's raining, then it's raining...you don't have to be a meteorologist, or how clouds form to know you're getting wet.

"I mean, raising the PSU voltages affects the whole amp," changing anything has a knock on effect...but changing the preamp voltages mostly just changes the preamp voltages (as close as you can get to "all things being equal"), add another node if you need to. On a 2 channel amp with a shared input tube you can run both halves at whatever voltage you want and hear the differences.

You make "messing with the PSU" sound like a big deal? Changing a resistor is not a big job, you don't HAVE to write a thesis on it first, you can just heat up the iron and do it.

"They belong to the subjectivists, not us audio engineers". Tone is subjective, you need to balance the theory with what the player wants to hear, most players aren't audio engineers either but they can still decide on what kind of tone they like. That's the job, to build/service/mod an amp that sounds better to the user than when it did before/what they used before.

You seem also to have failed, up to this point, to make any suggestions yourself as to how the OP might brighten their amp. This is where people come for suggestions/help/tips ...they're free to use them/try them/ignore them, whatever, but I'm not keen on this fad for hijacking threads purely for grandstanding. If you want to carry out my suggestions and make some audio clips to "prove me wrong" then do so, but you might, as a matter of courtesey, withhold "judgement" until you have something tangible to display. FWIW I have nothing to prove to you and I am not asking your opinion on the subject at hand...if I can help you get your head around the scenario then great, perhaps I may in turn learn something, but I'm not prepared to listen to you tell me that such & such does not happen, just because you don't see how it could...without trying it.

[MWJB]

"To restore the headroom I had to drop the main HT from 475V to 400" interesting but, just to be clear, I am not suggesting changing any dc voltage other than that supplied specifically to the preamp tube. This is being achieved in my scenario by reducing the value of the preamp dropper resistor (not the plate resistors, not the cathode resistor).

[Merlinb]

This is being achieved in my scenario by reducing the value of the preamp dropper resistor .

Aha! We may have found another cause...
By lowering the dropping resistor you raise the pole of the PSU smoothing cap (in many amps these are pretty puny to begin with). So what may be happening is that you're reducing the bass response of the PSU, rather than increasing the treble response of the preamp tube! If that's the case then leaving the voltages the same but reducing the size of the smoothing cap would have the same effect...

Oh, and if I tell you it's raining its and it is indeed raining then it is a fact.

But if I tell you that clapping my hands causes it to rain, and it does happen to start raining when I clap my hands, is what I said a fact?

I think you can see how the the two are different. This is an electronics forum, rafter all, not just any old here-say forum.

but I'm not prepared to listen to you tell me that such & such does not happen, just because you don't see how it could...without trying it.

I think youneed to re-read the thread. I never once said you were wrong.

[MWJB]

"If that's the case then leaving the voltages the same but reducing the size of the smoothing cap would have the same effect..." It has an effect, and is a valid suggestion, but it's not "the same" effect and does not fully explain the difference in tone. You're also relatively limited in useful cap values, given their sonic impact. I am not closed to the idea that there is more than one factor at work, however the obvious one is the change in plate voltage, there will undoubtedly be other contributory factors.

Why is what I say "hear say" & what you say something else?

[Old Tele man]

Why is what I say "hear say" & what you say something else?

...as it was once bluntly put to me: "...because my SIX is not the same as your HALF-DOZEN until you can show me how you got SIX doubled to TWELVE, equated to DOZEN, then HALVED back down to equal my SIX."

...just my worthless 2¢.

[loudthud]

When I started this experiment, I thought the results would clearly show a bandwidth reduction in step with B+ reduction. I didn't notice until the end that the heater voltage was 7.01VAC coming from my Heathkit bench supply. I used a GE 12AX7A/7025 that I got a big batch back in the 80's. The plate resistor was 100K and cathode resistor 1.5K bypassed with 100uF. The output was taken on an oscilloscope with a 10X 10Meg probe AC coupled. The input to the tube is from a function generator through a .022 with a 1Meg to ground on the grid to eliminate the DC coming out the function generator. The gain numbers are taken with an input of 100mV pk-pk. F(3db) is taken by setting the output to 6Vp-p at 1kHz and running the frequency up until the scope indicates 4.2Vp-p and reading the function generator dial. Note that at 150V I had to take the reading on a different frequency range so it could be off by a couple of kHz.

B+....V(k).......V(p).....Gain.....F(3db)

350...1.538.....245.1......58......102kHz
300...1.296.....211.5......56.8...102kHz
250...1.064.....177.8......55.2...101kHz
200...0.824.....143.3......52.8...100kHz
150...0.582.....109.3......46.6...98kHz

Conclusion: B+ reduction BY ITSELF does not significantly effect bandwidth.

[Merlinb]

Why is what I say "hear say" & what you say something else?

Well, ok, hear-say might be too strong, but what I said was basic electronic theory- and I can provide references to the effect, on request. You made a statement based only on observation (and I had to press you for that), but you said it as though it was a universal truth. It might be universally true of course, I just wondered if you could explain to me why it happens, but I guess it needs looking into.

[Old Tele man]

Conclusion: B+ reduction BY ITSELF does not significantly effect bandwidth.

...but, what's the content/composition of that bandwidth? Same number of harmonics at differing amplitudes?

[MWJB]

Loudthud, what is your conclusion therefore on how the tube sounded at the differing B+ you tried. Are you saying it sounds the same irrespective of voltage. What were the sonic effects of the differing voltages, curent draws & gain? There are some flaws in the test as compared to real life scenario, particularly with respect to heater voltage - when you tweak preamp voltage in an amp, heater voltage remains constant, it does not rise or fall, this will affect tone.

[guitarmike2107]

Interesting thread, I think Merlin is just trying to get a proper understanding of what you are reporting. It certianly is usefull to know that raising the voltages increases the brightness but for the Engineers amoungst us its also good to have an understanding as to why.

After reading this far I want to know why too?

Mike