Lets discuss amp tone and how it changes w/volume and why

[soundguruman]

Let me jump onto this dogpile and assert that instrumentation without a hypothesis is not science. It may be 'educational' but that's not the same thing. What I read into Daz's post is a request for discussion and the creation of hypotheses that might explain the auditory effects. Some ideas have already been suggested that can be turned into a testable hypothesis. That's an important step in the process for any kind of real investigation. I'd like to see this thread continue without sidetracking into the "corksniffer vs technician" debate.

Yeah, I mean I have a whole bunch of test gear.
But I realize, that tells me half of what I need to know.
I certainly do not discount the need for measurements. Every amp gets checked and measured, in all types of ways.
I wouldn't be fixing 3 amps a day without measurement...absolutely true. I would just be guessing!
But at the same time, I also know that just because it measures good, that don't mean it sounds good.
Especially from the standpoint of guitar player.
So to grasp the issue, you need to look from both perspectives. That's when success comes, as far as the guitar player is concerned.

You thought I didn't have test gear?
But my most important piece of test gear is: my electric guitar. By far. It fills in what all this other stuff can't tell me.

[loudthud]

I'd like to suggest some experiments, the purpose is to point us in the direction of the problem or possible solution. Looking for results on the order of A) Made no difference, B) Made it better, C) Made ot worse, D) Changed the tone but didn't affect the harsh tone, suff like that.

1) Operate the preamp from a B+ supply separate from the amp. Either another amp or a bench power supply. Perhaps vary the voltage to see what effect that has.

2) Connect a resistor in series with the speaker. Start with 1/4 the nominal impedance, 1 ohm for 4 ohm speaker 2 ohm for 8. The resistor needs to be high wattage. Perhaps move up one tap and use a resistor equal to the speaker impedance.

3) Change the pull down resistor in the cathode follower to 150K.

[daz]

I'd like to suggest some experiments, the purpose is to point us in the direction of the problem or possible solution. Looking for results on the order of A) Made no difference, B) Made it better, C) Made ot worse, D) Changed the tone but didn't affect the harsh tone, suff like that.

1) Operate the preamp from a B+ supply separate from the amp. Either another amp or a bench power supply. Perhaps vary the voltage to see what effect that has.

2) Connect a resistor in series with the speaker. Start with 1/4 the nominal impedance, 1 ohm for 4 ohm speaker 2 ohm for 8. The resistor needs to be high wattage. Perhaps move up one tap and use a resistor equal to the speaker impedance.

3) Change the pull down resistor in the cathode follower to 150K.

I can't do #1, but i'll see if i can find an appropriate resistor and try 2. On #3, do you mean the cathode resistor? I have to assume you do, but i never heard it called that. I can try that. I thought 100k was the max you could use there tho w/o problems.

[Chuck H]

I thought 100k was the max you could use there tho w/o problems.

It may be, according to some spec somewhere. But Fender used values as high as 470k here for tremolo circuits. The extra cathode voltage due to elevation over 0V is of more concern because it can stress the filament to cathode differential of some less tolerant tubes. I don't know if it's ever been addressed by the tube makers but I wouldn't do it with any Russian preamp tube. The Shuguang (Chinese) tubes seem to handle it just fine. As do NOS tubes from the known brands.

[d95err]

Seems like we could have avoided a bunch of arguing with just a quick audio recording. Throw a mic in front of your amp and actually capture the effect.

+1

When turning up an amp or listening to it for extended periods (more than a few minutes), ear fatigue plays a very important role in what we experience. The only way to tell if it's the sound of the amp changing or if it's just ear fatigue is to record it before and after we experience the change, and then listening to it (at reasonable volume) after our ears have had some time to recover.

[DrGonz78]

There is of course the proximity effect that comes into play with using the mic or even listening live with your ears. I think if you choose a good spot to place the mic it might give us some idea what you are hearing. I mean I would love to take the wave files and mess around with them in my studio. I would analyze the heck out of them and see if I could see this frequency change as it gets louder. It would be interesting to take the quieter sample and then do some altering of volume to analyze the difference in frequency spectrum. Then after you do the mods do some more samples to analyze the wave form again. Well that may be one way to see if that 1k frequency is ruling as you put it.

[daz]

It may be, according to some spec somewhere. But Fender used values as high as 470k here for tremolo circuits. The extra cathode voltage due to elevation over 0V is of more concern because it can stress the filament to cathode differential of some less tolerant tubes. I don't know if it's ever been addressed by the tube makers but I wouldn't do it with any Russian preamp tube. The Shuguang (Chinese) tubes seem to handle it just fine. As do NOS tubes from the known brands.

Ok, well last nite i added a 47k in series with the 100k. it was much too late to turn it up to the loudness i need to to test it. But I played it a minute at lower volume it seemed like if anything there was MORE gain. Shouldn't that have created LESS gain? Of is it opposite with a CF?

[Chuck H]

A cathode follower can only approach, and never actually reach a gain of 1. Your stock circuit has a gain of about .98 The extra resistance would not have noticeably increased gain or circuit impedance. LT is following some valid notion but I'm not clever enough to know what it is!?! That you can hear a difference is telling though and I have to wonder what's going on here. I'm hoping loudthud will enlighten us.

[daz]

That you can hear a difference is telling though and I have to wonder what's going on here. I'm hoping loudthud will enlighten us.

Maybe I'm not. This as i said was late and low volume, and i can't say that i really heard a difference. Seemed like i did but this was not one of those things i'm sure of at all. Too low volume, to short. It seemed different but i may be wrong. I will check it today after work of course. I too would like to know what his idea is behind trying that. I figured more R would mean less gain and i figured his reasoning was that that was a simple way to reduce gain and see if the issue was too much. I have tried reducing gain before the PI before tho number of times/ways and it never seemed to matter. On a side note, i always thought 100k was the max and that you'd kill the tube with anything more. So now i will experiment with that. My amp is so loud i have no worry about losing gain thru the tone stack ! If anything less gain would probably be good if for nothing more than making the master less touchy at low levels.

[pdf64]

Increasing the cathode resistor value may not increase the gain but it may allow a greater output voltage swing.
My understanding is that the stock circuit acts to pull down the voltage from the preceding plate when it rises too high.
Thereby causing assymetric distortion.
Pete

[loudthud]

A cathode follower can only approach, and never actually reach a gain of 1. Your stock circuit has a gain of about .98 The extra resistance would not have noticeably increased gain or circuit impedance. LT is following some valid notion but I'm not clever enough to know what it is!?! That you can hear a difference is telling though and I have to wonder what's going on here. I'm hoping loudthud will enlighten us.

I've been working on a solid state amp design that uses the architecture of the 5F6A/JMP50. The follower stage is real sensitive, changes you'd think wouldn't make much difference have a big influence. You think that it drives the tone stack with a low impedance, but that's a small signal effect. Big signals don't behave in the same way. Strictly speaking, in this case the problem seems like it is after the Master Volume, or maybe at the interface between the Master Volume and the power amp, but I've been wrong before. Let's try something to narrow down the possibilities.

If daz had a scope, I'd ask him to jam a sine wave into this amp and look at the duty cycle of the squarewaves coming out. I don't know what "harsh" looks like on a scope. What I do know is that a 50% duty cycle sounds hollow, most good sounding guitar amps make something more like a 40/60 wave. Does the duty cycle change when the Master Volume is turned up?

One problem that can occur in an amp like this is that the tone controls become less effective as the power amp starts to clip. Is daz turning the amp up that far? A scope would tell us.

I have many scopes, but nobody has ever told me what good tone looks like on a scope. Sometimes I see things that look bad, and they sound bad, usually.

[daz]

Increasing the cathode resistor value may not increase the gain but it may allow a greater output voltage swing.
My understanding is that the stock circuit acts to pull down the voltage from the preceding plate when it rises too high.
Thereby causing assymetric distortion.
Pete

Hmmm....so does that mean it would likely make the amp even more touch sensitive? Thats maybe #1 on my list of important sonic traits, so now i'm very interested in seeing how this affects the amp in that respect as much as testing it for Loudthud. This always happens when i post about nearly anything. It always turns out something else comes up that gives me ideas concerning something other than what the thread was about. I've had the amp in a hyper state of touch sensitivity in the past and i loved it. Of course i couldn't stop F'ing with it to change other things and now it's lost some of that. Theres no sonic trait so far that i have not at one time or another had perfectly tweaked in this amp. The problem has been making them all happen simultaneously.

[daz]

Ok, i tried it with the 150k CF load R and nothing changed aside from more gain. Tried a 3 ohm (all i had) resistor across the speaker and that did smooth out the highs a lot. But i only played it a few seconds because it freaks me out a bit lowering the load that much and the resistor was only 5 w. But that DID indeed cause much less of a change as i turned it up. Now i just need LT to explain to me what that means and what sort of changes i might be able to make to make that happen w/o changing the speaker load. I must also add it still sounds better low, but that harsh hi mid was mostly gone and the saggy feel was there to some decent degree at least. A definite improvement.

[eschertron]

Now i just need LT to explain to me what that means and what sort of changes i might be able to make to make that happen w/o changing the speaker load.

[I'm asking because I don't know the answer] How does changing the speaker load - in the way just tested - affect the NFB? Is there a relationship worth exploring?

[loudthud]

I was suggesting that the resistor be added in series with the speaker. When an output stage has global negative feedback, it lowers the output impedance. (Don't confuse output impedance with intended load impedance.) When the output clips, the feedback stops working and the output impedance gets fairly high in a tube amp. This impedance discontinunity has some effect on the tone, I'm not sure if it's good or bad. Many solid state amps made today have what is called mixed mode feedback. This raises the otherwise very low output impedance for small signals to several ohms, then when the amp clips the output impedance is low, opposite of a tube amp, but it's still an impedance discontinunity. Most people agree that this makes the solid state amp sound better, or more tube amp like.

I must say that I always think an amp sounds better when the output stage is clipping. I theorize that the speaker(s) create a better tone when driven with a discontinuous impedance. But it could be that the amp just feels better because the power supply is sagging. Most solid state amps miss the mark here because any compression is caused by distortion and not sag related gain reduction like in a tube amp.

Daz, I don't remember you saying much about the amp in this thread. I'll go back and re-read your posts. If you can provide links to other threads about the same amp, I would appreciate it.