cathodyne phase inverter ?

[danger-russ]

This is a some what hypothectical question.

Ok I'm in the planing for a new amp. It will be more of modern metal amp. I want to have a ppimv on it, so I can crank up the pre amp and cut back the power amp to keep the volume down.
Problem is when the power amp is cut back there is not much nfb for the presence control. So the ppimv changes the tone more than I would like.
So I'v been thinking about getting the nfb from the pre amp. basicly by changing the cathode follower that feeds the tone stack to a cathodyne pi, the cathode will still feed the tone stack. The plate would feed the the nfb circuit, and the nfb would be injected in to a earlier gain stage common to both channels in the preamp. Anyone ever try anything like that? Any reason it wouldn't work or drawbacks?

[HotBluePlates]

... I want to have a ppimv on it, so I can crank up the pre amp and cut back the power amp to keep the volume down.
Problem is when the power amp is cut back there is not much nfb for the presence control. So the ppimv changes the tone more than I would like. ...

While reading just this, it sounds like you would like a post-phase inverter master volume (arbitrary choice), which causes a change in how your presence control works.

So what's more important to you? The ppimv, or the presence control?

I can see how the ppimv could change the apparent operation of the presence control, because it's inside the feedback loop. This is a rule you should learn thoroughly from this experience: changes inside a feedback loop might produce unforeseen consequences; or, from a different perspective, wrapping a feedback loop around an existing circuit may change how that circuit seems to work.

Do you really use presence that much? I ask because I guess I'm one of those rare guys that keeps the presence (on an amp with it) turned all the way off, or close to it. But I typically play single-coil guitars, so I'm usually looking to lose some treble. I can see how that might not work for you.

The easiest, maybe best, fix is to move the master outside the loop. The the feedback and the presence control works as predicted.

... Problem is when the power amp is cut back there is not much nfb for the presence control. ... So I'v been thinking about getting the nfb from the pre amp. ... The plate would feed the the nfb circuit, and the nfb would be injected in to a earlier gain stage common to both channels in the preamp. ...

What is the issue you want to fix? Presence control operation? What will "more feedback" get that you don't have now?

I ask because it seems you're describing adding a new feedback loop to the amp. Unless you need less gain, less distortion, adding more feedback doesn't make sense (however you accomplish it). This seems possibly inconsistent with a metal amp.

What do you think?

[danger-russ]

I thought about taking it out of the loop doing power scaling, But I really like the lamar ppimv.
I use the bass, treble and presence together to set the gain, smooth it out sounds like the right way to say it.
problem is the master like you said is in nfb loop and I have to re adjust everything at different mv settings.
I'm trying to fix the presence and get everything more consistant.

[HotBluePlates]

I use the bass, treble and presence together to set the gain, smooth it out sounds like the right way to say it.

This is the thing I've probably never gotten the hang of. I sometimes hear people say it, but have never learned to do it myself. So I'd really love to learn how the presence control figures into this equation.

Forgetting every other consideration, a presence control works as a treble boost by reducing the amount of feedback of high frequencies, depending on the setting of the presence control. LEss feedback = more amplification of highs and possibly more distortion.

What I've been missing is that when I turn up a typical presence control, the sound seems more ice-picky to me. Since that's not a sound I want, I've never learned to turn up the presence to get more distortion. The path I'd usually take is a feedback control which doesn't use a cap and reduces the feedback at all frequencies. That gives more distortion, a looser, rawer sound, but not more treble.

So here's the thing: output stage feedback allows the amp to have tighter control over speaker flap. That seems like a good thing for a metal amp; "metal" and "flabby bass" don't seem to go together. So we can't get rid of the loop.

But having that type of master volume inside the loop will always have some kind of interaction. I'd argue adding another loop to the preamp won't solve the basic problem for you, at least not if you honestly use your presence control as part of the adjustment of the gain/distortion of your amp.

That's why I ask up front if you really use the presence control, or if the particular master you're using is really critical to keep. If yes to both, you probably have to live with it being fiddly.

There are amps out there whose tone controls are inside a feedback loop. When you change the setting of one control, it changes the apparent effect of the other controls. The whole approach is "fiddly" and its what players of those amps accept to get the unique sound offered. The only way to make that circuit non-fiddly is to remove what makes it tonally cool to the guys who use it.

[FYL]

Problem is when the power amp is cut back there is not much nfb for the presence control. So the ppimv changes the tone more than I would like.

Just use a bootstrapped MV in the cathodyne.

[HotBluePlates]

The problem is I think his amp uses a long-tail; he's considering adding a split-load in the preamp as a means of creating a second feedback loop, not around the output stage.

I simply think that's a complication that won't address his original complaint.

[FYL]

The problem is I think his amp uses a long-tail

Then the LTP bootstrapped MV would be fine.

[danger-russ]

Fyl I have normal master before the pi whats added is a master between pi and power tubes. It's the side effects of that I'm trying to get around.

HotBluePlates good point about speaker flap, I wonder if that is caused by bleeding the signal from between the OT and speakers or from the feedback going into the pi

[HotBluePlates]

HotBluePlates good point about speaker flap, I wonder if that is caused by bleeding the signal from between the OT and speakers or from the feedback going into the pi

"Speaker flap" is something you probably haven't heard in your amp. It's over-excursion of the speaker cone because of the amp's poor speaking damping factor and an output signal that is at the speaker's resonant frequency.

"Resonant frequency" is the frequency at which the speaker's cone and suspension moves most freely. Compared to transistor amp's, tube amps have generally poor speaker damping, which is a measure of the degree of the amp's control over the speaker's movement. An amp signal causes the speaker to hurtle from still forward and back; when the signal is removed, inertia makes the speaker try to keep moving. Good speaker damping results in the amp quickly stopping the speaker movement, they way it quickly started the speaker movement.

Feedback, if properly applied within the output stage, lowers the apparent source resistance of the amp and improve speaker damping. So you likely haven't heard speaker flap unless you've played through a no-negative feedback amp.

Fyl I have normal master before the pi whats added is a master between pi and power tubes. It's the side effects of that I'm trying to get around.

My questions have been trying to get you to describe the side effects that you're trying to minimize, as well as to find out if you really turn up the presence control that much.

If you don't turn the presence control full-up normally, the answer could be as simple as "turn it up more in this new situation". If you're like me and never use it, then there's no problem. If you turn it all the way up now, and it still doesn't do what you need it to (and I need to know what you're not getting that you need), the the real answer is get rid of the ppimv. If the ppimv is critical to keep, then I need to know exactly what you need different in your amp's performance to know what to suggest.

If you simply need "more treble" you don't need an additional feedback loop to get it. There are other ways.

If there is some critical interaction between the amp's power stage distortion and the presence control normally, which is removed by adding the ppimv, making changes in the preamp won't help, because it doesn't address power stage distortion. In this case, removing the ppimv seems to be the answer, or otherwise accepting the changed interaction.

Or if this is all hypothetical, then we need to define a clear objective. There may be other ways of doing things that we're not considering under the present constraints.

[FYL]

I have normal master before the pi whats added is a master between pi and power tubes. It's the side effects of that I'm trying to get around.

A bootstrapped MV has very little effect on NFB. Simple, effective and elegant.

[danger-russ]

You have been real helpful I understand the problem alot better now.
Still hypothectical at this point based on mods my friend has made to his 2204.

I'm going to stay with 2204/2203 PA scheme but possibly with an PT/OT and bias setup that can take 6v6 or 6l6 tubes.
My goals are
A: A clean / blues overdrive channel
B: A channel with mordern distortion at low volumes
consitant tone across different volumes.
Keep as much of tube feel and dynamics as I can.

A: Is pretty strait forward.

B: is the problem.
First I consideried more stages biased to the extreams cold and hot, for more distortion with less gain. This seems to lose feel and dynamics.
Second was the ppimv this works fairly well but has the nfb problem. So I thought get nfb from the pre amp. But it seems that the real purpose of the nfb is to control speaker flap.
I have to do some more reading on controling speaker flap.

[danger-russ]

Ok I think I got her figured out. I think the best solution is to get rid of the cap on the presence.
I think what I have really been using it for is speaker damping. The distortion gets smoother because the speaker's response gets better, but the change in treble bothers me.
hot blues plate you answered my question but I had to think about it a bit more to see it.

[HotBluePlates]

... The distortion gets smoother because the speaker's response gets better, but the change in treble bothers me. ...

Probably not.

Speaker control is but one aspect of the reason to use the feedback loop around the output stage. The main intent is to reduce distortion.

Presence works because you're selectively un-doing what the feedback loop is doing. Negative feedback reduces gain, so when the presence control bypasses some of the highest-frequency feedback signal to ground, that signal is no longer reducing output stage gain in that frequency range. As a result, your treble increases.

If you notice more or different distortion as a result, that simply because there's somewhat less action by the feedback loop to reduce distortion.

Ok I think I got her figured out. I think the best solution is to get rid of the cap on the presence.

Look up a variable negative feedback mod or circuit. Removing the cap alone will make most presence circuits do nothing (even for all freqeucnies, not just treble). And I don't want you to replace the cap with a wire, because in most circuits that will ground the feedback. What you'd like to do is make one of the parts determining operation of the feedback loop variable, so you can dial in more or less feedback. A good way to approach it would be to have stock feedback at one end of the knob, and no feedback at the other end.

[danger-russ]

Sorry by remove the cap, I ment changing it to more of a volume type circuit.
I will look up the variable nfb mod/circuit.

My reasoning about thinking its speakers Im useing the presence to control.
Sometimes when I'm double picking really fast and palm muting off and on.the sound seems to lose definition and I start to get a wumf or maybe wump sound at the muted part instead of individual notes. Adding presence cures it but then I have to cut the treble and or add bass to get the tone back. But really getting rid of the treble boost fixes the problem either way.

[HotBluePlates]

Well, the feedback is always in effect when you have a presence pot, so the control of the speaker is not being changed by the presence setting.

However, presence is boosting frequencies around/above ~1kHz. If you look into psychoacoustic effects, or recording/mixing, the transients that make up the leading edge of your sound fall into this area, even though a guitar have very few fundamental tones above 1kHz. In other words, the presence control is boosting the harmonics of your signal, which sound like increased definition, attack, etc.

The same trick is applied to bass tracks; you might apply a little boost around 1-3kHz to bring out the slap/pop of the bass attack, or on a kick drum (maybe slightly lower) toto bring out the percussive attack of the beater striking the drum head.