Resonance Control

[Boy Howdy]

I'm building an amp into an old Garnet PA head. (Yes, I feel guilty gutting one of Gar Gillies creations, with him so recently departed. I gutted it long before he died and am just now getting around to building something in it. It was unusable as a guitar amp, just didn't sound good, and nobody needs a 30 watt tube public address system that only accepts high impedance mics.)

The chassis has lots of holes (Looks like this one but it's all tube and has a reverb knob where the slave output is. Garnet Rebel - Welcome to garnetamps.com - Home of the Garnet Amplifier Company). I've had a field day coming up with stuff to fill those holes with (two channels (one tone stack), a tone stack bypass fader, a control to allow mixing of two different treble caps, post PI master, Vox hi-cut control and a resonance control). I'm pretty happy with how it's sounding.

Problem though, I can't get much of anything from the resonance control. I am completely inexperienced with these, having never heard one. I decided that I preferred not to use any negative feedback or a presence control. I'm starting to think that one or the other is a must in order for the resonance control to work. I'm getting nothing that I can say for sure is a change in tone. Here's what I've implemented Resonance Control for Fender Amps.

I'm not 100% sure where to hook the "feedback wire" end. I've used a standard Marshall PI and presently have the wire hooked to the ground end of the .1 cap/10K junction. I understand these things can be quite finicky and can vary depending on output impedance, etc. The amp has no impedance selector, it's 4 ohms all day, every day.

Can you guys help me get this thing working?

[guitician]

That linked circuit has 100% negative feedback when the control is towards the cap side. The feedback wire feeds back to an inverting point at the front of the power section. You will know when you connect it to a non-inverting point as it will HOWL... I've seen this called "Damping" in some amps.
https://en.wikipedia.org/wiki/Damping_factor

[Boy Howdy]

Thanks, G. I'll see what I can figure out and get back in a couple days.

[Enzo]

The resonance control is the bass equivalent of the treble's presence control. it is in the NFB loop, so if you eliminate the NFB, you will have no resonance.

[Boy Howdy]

Well, when you put it like that.... lol.

Yes, of course, the resonance is a bass presence control. What I can't figure out is, whether a treble presence control is required in addition to the resonance control or if not what is missing from the above linked diagram. I have it where the 27K resistor is in this schematic http://www.classictubeamps.com/schem...hall/JTM50.pdf . In my amp there is currently no 27K resistor and where the presence control is in the schematic I have a 4K7 resistor and nothing else. I've tried putting one lead from the presence control on both legs of the 4K7, but so far nothing's happening. What am I missing?

[guitician]

I would say insert the pot/cap in series with the 27K resistor. That way, when you turn the pot fully it just returns the feedback to the normal circuit. Fully in the opposite direction will be like no negative feedback loop.

[guitician]

It looks like that that linked post has it wrong, I thought something looked funny. AMPAGE Archive: Re: Resonance control, anyone?
The cap should parallel the pot from pins 1 & 3.

[pdf64]

Without the 27k feedback resistor, there's no feedback loop.

[guitician]

Another old Ampage post talks about it... AMPAGE Archive: Re: SLO clone with resonance control

[Boy Howdy]

Ah, now we're gettin' somewhere!

I'll be offline till Thor'sday, hopefully I'll bring good news then.

[Boy Howdy]

I wondered about that too. I saw the Peavey 5150, Satriani model schems, but couldn't decide if it was hooked up wrong or what. I had it so neatly installed I didn't want to mess with it.

I'll re-do it.

[Boy Howdy]

Okay, so I've given up on the resonance control. I reconfigured to match the Peavey schems, and nothing happened. Tried upping the resistor value to 100K, tried everything I could figure to do, and nothing.

So then I thought I would try a presence control (w/ a 5K pot), figured there might be something to be learned by doing that. And there was. I couldn't get it to work either! so I upped the resistor value to 220K..., and nothing. 330K..., and nothing. But at 680K I started to hear something. Upped it to 1 and then 1.5 meg. These sounded about right. I think I prefer the 1meg. It's not a dramatic effect (my Traynor's presence control is very dramatic and sounds great), but it gets me a little beyond where I would want to go, so good enough.

So then, figuring that it was just a matter of the resistor value being too low, I changed back to the resonance control (1 meg pot). I did manage at one point to get it to reduce the bass ... along with everything else. Little if any difference between that and a volume control. I'm tired of messing with it, and as I've decided I want a presence control after all, I'll be switching back to that.

I am puzzled though as to why it took such a large resistor value to get a noticeable effect on that presence control. Any thoughts?

Interestingly, perhaps, I tried messing with the cap value on the presence control. I noticed that on the Marshall Major the presence control (which is an entirely different setup) cap value is .68, so I tried it. Nice! but it took away too much bass. So I tried .3 and .2, but ultimately decided that the original recipe (.1) was the best.

[pdf64]

Does not compute - Really need you to sketch and upload a schematic of exactly what you've done.

[Boy Howdy]

Exactly this: http://www.classictubeamps.com/schem...hall/JTM50.pdf. Except for changing the resistor value and the fact that I only have the 4 ohm tap.

As for the resonance control, at this point I couldn't tell you exactly what I did.

[Tom Phillips]

Exactly this: http://www.classictubeamps.com/schem...hall/JTM50.pdf. Except for changing the resistor value and the fact that I only have the 4 ohm tap...

If you use the same parts values as shown on that schematic AND you take the feedback signal from a 4 Ohm tap instead of the 8 Ohm tap THEN you have reduced the amount of feedback compared to the "exactly this" circuit. As mentioned in earlier postings of this thread any circuit that depends on the feedback signal to operate, such as presence or resonance, will be less effective when you reduce the feedback. This is most likely why you are noticing less effect than you expected. One way to compensate for the tap change is to reduce the value of the 27k resistor in the feedback loop. Maybe try 10k to 15k. This does add another variable so keep in mind that, if you do this, you may need to switch back to the stock presence control pot value.