"Presonance" Control Mod

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martin manning
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"Presonance" Control Mod

Post by martin manning »

I came up with this wrinkle for combining a presence control and a resonance control on one knob over in the TW section of the forum, but it can of course be applied to any amp. Based on Colossal's bench testing using a Liverpool-type circuit, it seems to be an effective and useful mod. Knowing that, I've come up with some scaling rules for amps with different feedback ratios and collected it in this .pdf. I'm interested in any reports of more testing and listening should anyone else try it. Original thread is here: http://ampgarage.com/forum/viewtopic.ph ... highlight=

Edit: Some discussion about possible stability issues using this control is here:

http://ampgarage.com/forum/viewtopic.ph ... highlight=

A revised schematic with an added resistor R3 replaces the original version posted in this thread. At this time it seems that some amps may have these issues while others do not, but the sound is as good or better with the addition of R3.
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Last edited by martin manning on Wed Mar 21, 2012 12:36 am, edited 3 times in total.
John_P_WI
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Re: "Presonance" Control Mod

Post by John_P_WI »

Martin,

Looks great, I'll have to give it a go in one of my builds, maybe with a twist or two...

Quick question, where did the the "40" come from in the equations?

Thanks, John
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Colossal
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Re: "Presonance" Control Mod

Post by Colossal »

Yeah, give it a shot John. It is very cool because you get two-for-one functionality on a single knob. Does exactly what Martin shows in the thread. I am using version 3 which is what is shown in the document Martin just posted.

Easy and fun for the whole family. I think I'll go crank that amp up right now!
9pins
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Re: "Presonance" Control Mod

Post by 9pins »

check the available peavey schemos, I've played with a resonance presence thing a ma bob on a van halen model 5150 , worked pretty good
there is a right way to mic a musical saw
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rawnster
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Re: "Presonance" Control Mod

Post by rawnster »

This looks cool. I'll give it a try on an upcoming "fender-type" build.

Thanks!
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Structo
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Re: "Presonance" Control Mod

Post by Structo »

That is great Martin.

I will make this a sticky post for the time being.
Tom

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John_P_WI
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Re: "Presonance" Control Mod

Post by John_P_WI »

Yeah Dave, IF you give it the thumbs up then I gotta give it a go.

Think two "presonance" networks with different FB resistors to give a different feeling and setting for clean and lead channels.... Very Cool and easy to do.

Thanks again Martin!
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martin manning
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Re: "Presonance" Control Mod

Post by martin manning »

John_P_WI wrote:...where did the the "40" come from in the equations?
John, I determined that the original presence cap should be split equally between C1 and C2 for a 1:20 FB ratio, and Dave confirmed that this gave a balanced result. The "40" makes that happen and redistributes the capacitance appropriately for other FB ratios, I hope.

Looking forward to hearing how this thing works out for others!
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Re: "Presonance" Control Mod

Post by John_P_WI »

OK, thanks Martin. Will let you know how it works when I get around to it.
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Colossal
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Re: "Presonance" Control Mod

Post by Colossal »

Martin,

I have been considering a modified layout for the stock Trainwreck turret board to accommodate this mod. Mark Fowler and others have reported challenges with oscillation in the presence portion of the Express circuit. I am now encountering oscillation myself but this is in no way the fault of the mod, but due to my test chassis being a work space for rapidly testing ideas. Proper lead dress has gone out the window in favor of expedience so I'm sure the angles at which I have mounted the caps is inciting chaos. It only manifests itself at high PPIMV settings and with the Presonance rolled in favor of more high end. Maybe I need to add some shielding...

Anyway, I am thinking about how best to add the extra components either to the board or on the back of the Presonance pot, or a combination of the two. Both my "permanent" Liverpool and Express will get this mod.

My wife confirmed the utility of the presonance control (so it officially passes muster haha :)). She has a great ear and I use her sparingly to evaluate my work. She says that the amp sounds really clear while sounding very mean so the resonance side is certainly doing its job.
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martin manning
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Re: "Presonance" Control Mod

Post by martin manning »

Looking at the Express and Liverpool layouts I see the NFB wire is run way over by the cap stack, which was probably done for a good reason. Can you confirm that the oscillation in your mule is in the power amp section by pulling the preamp tubes and then seeing if it still whistles? Re the layout for the presonance, it's only one extra part, but the arrangement is such that another tie point is needed somewhere, and you can't hang the parts off the pot like the KF layout. That'll take some thought...
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Reeltarded
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Re: "Presonance" Control Mod

Post by Reeltarded »

The only thing that doesn't have a home on a Marshall layout is the right side of the tail. The rest fits either where it is supposed to go, or on the pot.
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Structo
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Re: "Presonance" Control Mod

Post by Structo »

I wonder if a "tone" control such as this could be used on a PAB on a non HRM to tweak the tone?

Or would it be just as lossy as the tone stack in this case?
Tom

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Reeltarded
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Re: "Presonance" Control Mod

Post by Reeltarded »

Installed and tested as absolutely working in my Marshall mod mule.

I used a shielded cable on the NFB to 4 ohm and used the control end of the NFB loop from PI tail as feed, split the tail 5k to ground and built an octopus on the 250k pot with a 25k trimmer across its outside lugs with much success.

This control rules.

Can't thank you enough Martin, and Dave too for stepping me through poop!

Amazing!

So, change to pre PI MV or should I add one instead to the PPIMV? I like being able to scale this along with the volume.. add prolly!

Oh! So if I change the ratio on those caps, what to expect?
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martin manning
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Re: "Presonance" Control Mod

Post by martin manning »

This is good! I believe a 10% log taper pot will produce a more even variation in the effect of this thing (and that is what Dave used), try that when you can get one. No stability issues, I guess, so that's good too. Post-PI MV will reduce the effectiveness of presence or presonance, so that's a trade-off.
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