"Presonance" Control Mod

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bcmatt
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Re: "Presonance" Control Mod

Post by bcmatt »

Well, I won't be getting to it right away. My tools and parts are not with me in the touring motorhome. I'll hopefully get time to check if I have the parts in another week or so. I am anxious to give it a go.
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bcmatt
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Re: "Presonance" Control Mod

Post by bcmatt »

Ok, so I installed it yesterday when I visited my folks place and had access to my tools and parts.

Used a PEC pot too that I had (fancy schmancy!). I'm not too pleased with the fact that I had to use my old crappy soldering iron that doesn't quite get hot enough, and then I had to cram so many things into(the blob at) that one eyelet, so it does not look pretty. But, it does seem to work.

I haven't had a chance to play at any significant volume, but even at low voltages (VVR) I notice a huge variance in sound this new control gets. I'm looking forward to possibly trying it out today. I used to run the presence control anywhere between 0 and about 2 depending on the speakers I was using, so I basically just wanted it al the way off. I can tell I won't be doing that with the presonance.
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martin manning
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Re: "Presonance" Control Mod

Post by martin manning »

So it seems to play well with VVR then... Good to know! Looking forward to your impressions at higher power.
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bcmatt
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Re: "Presonance" Control Mod

Post by bcmatt »

Yes, I think that it works well with VVR.
It does definitely change the EQing on this amp and that will take some getting used to. I was previously always trying to manage treble and sizzle and I guess I got good at it with my speaker selections and so on. But now, it is a different amp to get used to and I'm not sure yet how to set it. The presonance control sure has quite the range and (using a linear 10k pot) I am tempted to run it quite high so far... but I need to play around with the other controls more (maybe I'll be running the treble boost switch all the way up now).

I'm going to dig out the 4x10 I built for this amp but couldn't use because the speakers didn't seem to match it. Maybe now it will work well with it...
[IMG:640:480]http://i45.tinypic.com/14kxvfn.jpg[/img]
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bcmatt
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Re: "Presonance" Control Mod

Post by bcmatt »

So, for me the results were disappointing.
I still don't like the way my 4x10 breaks up... so it will remain a cab to use with my Rhodes, seems to sound great for that application...

But yeah, even though this presonance control gives quite a range tonally for the amp, it seems to alter the gain structure in such a way that it adds an unwanted "fizzle" sort of sound to the gainier stuff I'm playing. It is possible that it is not the new circuit, but perhaps a lead dress issue created by the mod in a tight place. I can dial it out by turning the presonance pretty much all the way down, but then it also seems to dial out that "open and lively" sound that I am used to from my liverpool, and no amount of turning up treble or treble boost switch seems to compensate. It seems to either suffocate or fizzle. I think Presonance is probably quite a useful mod for many amps, but probably a poor choice for me and my Liverpool that I have become quite dependent on for a very specific range of tones. The sound of the amp is just altered too much. Somehow, the original Liverpool design (with VVR) is a delicate balance of a circuit that I was already pleased with and I shouldn't fix if it ain't broke. It's not like I am a purist or anything though. I do like to try mods. I use a 5751 in V1, and I use the 5k1 OT tap, and I did do the update in that note that KF wrote. It's just this mod was too much for me.
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martin manning
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Re: "Presonance" Control Mod

Post by martin manning »

I'm about to convert my Express to old-school presence, mostly to see what that sounds like in comparison. At the high end the Presonance is fine and has the desired effect, but turned all the way down it still has a breif oscillation that attaches to the end of notes. That can be killed by cracking the cut control, however, and it is fine when turned up. There have been varying results with this thing; fortunately it's easy to try out and revert back if it doesn't deliver.
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Reeltarded
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Re: "Presonance" Control Mod

Post by Reeltarded »

Anyone else ever squelch that meltdown on the bottom of travel? Love the control. I worry about Dorko Notknowmyamp burning the studio down when I am not around. :D

Martin, did you ever measure the resistance on the RES side when your's started with the bucking like a gamma ray pony? I think 750R would govern that side well for the amp here.

aha! I need a trimpot on the res cap to the resistor tail! Duh!!

Also, cap ratios. My standard for res is 4700pf, and pres is .1uF/56k nfb on the 8R.

Did you ever experiment with values lower than 100% between the caps? I would like to keep the high side but narrow the low side a bit. Maybe this also gets us out of poscillation, the possibility of oscillation. It sounds like subsonic runaway negative making the positive wiggle like a worm if it happens. Eeeks.
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
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martin manning
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Re: "Presonance" Control Mod

Post by martin manning »

I have not fooled around with this for a while. Curiously there are a number of people using this without oscillation on a variety of amps.

There are a couple of ways to change the balance between pres and res. In the circuit shown there is already a resistor between the pot and the FB which limits the level going through the resonance cap by forming a divider with the pot. You could increase that value, or you can increase the presence cap and decrease the resonance cap such that when they are paralleled (full presence) they add to the same value. With the former its reducing the level and with the later the frequency shifts up. On my Express the little glitch seemed to occur not at the stop but a little above. That didn't bother me but I didn't like the whistle that occured at full resonance with high treble and volume.
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Reeltarded
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Re: "Presonance" Control Mod

Post by Reeltarded »

Very good. Thanks for everything.

Build an amp! :)
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
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M Fowler
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Re: "Presonance" Control Mod

Post by M Fowler »

For some reason the noise comes and goes, recently I have been getting oscillation. Guys like the out of control noise so I haven't tried to change the amp.

This Martin Manning version of a PTP (eyelet) Express just kicks ass with nice harmonics fun to pretend your some super star I had to get some shades and large mirror. :lol:

Mark
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Reeltarded
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Re: "Presonance" Control Mod

Post by Reeltarded »

haha

Mark said the squeally quits with an at7 PI. Martin, would raising the tail have the right effect there instead? I must experiment with this other PI Colassal told me.
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
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martin manning
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Re: "Presonance" Control Mod

Post by martin manning »

A larger tail resistor means better balance in the PI at the expense of headroom... potentially trading one kind of distortion for another. It also means more feedback (unless the other components are adjusted proportionally), so this gets complicated. With no other adjustments, I'd expect that increasing the tail resistor would make any positive feedback oscillation worse.
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pablogt
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Re: "Presonance" Control Mod

Post by pablogt »

If you make the tail resistor variable, you get what Mojave calls "Power Dampening". Sort of a poor man's VVR

http://mojaveampworks.com/amplifiers/scorpion/

It works fine, although cleans get hairy at high "power dampening" (large tail resistor)

Pablo
EtherealWidow
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Re: "Presonance" Control Mod

Post by EtherealWidow »

Hey guys. So I'm trying to design the presonance circuit for a 6G3 build I'd like to start. Running into what seems like some design flaws. Although the 6G3 doesn't have a presence control I used the pretty standard value of .1uf. The problem I run into is that the caps aren't even close to being equally split and they don't come close to the original value of the cap when added. I'm wondering if that will affect the way the circuit performs.

R FB=56k
R Tail=1k5
C1=.0068uF
C2=.082uF
RV1=5k-A
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martin manning
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Re: "Presonance" Control Mod

Post by martin manning »

I would try these values:

RFB = 47k - 82k (the original 6G3 FB ratio is reproduced using 82k, but you might want to tweak to taste)
Rtail = 2k2
C1 = C2 = 0.1uF (this combination of Rtail and C1 + C2 gets the presence frequency response in the right ballpark)
R2 = 4k7
RV1 = 5kA
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