NFB cap observation

[iknowjohnny]

I never saw a schematic with a NFB cap bigger than .1UF. But i recently have been thinking of ways to beef up the tone in the PA section of my 2xEL34 build and i began thinking why not bend the rules and try a larger cap. For one thing the presence is very toppy and i thought what if it was set to a lower frequency. So i had to use an electrolytic, but it worked so i tried a 2.2uf then a 4.7. But i tried it on a different amp....my 18 watt converted to 6v6 and with a high gain pre like my EL34's pre. I will try it on my other amp too. But what i found was that it really beefed up the tone big time. Switching back and fourth from the .1 to a 2.2uf made me wonder why no one seems to do this. Really thickens up the amp's tone while allowing me to still get all the brightness i'd ever want it seems. of course this is preliminary so maybe my feelings about this will change. But what are your thoughts about this? Anyone ever try it?

EDIT: first of all, i meant 3.3uf, not 2.2. And that seemed to work better than the 4.7. Anyways, i've been playing it a good while and it seems to be just the thing i've been looking for. i just have to see if it works as well on my main amp. but i have tried everything in the book to get that thick meaty sound i've rarely been able to get from many amps, and it's just awesome so far. i've tried everything you can imagine including tone stack tweaks of course, but nothing has thickened up the tone without also deadening it in some way. Can't wait till tonite to try it on my main. I just can't figure out why i never see this in other amps.

[Structo]

The Dumble ODS has a 1uf cap.

I just built a small 2x6v6 amp which I put a .1uf cap on the presence cap.
I think I will experiment with that because there doesn't seem to be a lot of adjustment on the tone.

[iknowjohnny]

I tried this just now on my other amp and similar improvment. i like it ! I found 3.3uf to be the highest i can go before it loses something in the high end. 3.3uf is just right, but at times i might like lower values. A rotary switch may be in the cards !

[Jana]

Early plexi marshalls used a .68uf.

[iknowjohnny]

Early plexi marshalls used a .68uf.

Are you referring to the preamp? I know they used a .68 there, but i never saw a schematic with a .68 in the NFB. At least the plexi schematic i have shows the typical .1uf.

[Jana]

Not the Pre-amp, we are talking about the Presence cap, right? I don't know which years specifically, but I have seen a .68uf there.

[iknowjohnny]

Ok, just wanted to be sure you didn't mis read my post cuz i never saw that but have of course seen they all have that .68 on the first preamp stage. Anyways, i guess that may be part of the plexi magic because most loops use .1uf. I think i'm going to put several caps on a rotary switch. The original .1uf, probably a 1uf or possible a .68, and the 2.2uf i have now.

[ampdoc1]

It sounds like you might want to install a "resonance" control,..kinda does the same thing to lows as the presence does for highs. Attached is one design. I believe the jack is to make the resonance footswitchable.

[iknowjohnny]

I had a resonance for a while, but it's just one of those things i grew tired of. I could see it for certain heavy styles, but it's not my sound.

[Structo]

Johnny, did any of your values you tried make the amp squeal with high freq oscillation?

I'm still trying to figure out why my amp does that when the treble is maxed and I turn the presence up. Only does it when the volume is way high.

I may be a result of several things including the power tubes but I was just curious if you ran into this?

[Structo]

It sounds like you might want to install a "resonance" control,..kinda does the same thing to lows as the presence does for highs. Attached is one design. I believe the jack is to make the resonance footswitchable.

Doc so that just hooks up on the feedback wire that comes from the output tap?

I think that jack is just the speaker jack since it says 4 ohms.

[iknowjohnny]

Johnny, did any of your values you tried make the amp squeal with high freq oscillation?

I'm still trying to figure out why my amp does that when the treble is maxed and I turn the presence up. Only does it when the volume is way high.

I may be a result of several things including the power tubes but I was just curious if you ran into this?

No, but then i haven't cranked it. But knowing the amp i'm sure it won't. However, if you crank the presence with say a .047 cap or such it may. I've tried a .047 and it's nice too. I like all the values i've tried for different things/moods. Thats why i think i'm gonna get me a rotary switch.

[ampdoc1]

Structo,

You are correct,..I just didn't examine the drawing carefully. And yes, that all hooks up on the feedback loop.

ampdoc1

[Structo]

Johnny, can you post a picture or reference to the type presence circuit you are using?
What circuit did you follow for your 6V6 Plexi?

What size feedback resistor are you using?

I think this was the circuit I copied for mine.

[iknowjohnny]

It's just the same circuit marshall always uses with a 100k FB resistor. Just look at a JCM2204. Used the same with the 6V6. I gotta say tho, i'm starting to realize it's not really happening with the el34 amp. It works great on the 6v6 amp for some reason tho.