non-presence 12ax7 phase inverter

[kingswayguitar]

Does such a thing exist? Does it still retain all the gain and brightness of a silverface or a presence control on 10?

FIRST - thanks to everyone for all the help to date. This forum has yielded more help than I expected. I did a bit of research on this subject and can't find an exact match. I took the 12at7 PI out of my '77 bassman ten and have a new 12ax7 to use instead. But every 12ax7 PI schematic I see has a presence control. Can I omit this additional knob and just set the presence control wide open with a fixed resistor, or no resistor at all after the 10k tail? But then where do I connect the NFB resistor? Maybe just keep the big 100ohm resistor? Help! I can't follow my revererocket to save my life but maybe there's an example there.

EDIT

What about this example?
http://www.el34world.com/charts/Sche...UITAR_MATE.pdf

[frus]

but what's the problem with the amp when you do put a 12ax7 in? does it soud too dull? too shrill? too much gain? too little?

[Chuck H]

Indeed! what exactly is the goal? How/why do you need the amp to behave as it would with a presence control on ten?

There are a ton of circuits to manipulate NFB. It's even easier to eliminate NFB. A presence control eliminates NFB on the HF in a variable control.

The difference between an AT7 and an AX7 for the PI is going to be gain. The AX7 will have a tad more. Worthy of note is that it won't change a thing about the amount of NFB as it relates to gain. They are analogous with each other in this case because the PI is within the loop.

[frus]

btw, those black/silverface circuits with no presence control behave as if the presence is all the way down, not up

[kingswayguitar]

btw, those black/silverface circuits with no presence control behave as if the presence is all the way down, not up

my mistake - thanks. i just want to keep as much high end as possible. and i really have no idea when it comes to nfb. just going from the schematic comparisons i did, i thought i saw a difference.
to be honest, when i popped the 12ax7 in with the stock resistors it sounded better than the very old 12at7. so i'd say good. it sounded cleaner and louder, although the meters on my mix board showed it was quieter - go figure.

i'm putting together a comparison of the two which I'll upload tonight.

cheers to me still learning


EDIT#2 - I'll get this link right eventually http://www.youtube.com/watch?v=dX6j7vcXkLU

[daz]

heres the thing.....if you eliminate NFB you will NOT want to have presence ! Of course presence is part of the NFB loop, but my point is even if it were possible to eliminate NFB and still have a presence control, you'd never want one ! The reason being is that just eliminating NFB will make the amp brighter, depending on the amp possible TOO much so. If u read my post on master volumes i mentioned that i have been playing around with NFB and cut controls trying to decide what works best. I think you'd do good to experiment by disabling NFB and trying to find ways to reduce the excessive high end that will bring. I have pots to vary both NFB and cut right now with the ability to 100% remove either or both. And however i set things i'm finding i always come back to turning both NFB and cut off and lowering the tone controls. It's just a lot livelier and punchier. I'm still experimenting tho, and as you get louder you may need some NFB or a cut control. I'm still experimenting with that.

[g1]

Not sure exactly what you are looking for, but Fender Deluxe 6G3 (and some other older Fenders) used 12AX7 phase splitter without presence control.
http://music-electronics-forum.com/a..._6g3_schem.gif

[isaac]

Is it just me, or in that 6G3 schematic, shouldn't the bright channel be the one with the .01 cap on the tone control and not the .02? NEVERMIND, I just saw the cap on the plate resistor...

I do find it very interesting that on the normal channel with the .002 across the plate resistor that there is a .01 cap on the tone control. And on the bright channel, with no plate resistor bypass cap, theres a .022 cap on the tone control. So effectively, (yet not as drastic as this explanation OBVIOUSLY) on one channel it's darker because of one thing, but it also has a brighter tone control. But the other one has more treble, there's a darker tone control. So that way they would be similar in tone due to the way it works. Kinda funny, one has a thing to make it darker, then right after that it has a thing to make it brighter, and the other one is just bright, but with a tone control to make it darker. So they have an interesting equaling out effect, but not quite to that degree.....

Oh well, hope you get why I find that kind of funny.... lol


And to stay on thread subject, the AB763 PI has the same thing. NFB, but with no presence control and no set cap, it's just a resistor. That might be a reason why the deluxe is a little more sparkly than some amps. But I suppose that the other AB763's have the same thing, no presence pot... Oh well.

[kingswayguitar]

isaac
i just got the reply now. i'll try to follow your description, but what I can say now is that the two channels are equally useful for guitar. the tone controls seem to reach similar settings to my ear. the channel with the bright switch does get a lot brighter though when that switch is engaged. it makes the biggest difference.