Shielded cable for NFB?

[12AX7]

I have a pot to control the amount of NFB, so theres about a 6" wire from the impedance selector to the pot. Being a signal wire is it important to use shielded? Also while i'm asking, besides the initial grids on the first stages in a cascaded 3 stage/CF preamp, (which o DO have shielded) are there any other places i should shield if the wires are over a certain length, and what length? Tone control wires maybe? Or treble to master (pre PI) and master to cap? Just trying to get the amp as quiet as i can, as it buzzes a bit and i've tried everything else.

[The_Gaz]

I once had a NFB wire that would cause motor boating when routed to to close to other signal wires. What I had to do was run it waaaaay around near the PT it get it to go bye-bye. I mention it because I played with shielding the wire, and there was no difference in the motor boating or any other noise for that matter. During that time I remember posting on a forum, and some knowledgeable person said that shielding the wire would have little to absolutely no effect. I know, I know , pretty vague, but I have tried it...

If your buzz is caused by signal wires you should be able to narrow down noisy wires by shorting each stage's grid with a little alligator clip. That way you can at least know where the buzz is originating. It really depends on how long your wire runs are, but a quick and dirty way to 'shield' the tone control wires is to twist them altogether going to the pots, a technique Kevin O'Connor advocates.

Also depending on control panel layout, I've seen bother treble pot to pre-PI master sheilded as well as master to PI input coupling cap shielded.

And in general, if the wires to the pots are longish, make sure they hit the chassis off the board ASAP to gives a psuedo-shielded effect. Of course, this is not always necessary or helpful!

Could there be a possibility your grounding is a little sketchy? If it's not by the book like the Valve Wizard suggests, you may get some little buzzes here and there. Doug's method works most of the time, but in general I've found The Valve Wizard's quieter. Here's an article here:

http://www.freewebs.com/valvewizard2/Grounding.html

[HotBluePlates]

I have a pot to control the amount of NFB, so theres about a 6" wire from the impedance selector to the pot. Being a signal wire is it important to use shielded?

The wires that typically pick up noise are those connected to high-impedance circuits. NFB is coming from the speaker output, and is therefore a low impedance signal. So the problem might not be that the NFB isn't shielded, but what (high impedance) circuits it passes.

[12AX7]

If your buzz is caused by signal wires you should be able to narrow down noisy wires by shorting each stage's grid with a little alligator clip. That way you can at least know where the buzz is originating. It really depends on how long your wire runs are, but a quick and dirty way to 'shield' the tone control wires is to twist them altogether going to the pots, a technique Kevin O'Connor advocates.

Well, heres what i've done along those lines. I lifted the input stage to the gain pot and injected the guitar at that point. Less gain by a good margin of course, and maybe thats why. But it loses 95% of the buzz. At first i figured it was an issue with the first stage which led me to replace everything even the tube socket. But no joy so i then just figured it must be the 1st stage just picking up a tiny bit of buzz but with all the gain it makes it multiplies it a lot.
Off topic maybe, but heres a really bizzare thing i noticed. Maybe it's somehow related. If i touch the signal at a point after the coupling cap (yes, i know the other side of it it deadly)  from the input stage to the gain pot with my finger, it of course causes a huge loud hum. But if i touch the signal after the coupling cap of V2, it causes absolutly no noise whatsoever ! Absolutly nothing changes when i touch it, not even the slightest hum. As tho my finger is not there.

[HotBluePlates]

Can you post a photo of the first few stages of your preamp?

The test you described may be a red herring. Or not, depending on the specifics. Annotating your photo to show where you touched both times may help.

And, of course, a schematic with the photo simplifies head-scratching a lot.

[12AX7]

I don't have a camera or a cell phone. Lame, i know, but i DID have a cam which went somewhere without my permission. In any case, i can scribble up a schematic 4u and post it when i get a chance. One thing i just thought of which i suppose could be the issue. But that thing i mentioned about touching the circuit i think may itself prove it's not and issue. I'm talking the 3rd stage grid and the connection after the coupling cap from the plate to the 3rd stage grid. From the coupler i have a 100k to ground and then a grid stopper on the 3rd stage grid. Before you ask, i did that because a voltage divider there just did sound as good, and the grid stopper is only 2.2k just as a bit of insurance against ocsilation or blocking even tho it doesn't seem to need it. Thats why such a low value. Anyways, what i'm getting at ever so slowly here is the grid stopper is not on the socket but on the board. So theres maybe 4" of distance between the cap and the grid thats unshielded. about 2 or 3" from 2.2k to grid.  the point after the cap and b3 the stopper is where i touched the circuit with no noise at all, which is why i wonder if it even matters that it's not shielded cable with the stopper on the socket pin.

Anyways, i think when i get home tonite i will put the stopper on the socket and use shielded cable from the cao. may not help but it can't hurt. I'll post a drawing in a few unless i get busy here.

here ya go. the red dot is where i can touch the circuit without any noise being generated. If i touch it in the same place a stage earlier it's like WW2. I think it may be due to the 100k ground reference, tho i would think there'd be at least SOME noise.

[jojokeo]

high level signal and low impedance is what you're describing when touching and nothing happening. This type of signal is not as sensitive as earlier in circuit where it's the reverse and you become like the antenna when touching. As you know(?) grid resistors should be directly on the socket pin & shielding the particular wire you're talking about needn't be required.

[12AX7]

Thanks, i figured the low impedance might be the reason. I tried putting the stopped on the socket last nite and even using shielded wire and nothing changed in any way. But i'm going to try experimenting with many different values of voltage dividers with and w/o a bypass cap and see if i can get better results in several ways. Did this before many times but never tried many values of resistors and caps.