Where to use shielded

[iknowjohnny]

here is a schematic i finally got to updating and i put red in the places that i'm using shielded wire. Any other places you would use it? granted, you can't see the actual amp to know how long any run is, but assuming they are long enough to warrant shielding, where would it be a good idea?

Since i started that thread on my amp getting noisy (buzzing like SC noise) after i lifted the board, even tho i said it's better now after trying several things, it's still bothering me because it used to be dead quiet when using humbuckers or singles with one thats reverse would together. I tried everything imaginable, so now that i gave up on a fix i want to try and at least reduce it by means of shielding.

http://i24.photobucket.com/albums/c39/dazco/amp.jpg

Oh, and, yes, those grid stoppers are on the tube pins.

[surfsup]

I know the tone stack is susceptible (leads to the pots) but usually only the MV pot? the wiper middle lug...

But I think if the leads are not terribly long you don't need to shield em. I also "think" that too much shielding can add noise? But I'm just a noob and this could be due to grounding the shield itself,

...so perhaps running shielded and not grounding would be a potential hiss/hum saver?

I have a little bit of hiss/very high pitched oscillation in my ax84 HO still that I have been taking a break from solving. I am thinking for me the solution would be to re-think the grounding scheme.

I might break up the grounds into a PS ground and one for the input/preamp. In fact I might post a question about that today...

[iknowjohnny]

...so perhaps running shielded and not grounding would be a potential hiss/hum saver?

no, won't do anything. But you only ground one end when using shielded in an amp and it should then never contribute to noise, only reduce it or leave it unchanged.

[Alexo]

I usually build with no shielded wire, and then add it in as a last resort if I get oscillations. Just takes a couple nudges with a chopstick to find the offending wires, usually.

But you're on the right track - leads going to the grids of early preamp stages.

...squeals are usually lead dress issues, and not grounding issues, IME.

[surfsup]

I have a little bit of hiss/very high pitched oscillation

I am curious if you think the grounding design would cause that or lead dress? I have some photos still of my amp here:

http://chicagocadcam.com/ChrisHahn/amp.html

I don't want to hijack the thread but it is about shielded runs so maybe it is appropriate....

[rp]

Alexo's got the right idea if you ask me. Build w/o out it and then see. It's bad engineering to add stuff you don't need, I think most people do it 'cause it looks pro and keeps them busy. They see it in snazzy high gain amps and want to copy the snazz. For the normal gain stuff most people do, Fender and Marshall certainly proved that with proprer layout you don't need it.

There was a post a while back about how much capacitance this stuff has, 70pf/foot for the common teflon stuff! I was shocked, think about how noticeable a 100pf cap can be. I'll use canare thin MI cable if I have a long run front to back, otherwise I avoid. Never built a high gain amp, however.

[David Root]

Shielding from MV wiper to the .0047uF PI entry cap might help.

If the noise is happening only after you lifted the board, maybe you stressed a ground solder point on the board, or some other solder joint for that matter.

[iknowjohnny]

There was a post a while back about how much capacitance this stuff has, 70pf/foot for the common teflon stuff! I was shocked, think about how noticeable a 100pf cap can be. I'll use canare thin mic cable if I have a long run front to back, otherwise I avoid. Never built a high gain amp, however.

Wow, thats good to know. I have 3 in mine and 2 are about 6-7" long and the 3rd maybe 4". Think i'll replace with regular wire and see what happens because i have never been a fan of putting caps to ground and this is like doing that. I had no idea it could add that much capacitance.

[iknowjohnny]

Well well well, as steve marriot often said. Looks like you were right. I removed the FOUR (yep, thought i had 3 but forgot about one) shielded cables and sure enough i got extra brightness. But it's that kind you usuallu strat a 100 pf across the PI plates to get rid of ! Now i know why i never needed one of those. Now to decide....put the shielded cable back or add a 100 pf. I think the latter because heres the weird thing....it seems QUIETER with no shields ! I'm tellin' you....electronics are harder to figure than women !!! Uh.....sorry Jana....i uh, ummm.well, oh, nevermind.

[Jana]

Well, no one ever said I was easy to figure out!

[Super_Reverb]

Any other places you would use it?

We need to think about how noise voltage makes its way into the signal path. Induced current from EM fields (EM fields from OT, PT, or other signals where high current is flowing and capacitive coupling are a few that we try to guard against. Properly shielded cable reduces the effect of magnetic flux inducing noise current (and ultimately noise voltage) in the signal path. It also reduces capacitive coupling effects due to outer shield being grounded.

High impedance nodes, such as gates can be vulnerable. Lower impedance nodes are not as susceptible. Nodes with lower voltage levels are more susceptible to noise than higher voltage nodes, all other conditions being equal.

Think of line level analog audio signals vs. speaker signals in an audio system: we use shielded line cable because impedance is high and voltage is relatively low. Few people use shielded speaker wire due to high voltage levels and extremely low impedances (4-16 Ohms), i.e., small induced noise currents will generate small noise voltage due to low characteristic impedance and due to large current signal.

We go to great lengths to keep OT EM fields away from pre-amp signals to avoid parasitic coupling and the possibility of oscillation. We orient PTs and OTs to minimize EM field effects on poweramp/preamp circuitry and each other. We cross wires, like heater voltage and signals 90deg angle to minimize inductive (magnetic lines of force) and capacitive (plate area) coupling.

So the short answer is use shielded cable for the input jack to 1st grid connection. If you have 60Hz, 120Hz, or other noise, you know where to look for solutions.

cheers,

rob

[iknowjohnny]

Well, i have to go with my ears since my knowledge is minimal, so i put the shielded cables back in and it's much better. Maybe i could have used a bigger cap across the PI than the 100pf, but the 100pf sure didn't even begin to do as much as the shielded cables do. But i think it was quieter without them ! However, i can deal with some buzz a lot easier than tone that has that uber high end fizz like i got w/o the shielded cable.