PAB Hummmm

[grumpy]

Well I have my Vibrolux donor up and running, I have bypassed any FX loop/Dumbleator extras for now just so I can get to a happy place first before I complicate things.
When I engage the PAB I'm getting a hum which sounds like an earth loop, I can't figure why? I have the PAB switched as in the HybridA schem but without the Rock/Jazz switch. So My DPDT relay breaks the connections between Bass pot CW and trebleCCW, and bass pot CW and bass pot wiper.
Is this correct? I read in an earlier post that the PAB takes the bass cap completely out of the circuit which in my case is not true. It's still in series with 510K to ground.
Any ideas on the hum and the wiring I have used?

Cheers all John

[grumpy]

Actually on further investigation it may be that the runs of wire to and from the relay may be causing it. If I disconnect the wire going from the treble pot to the relay at the treble pot itself, the hum almost goes completely. I have my relays a little further away from the pots than would be ideal. I have both relays at the input end of the board as close as I could get them.
Using a Fender Vibrolux chassis didn't lend itself to any other position for the relays. the narrowness and shallowness plus the tilted front make it a little tough.
Has anyone used shielded here? Would that be a no no? They would only be short runs but I'd have to use some light guage shielded. I have some 30pF per foot.
As always, all advice much appreciated!

Humming away in Oz, John

[paddy]

hey john
i had the exact same problem,so i ran shielded from the relay
to the treble pot and the problem was solved.
just remember to ground only one end of the sheilded cable,i grounded
the relay end.also try to use a low capacitance cable so you don't bleed
off to much of the high frequency content.
where in oz are you? i'm in tassie.

cheers
paddy

[grumpy]

Hey Paddy, thanks mate, I'll try that then and see what happens.
I live in Ourimbah about 1 hr north of Sydney. Pretty rural where I am, I can turn my amps up full blast when my wife is out! and I do!!
Whereabouts are you in Taz?
Don't you love the shipping charges on components for amps????

Cheers mate, John

[paddy]

hi john
i'm in devonport.it's a pity we're not a bit closer as we could check out
each others amps.i'm a shift worker so plenty of time off during the week
when the wife and neighbours are at work---turn up the volume!
i have some very low capacitance sheilded cable so if you don't have
any,send me your address in a p.m. and i'll shoot you some up in an envelope-no charge.
are there any other aussie builders out there?

cheers
paddy

[Luthierwnc]

Hi Grumpy;

I did the same circuit (no R/J switch) with a PAB and haven't had any trouble. On one side of the switch, the pole is connected to the #1 lug on the treble pot (on the left as you look at the back of it). The normally closed throw is connected to the .1 cap. The normally open side has no connection. On the other side, the pole is connected to the bass pot. The normally closed side goes to the .1 cap which means you jump directly across to its opposite number on the first switch. The normally open side is not connected. My description refers to relays where the pole and the NC side are connected until the coil is activated. If is the same idea if you are using panel switches.

When the circuit is not activated, the treble pot connects to the input of the bass pot. When the PAB is on, only the mid control is unaffected and the higher frequencies you would have grounded through the bass cap (or through the pots from the treble cap) go straight to the volume control.

I hadn't seen a schematic with this set up so I just winged it. Since all of the active contacts reference ground, I didn't need to add bleeder resistors to keep it from popping. YMMV.

Attached is a rough schematic if that helps.

Skip

[Luthierwnc]

I made the picture too big. If it doesn't take this time, look up my email address at http://www.ziaguitars.com and I'll send it to you directly.

Skip

[grumpy]

I thought I'd just update on this one, since I have fixed the issue I was having.
I wracked my brain over this one and still don't know the technical reason for it, so if someone could enlighten me I'd be delighted.
I could not get the hum to go while using a relay supply derived from either 6.3v heater winding or the 5v which is present on my PT. I tried everything, no luck, chassis earth, no earth, referenced to earth through resistance. If the circuit was quiet when the relays were not engaged it would play up when I engaged them and vice versa.
I ended up using a separate filament Tx and referencing it to earth through a 2.7k resistor. It was very noisy with no reference to earth but it is silent with one. I removed the shielded cable I used going to the relay from the bottom of the treble pot as it was not the solution and the noise level is fine without it now.
Thanks to luthier and Paddy for their help

John

[Luthierwnc]

You didn't say how you were filtering the relay supply. I use a separate transformer. The center tap is taped-off and the outside leads go through a FWBR. From there it goes to a 7815 voltage regulator and a 100/50 filter cap. After the regulator it goes to the relay coils and another 100/50 cap. The caps, the regulator and the rectifier are all connected to the same ground wire which is tied to the main buss near the HV center tap.

My amp only has two relays so I used a common stereo jack on the back panel that grounds on the chassis. It works fine. I was prepared to isolate the jack and run a ground wire back to momma but never needed to. I'll use an XLR when I get around to a fancier version.

Someone in another thread had a great idea for a hum tester. Solder a few feet of wire to a .1 630v cap. Solder the other end of the wire to an alligator (croc to you) clip. Put the unused end of the cap on a chopstick and wrap it with electrical tape, shrink tube, what-have-you. Ground the clip and touch suspected hum points with the cap. It will bleed the AC off. If it quiets the hum, you know where to work. Thanks again whoever posted that!

Skip

[grumpy]

Hey Skip
Thanks for your replies!
I originally used 6.3v heater tap with voltage doubler, 1,000uf on both halves of the doubler into 7812 regulator. ground isolated from chassis.
This system was silent until I engaged the relay for PAB, then it was hum city. If I grounded the supply to chassis the hum mostly went when PAB was active, but noise would be a problem when the relay was off. As I mentioned in my first post all noise would go when I lifted the wire from the relay to the bottom of the treble pot.
I am presently using 6v filament Tx with voltage doubler into 7812 and 1000uf on both halves of the doubler, pretty much the same as in Gil's pics. If I need the extra post 7812 filtering later, I'll put it in.
Thanks again, John

[Luthierwnc]

I've been leery of using spare PT taps for relays just because I didn't trust the flux patterns on the chassis grounds. I forget where (maybe AGA) but someone posted what the electron draw looked like a various points of a Fender amp chassis. It was drawn with various sized arrows pointing to the grounding points based on the current draw. Kick that pig and they overlap. I make my own chassis' out of .125 plate aluminum and use a star ground off one of the PT bolts. One of the stars is a hunk of romex with a spade lug that runs behind the pots for all the preamp grounds. Not pretty but effective.

If you really get stumped, disconnect the relay supply and run the circuit with a lantern battery. If the hum is gone, you know the culprit. You can't rule out that the relay is only exposing another ground loop.

Cheers, Skip