hum introduced by channel switcher

[Auke Jolman]

Sometime ago I build the dual preamp marshall as was develloped by sluckey and I really love how it has turned out. Lately I've added a relay channel switcher (http://www.tube-town.net/ttstore/Kits/Needfull-Things/Kit-Double-Relais-Switcher-6-V::5910.html). I made a simple footswitch pedal with 2 dtpt footswitches and a stereo plug.

When I turned the amp up really loud last weekend I noticed a hum. When I checked it out this evening I discoverd that when I took out the footswich plug the hum diapeard. I don't understand why this is happening. I hope somebbody can help me solve this problem.

With kind regards,

Auke

[shooter]

My guess, the footswitch box and cable are acting like an antenna.  I'd check the cable shielding, especially at the relay board, could loose/bad solder connection.  Is the board grounded to the footswitch chassis?  maybe an ohms check between the footswitch chassis and your amp chassis with everything plugged in, I would expect real low ohms, unless it's an isolated circuit, then it should be open.

[sean_thornton]

I have used the same channel switcher from Tube-Town and had no problems with it at all. It MAY be acting as an antenna (the cable) but the cable should be shielded and the shield goes to ground when plugged in so I can't imagine that being the cause but anything is possible.
I also used shielded wire to connect the kit with the rest of the circuit.
The footswitch cable is only carrying low voltage (6v) dc current, not in the signal path at all, it just feeds the relays.
Check the cable shield is actually grounded first then go from there. That switching kit has never given me any problems the way you describe.

[Auke Jolman]

Thanks for the replies sofar.

I'm glad to see that the kit has been used without any problems, which means that whatever problem(s) I have, can be fixed.

Reading the replies and the increased hum when plugged in, I think the problem lies wthin the used cable and/or the way I connected it.

Wich cable should I use and how to connect it?

Thanks in advance for your replies.

With regards,
Auke

[Willabe]

Wich cable should I use and how to connect it?

What cable do you have now?

And;

I'd check the cable shielding, especially at the relay board, could loose/bad solder connection.  Is the board grounded to the footswitch chassis?

Go back and re-read the answers that the guys have posted already. 


             Brad     

[sean_thornton]

Shielded twin conductor with a 1/4" TRS (tip, ring, and sleeve) jack on the end you plug in. A Marshall two-button footswitch from Doug should work.

[Auke Jolman]

This is a link to the cable I used:http://www.tube-town.net/ttstore/Cables-Wires/Guitar/Bass-Wires/Bulk-Cables/SOMMER-CABLE-SC-TRICONE-MKII-symmetrically-2-x-0-34-mm::4186.html. It has 2 core wires and 1 shield.

I checked all the solderings and could not find anything out of order.

Using my multimeter I tested whether the footswitch cage was connected to the outer shield, which was not. I connected the outer shield to the cage of the footswitch.

I plugged in to the amp and tested if the footswitchcage and amp chassis were connected and found that there's a resistans of about 470k.  When I powered her up, it changed to - 320k!? Now what?

[shooter]

It looks like your switchbox is jacked vs hardwired, if that's the case, your switchbox might have an isolated jack.  If it is 1/4" to 1/4", find a ground-point on the pcb inside and ohm it to the Footswitch chassis.  The problem though could be there might be a reason for it being isolated from the amp chassis, so you might wait to "ground everything" till more folks chime in.

[kohvakka]

Replying to an old post, but I have the same problem with my amp and the Tube Town relay kit: Lot's of hum when the footswitch is connected. When I touch any metal part on the cable or the footswitch, the noise increases.

I guess it might be because the relay board is not connected to chassis ground, and therefore the cable and the pedal are functioning as an antenna. That might cause 50Hz noise coupling to the signal. The filament winding of my amp is center tapped to ground, and the relay board has a full wave rectifier to make DC voltage for the relay. I cannot connect the relay board ground or cable shield to chassis ground because that would short circuit the rectifier.

Have you found any solution to this problem?

Schematic:
http://www.tube-town.net/info/datenblaetter/kits/kit-chsw2-6.pdf

[kagliostro]

How many wires has your FS cable ?

to me you must use a 3 wire cable plus shield and you have to connect the shield of the cable to the chassis ground

(not the PS of the relays, only the shield of the cable at the amp side to ground)

Franco

[kohvakka]

The Tube Town relay board includes a stereo 1/4" connector that is soldered straight to the pcb. At he pcb the connector shield is connected to the pcb ground. The two terminals of the stero connector are for control signals of two relays. I'm using only the other relay and a "mono" cable. The connector itself is plastic, so it doesn't connect to the amp chassis. Maybe I should use a stereo cable and modify the board so that the cable shield connects to chassis. And have the pcb ground and the relay control signal go in the two wires.

Thanks.

How many wires has your FS cable ?

to me you must use a 3 wire cable plus shield and you have to connect the shield of the cable to the chassis ground

(not the PS of the relays, only the shield of the cable at the amp side to ground)

Franco

[kagliostro]

You can also try to ask to Tube Town if there are other solutions


K

[sluckey]

... stereo 1/4" connector that is soldered straight to the pcb... I'm using only the other relay and a "mono" cable.

Doesn't that cause one of the relays to be energized all the time?

Try this temporarily...

Use a separate filament transformer to power the bridge. This will allow you to connect the relay pcb ground to chassis ground. Does this cure the hum?
 

[kohvakka]

... stereo 1/4" connector that is soldered straight to the pcb... I'm using only the other relay and a "mono" cable.

Doesn't that cause one of the relays to be energized all the time?

Try this temporarily...

Use a separate filament transformer to power the bridge. This will allow you to connect the relay pcb ground to chassis ground. Does this cure the hum?

I think I will try to modify the rectifier to this:
http://music-electronics-forum.com/attachments/7316d1261161525-imgp1009_tripler.jpg

I think with this I can connect the board ground to chassis.

Thanks.

[Auke Jolman]

Hi all,

I learned to live with it, more or less.

But then about a fortnight ago it struck me. I was using a mono 1/4 jack instead of a stereo jack. I changed the mono out for a stereo jack and hum went away!
 
Auke

[kagliostro]

I changed the mono out for a stereo jack and hum went away!

Please, can you specify further ?

Thanks

Franco

[Auke Jolman]

I was using a footswitch with 1 knob and a mono connector. I still use that footswitch but replaced the mono connector with a stereo connector.

In that amp I'm using just 1 of the 2 relais that are on the board and therefore thought of using that footswitch. Turns out that using a mono connector made a unwanted shortcut of somw sort.

Auke

[kagliostro]

Thanks, now I understand

Franco

[kohvakka]

Thanks for the tip! 

That also solved my problem. I didn't use stereo cable but i cut the trace on the pcb.

I was using a footswitch with 1 knob and a mono connector. I still use that footswitch but replaced the mono connector with a stereo connector.

In that amp I'm using just 1 of the 2 relais that are on the board and therefore thought of using that footswitch. Turns out that using a mono connector made a unwanted shortcut of somw sort.

Auke

[warioblast]

I'm bumping this old thread. I have a related problem with the same relay kit.



The short version:
I have hum when I use a footswitch, but only on 1 channel.



The long version:
I modified the pcb to be able to use my Marshall footswitch (mono) into the relay kit stereo jack, and switch both relays at the same time.
I've done some tests outside of the amp and everything was working fine.
My amp is based on the 2204 JCM800 circuit. I'm using the relay to bypass the 2nd preamp  stage and get close to a 1987/1959 plexi; that I will refer as my clean channel; with switchable gains & masters.
When I first put the kit in my amp, my footswicth wasn't working. I only had the clean channel. I did a quick test, bridged s1 & s2 on the relay pcb, and the LEDs lit on. So I decided to solder a mono jack on s1 & s2. Now I could switched between clean and dirty. But my clean was noisier than my dirty. I unplugged my footswitch and realized it was adding hum.
I plugged my footswitch into the stereo jack of the relay. And now it's working. I can switch between channels. Still, the clean is humming.
I went back and forth between the stereo and mono jacks a couple of times. I played a bit (I didnt have an amp for 2 years) and I realized I couldn't switch any more. Nor on the mono or the stereo jacks.


On the next day:
In the meantime I came across this thread. I had an old extension cable lying around, that I used to connect my headphones to my hifi. It had a stereo jack on one end, and on the other end, the wires were naked. So I tied the wires, plug the cable, and it could switched again from clean to dirty but still with hum on the clean. I was glad  the relay kit was still working.


On the next day:
I unsoldered the heater wires that were powering the kit. I used an external 6v power adapter for my power source.
My Marshall footswitch was working again. Plugged to the mono or the stereo jack.
But the hum is way louder than before. Like before, I had hum on the clean channel when footswitch was plugged in; even more hum with the 6V power adaptor... but only on the clean channel. The JCM / dirty channel is totally hum free.
When the relay isnt powered at all, I have hum as soon as I plugged the footswitch.


Today:
Still with my 6V power adaptor. My Marshall footswitch isnt working ... again. I can switch channels with my audio extension cable.




I'm going to unmod the relay and put it back to stock, rewire the heater to the relay, though I dont think it will make a difference. I can understand the idea that the footswitch or cable act as an antenna, but why on earth, do I get only hum on the clean channel. The channel that doesn't need the relay to be powered to work. 

[shooter]

but why on earth, do I get only hum on the clean channel

the clean channel is probably "receiving" the hum better, It's a better receiver, since it's probably air coupled, move wires, chopstick, move "pedel/relay" feed somewhere new, "the usual suspects list" of fun things to try while humming 

[Auke Jolman]

Good luck in sorting it out.

You could temporary replace the relais with a switch. Put the switch at the same spot as the relaisboard so the signal wires are more or less the same as with the board in place.

[warioblast]

  It's working !!!!!!

I rewired the heater as a power source for the relays. I unmodded the pcb (though it probably has nothing to do the fix...) I inverted the "clean" & "dirty" wires that went to the pcb. I moved a few ground wires from the shielded cables.
I added a mono jack for my footswitch.... who appeared to have a shortcut. Hence why it wasnt working all the time.
I have 0 noise when I insert a mono switch on my added mono jack. No noise when I insert a stereo switch into the stereo jack of the pcb. 

I'm super stoke, I have no popping too. 

[warsavius]

I'm resurrecting this topic because I have a similar noise issue with the footswitch plugged in:

I built a 18w Tube town kit, with 3 mods, PPIMV, Cascade (JCM800 mode i call it), Boost (Cathode cap bypass).
Cascade is switched with 2 relays (same tube town 2 relays board of this topic) both switched simultaneously with a small variation in the board circuit. The boost is switched with another relay in its board.

As usual the relays can be activated with 2 simple spdt switches on the rear panel of the amp.
They can also be activated with the footswitch.

With the footswitch unplugged, the amp is dead silent, absolutely quiet in every modality, cascade, tmb, boost. I respected all the good grounding rules. One star ground for the power section, another ground star for the preamp section.

The relays are feeded with 6,3v ac from heaters. The board has a full wave bridge rectifier. The Relays boards ground is referred to the negative side of the bridge, isolated from the chassis (isolated jacks). It's a "floating" circuit

When i plug the footswitch (stereo jack, three core cable) there's a buzz / hum / noise, obviously amplified when the cascade mode (high gain) is engaged.

I tried both a three core cable and a stereo shielded cable (shield to relay board ground) but the noise is always there.

Take note that for footswitch led purpose the relays are activated in standard mode, and inactive (nc normally closed) in cascade and boost mode.

What are the solutions to kill the noise?
Maybe can use 5V AC from the unused transformer output (see scheme attached)?

[Bieworm]

I have used the TubeTown relays board a few times. The hum will disappear when you ground the relays board. There are soldering connections for toggle switches. Ground the negative side of those and the him should be gone. It works on my amps

I find it hard to believe the cable to the footswitch could be a problem. There is no signal running through it. It’s only the power supply interruptor for the relays, so there’s 6VDC going through it.

[sluckey]

Maybe can use 5V AC from the unused transformer output (see scheme attached)?

That 5V tap is most likely just a tap on the 6.3V winding that's feeding the EZ81. I suggest swapping over to use the 6.3V winding that feeds the EZ81. Then you can connect the negative side of your power supply to chassis.

[warsavius]

Maybe can use 5V AC from the unused transformer output (see scheme attached)?

That 5V tap is most likely just a tap on the 6.3V winding that's feeding the EZ81. I suggest swapping over to use the 6.3V winding that feeds the EZ81. Then you can connect the negative side of your power supply to chassis.

Ok, I assumed to use the 5vAC tap to avoid interferences with the rectifier heater. But I now think it's not a real issue.

So, let me know if I understand it right: If I feed the relays rectifier with the 6,3VAC winding of the EZ81 heater, I can connect the common (negative) of the relays rectifier with the chassis, because that winding has no center tap. Note that the 0VAC of that winding is not connected to the chassis.
(image attached)

I can do it even if the others winding center tap are connected to the chassis? (Sorry but I have poor knowledge on transformers)

Thank you very much for the help!

[warsavius]

I have used the TubeTown relays board a few times. The hum will disappear when you ground the relays board. There are soldering connections for toggle switches. Ground the negative side of those and the him should be gone. It works on my amps

I find it hard to believe the cable to the footswitch could be a problem. There is no signal running through it. It’s only the power supply interruptor for the relays, so there’s 6VDC going through it.

The connections for toggle switches are already used to switch the relays when footswitch isn't plugged in.
The footswitch cable act as an antenna and bring in some noise. Not much indeed, but I'd like the amp to be dead silent like with the FW disconnected.

[sluckey]

So, let me know if I understand it right: If I feed the relays rectifier with the 6,3VAC winding of the EZ81 heater, I can connect the common (negative) of the relays rectifier with the chassis, because that winding has no center tap.

That's correct. Easy to try. No guarantee it will fix the issue, but worth a try.

[Tone Junkie]

Sometimes a reference ground with 100 ohm resister to ground can kill the problem I have one connected with wires to two alligator clips. That I will us to trace the noise. Not my trick but long time poster here. Gave it to me.I’m tired so can’t think of his name to give credit . One of tubnits good buddies but it works part of my noise reducing arsenal. Bill

[Tone Junkie]

Geezer was who I was trying to think of

[warsavius]

Sometimes a reference ground with 100 ohm resister to ground can kill the problem I have one connected with wires to two alligator clips. That I will us to trace the noise. Not my trick but long time poster here. Gave it to me.I’m tired so can’t think of his name to give credit . One of tubnits good buddies but it works part of my noise reducing arsenal. Bill

This is what my father (electronic/electrical engineer in the '70 and '80) suggested to try. I'll try both with ±3,15 VAC of tube heaters and with the 0-6,3 VAC of the rectifier heater.

I think also another solution, maybe the best, should be a 3 pole wire with shield braid. The 3 poles used for footswitch switching relays (RY1, RY2 and common of the RY rectifier) and the shield connected only on one side, to the chassis.

[warsavius]

So, let me know if I understand it right: If I feed the relays rectifier with the 6,3VAC winding of the EZ81 heater, I can connect the common (negative) of the relays rectifier with the chassis, because that winding has no center tap.

That's correct. Easy to try. No guarantee it will fix the issue, but worth a try.

It works fine.

Yesterday i found the time to hide from kids and wife in the cellar-lab.

I disconnected the +/- 3,15VAC heaters supply from the relay board,leaving it center tapped as required.

Then I used the 0-6.3 VAC of the EZ81 heater to feed the relay board (tube town) rectified with a full wave bridge.

To be sure, I connected the negative of the relay board to the chassis with a 1k resistor and checked with the multimeter if there was some current flowing at the ends of the resistor. no current - just some mV DC.

Then I removed the 1K res. and connected directly the neg of the 6VDC to the chassis.

With the footswitch plugged in, now is silent exactly like with the FW unplugged.

The FW is connected with a shielded stereo cable - with the shield now grounded to the chassis. The FW box is not grounded, don't think is necessary. The box is an original marshall 2 switches box. It controls 3 relays (placed on a single relay board + double relay board).
2 Relays for channel switching: Normal channel into TMB channel (i call it JCM800 mode)
1 Relay for switch cathode bypass capacitor.

As said, everything works fine, just a little messy wiring because of the quite complex switching scheme.

Thanks to all contributors! This forum is a great resource.