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Probably a ground loop problem

10/14/1999 8:43 AM
Blues Lyne	Probably a ground loop problem I have a SR head and BMR head. I have used both before switched between them with an ABY pedal, but it's been quite awhile. Since I take both to gigs just in case I figured I might as well use both. The BMR has a Post PI MV and lower B+ and Philips 6L6WGB's so it breaks up nice and the super is spanky and clean. When I tried this at practice tonight I kept getting a high pitched squeel that would start soft and get louder. Adjusting the BMR to less negative feedback made it worse. I tried a cable that I had lifted the ground from one end. This didn't work. The amp it was going to got very little signal and had very little sound even when it was on 10. Moving volume knobs would stop it but then it would start low and build again. When I would unplug the cable out from the front of one amp the ringing would stop completely and the other amp would sound normal, but if I touched the plug on the cable that I had just pulled out the ringing would start again. Also both amps sounded real thin and not much output when both were attached to the ABY box with normal cables. The ringing was worse on the BMR probably because the volumes were up higher and MV rolled down a bit. I at first thought some thing was wrong with my speakers because there was this strange metalic distortion before the ringing started. I tried lifting the ground on one amp and that did nothing. I switched cables around and nothing seemed to make any difference. Both amps have grounded power cords They were put on years ago by someone else and I'm not sure if they left the ground switch connected or bypassed it. Could this be causing something like this? I thought with a ground loop you got hum but this was more of a ringing or high pitched ocillation. Any ideas? It's the perfect spanky clean/bluesy drive combination if I can make it work. Thanks, I appreciate all of you guys on the ampage's help. Blues

10/14/1999 8:43 AM

Blues Lyne

Probably a ground loop problem

I have a SR head and BMR head. I have used both before switched between them with an ABY pedal, but it's been quite awhile. Since I take both to gigs just in case I figured I might as well use both. The BMR has a Post PI MV and lower B+ and Philips 6L6WGB's so it breaks up nice and the super is spanky and clean. When I tried this at practice tonight I kept getting a high pitched squeel that would start soft and get louder. Adjusting the BMR to less negative feedback made it worse. I tried a cable that I had lifted the ground from one end. This didn't work. The amp it was going to got very little signal and had very little sound even when it was on 10. Moving volume knobs would stop it but then it would start low and build again. When I would unplug the cable out from the front of one amp the ringing would stop completely and the other amp would sound normal, but if I touched the plug on the cable that I had just pulled out the ringing would start again. Also both amps sounded real thin and not much output when both were attached to the ABY box with normal cables. The ringing was worse on the BMR probably because the volumes were up higher and MV rolled down a bit. I at first thought some thing was wrong with my speakers because there was this strange metalic distortion before the ringing started. I tried lifting the ground on one amp and that did nothing. I switched cables around and nothing seemed to make any difference. Both amps have grounded power cords They were put on years ago by someone else and I'm not sure if they left the ground switch connected or bypassed it. Could this be causing something like this? I thought with a ground loop you got hum but this was more of a ringing or high pitched ocillation. Any ideas? It's the perfect spanky clean/bluesy drive combination if I can make it work.

Thanks, I appreciate all of you guys on the ampage's help.

Blues

10/14/1999 2:08 PM
Jukka	Are you using a box , which is capable of isolating its A/B outputs from each other so that the other amps preamp tube DC Bias voltage does not have an effect at the other ones input tube ? if not , that might be the reason to the ringing tones. Check RG's "The Splitter ABY" box schematic at the GEO , it is using transformers to get DC isolating. Helps hum problems too when splitting a signal , I mean if you have any . Using a cable with the lifted ground , won't help because you need a back route to the signal , so when lifted , no signal to the input. I hope this helps Jukka

10/14/1999 2:08 PM

Jukka

Are you using a box , which is capable of isolating its A/B outputs from each other so that the other amps preamp tube DC Bias voltage does not have an effect at the other ones input tube ? if not , that might be the reason to the ringing tones.
Check RG's "The Splitter ABY" box schematic at the GEO , it is using transformers to get DC isolating. Helps hum problems too when splitting a signal , I mean if you have any .
Using a cable with the lifted ground , won't help because you need a back route to the signal , so when lifted , no signal to the input.

I hope this helps
Jukka

10/15/1999 12:24 AM
Blues Lyne	I guess I don't really understand what is happening here. Couldn't you rewire the box so that what ever output is selected the other is totally disconnected from it? I assume that the dc is getting through because both jack share a common ground. What happens if the ground on the unselected output is unconnected by the switch? I guess my question is, is there any way to make this one work or is building a new A/B box my only option? Sorry, I remember some posts on this recently but they are not here now and I wasn't having this problem then. About 2 years ago I did several gigs using both amps and the same A/B box and had no problem so I'm confused about what has changed. Thanks Blues

10/15/1999 12:24 AM

Blues Lyne

I guess I don't really understand what is happening here. Couldn't you rewire the box so that what ever output is selected the other is totally disconnected from it? I assume that the dc is getting through because both jack share a common ground. What happens if the ground on the unselected output is unconnected by the switch? I guess my question is, is there any way to make this one work or is building a new A/B box my only option? Sorry, I remember some posts on this recently but they are not here now and I wasn't having this problem then. About 2 years ago I did several gigs using both amps and the same A/B box and had no problem so I'm confused about what has changed.

Thanks

Blues

10/15/1999 9:18 AM
Jukka	>Couldn't you rewire the box so that what ever output is >selected the other is totally disconnected from it? I guess that's how it should be wired. >I assume that the dc is getting through because both jack >share a common ground. No , only then , when both , the ground wires and the signal wires are common in a simple ,cheaply made box which include just a switch and no active electronics, capacitors or transformers to isolate. But a ground loop and hum is what you might get with a common jack gound. In a properly designed ABY box the jack grounds should be isolated from the ground of the box and from each other , in other words ,no "galvanic" = metal to metal connection between the grounds or the hot wires >What happens if the ground on the unselected output is >unconnected by the switch? That will help , but only if you are using an one amp at a time When splitting the signal for both amps , that's out of a question. And on a stage it's quite difficult to switch. Maybe I didn't get it clearly ? Are the squeel on , when whatever output is selected , or just when both are selected ? >About 2 years ago I did several gigs using both amps and >the same A/B box and had no problem so I'm confused about >what has changed. There also might be some corrosion on the jacks , switch or somewhere else , but thats got nothing to do with the DC levels , but it could be the reason to the odd behavior. Jukka

10/15/1999 9:18 AM

Jukka

>Couldn't you rewire the box so that what ever output is >selected the other is totally disconnected from it?

I guess that's how it should be wired.

>I assume that the dc is getting through because both jack >share a common ground.

No , only then , when both , the ground wires and the signal wires are common in a simple ,cheaply made box which include just a switch and no active electronics, capacitors or transformers to isolate.
But a ground loop and hum is what you might get with a common jack gound.

In a properly designed ABY box the jack grounds should be isolated from the ground of the box and from each other , in other words ,no "galvanic" = metal to metal connection between the grounds or the hot wires

>What happens if the ground on the unselected output is >unconnected by the switch?

That will help , but only if you are using an one amp at a time
When splitting the signal for both amps , that's out of a question. And on a stage it's quite difficult to switch.

Maybe I didn't get it clearly ? Are the squeel on , when whatever output is selected , or just when both are selected ?

>About 2 years ago I did several gigs using both amps and >the same A/B box and had no problem so I'm confused about >what has changed.

There also might be some corrosion on the jacks , switch or somewhere else , but thats got nothing to do with the DC levels , but it could be the reason to the odd behavior.

Jukka

10/15/1999 10:39 AM
Blues Lyne	It was happening with any setting, A alone, B alone or both. I'll open the box up today if I get some time. I know the jacks are isulated from the box where they attach but I'm not sure what happens on the pcb. It is a Morley and if I remember correctly it just has the jacks, two DPDT switches, two LED's and some resistors. It takes a 9 volt but I think that is just for the LED's. I don't necessarily care about running both at once. It could be cool at times, but my main intrest is to have the option of either or. Thanks for your help Blues

10/15/1999 10:39 AM

Blues Lyne

It was happening with any setting, A alone, B alone or both. I'll open the box up today if I get some time. I know the jacks are isulated from the box where they attach but I'm not sure what happens on the pcb. It is a Morley and if I remember correctly it just has the jacks, two DPDT switches, two LED's and some resistors. It takes a 9 volt but I think that is just for the LED's. I don't necessarily care about running both at once. It could be cool at times, but my main intrest is to have the option of either or.

Thanks for your help
Blues

10/15/1999 12:51 PM
Stephen Conner	Probably not a ground loop Hey Blues I think you've got a microphonic tube in your amp. It is picking up vibrations from the speakers and causing feedback. Take the head off the top of the cabinet, turn the volume up a bit and try tapping the tubes. If any is badly microphonic you will hear a loud clunk - replace that tube. All tubes are microphonic to some extent so it's a good idea not to put your head on top of your cabinet. Steve.

10/15/1999 12:51 PM

Stephen Conner

Probably not a ground loop

Hey Blues

I think you've got a microphonic tube in your amp. It is picking up vibrations from the speakers and causing feedback. Take the head off the top of the cabinet, turn the volume up a bit and try tapping the tubes. If any is badly microphonic you will hear a loud clunk - replace that tube.

All tubes are microphonic to some extent so it's a good idea not to put your head on top of your cabinet.

Steve.

10/15/1999 5:58 PM
Blues Lyne	This gets even stranger Thanks Steve, I did try that first thing and found one that was maybe borderline but not too bad. I replaced it and still had the problem. The sound that I was getting was similar to what you would expect from a microphonic tube though. It did seem to be coming from just one of the amps, the one that I had turned the volume up higher and rolled back the MV a bit (I assumed because of the extra gain), but it only did it if the other amp was connected to the A/B box. If I would unplug the cable from the other amp it would stop, unless I touched the plug on the end of the cable then it started up again. I could crank that amp up loud by itself and get no ringing. So I think if it was a tube it would have still caused problems with the amp by itself. Also it was doing this whether I was playing or not so I don't think the cabinet was vibrating it. Still I'll turn it up and tap the tubes again. Never hurts to double check. It just hit me, how could it be a ground loop or dc from the other amp if it happened with just me touching the plug of the other cable. In other words it still happened with guitar to effects to AB box, cable from output A to MV amp and cable from output B to my hands, with output A selected. I'm obviously no expert as I have proven many times here but that seems real strange to me. Any Ideas? Thanks Blues

10/15/1999 5:58 PM

Blues Lyne

This gets even stranger

Thanks Steve,

I did try that first thing and found one that was maybe borderline but not too bad. I replaced it and still had the problem. The sound that I was getting was similar to what you would expect from a microphonic tube though. It did seem to be coming from just one of the amps, the one that I had turned the volume up higher and rolled back the MV a bit (I assumed because of the extra gain), but it only did it if the other amp was connected to the A/B box. If I would unplug the cable from the other amp it would stop, unless I touched the plug on the end of the cable then it started up again. I could crank that amp up loud by itself and get no ringing. So I think if it was a tube it would have still caused problems with the amp by itself. Also it was doing this whether I was playing or not so I don't think the cabinet was vibrating it. Still I'll turn it up and tap the tubes again. Never hurts to double check.

It just hit me, how could it be a ground loop or dc from the other amp if it happened with just me touching the plug of the other cable. In other words it still happened with guitar to effects to AB box, cable from output A to MV amp and cable from output B to my hands, with output A selected. I'm obviously no expert as I have proven many times here but that seems real strange to me. Any Ideas?

Thanks
Blues

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