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RCA tube amp woes/hum/ground

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Hey All,

I just refinished rebuilding a pair of RCA SA1000 tube theater amplifiers. They use a pair of 6550's in the output. These are going to be used in a mobile DJ setup installed in a roll around box..

My problem is that I want to ground them, kids, schools, GFI outlets hate these, you get the idea.

However, when I ground them, I get 60 cycle hum in the audio. That's without any input connected, the amps are dead quiet with no/shorted input.

I included the schematic. The amps are right now modified with cannon inputs (microphone style) and are terminated properly, but are being run in unbalanced mode.

My Home Theater home stereo also uses a pair of these for the mains. HOWEVER, these amps have a optional matching transformer that's used when these are used in bridge mode (the amps are designed to be bridged for 200 watts). The transformer isolates the ground and they are dead quiet on the system. These amps are not grounded and still use two prong, none polarized plugs. The schematic includes the diagram for that matching transformer.

BOTH NEW RCA's have low level hum that goes up with the volume of the HK AVR 235 that's being used as the HT preamp. The hum gets worse when the new RCA's are grounded.


I'm completely at a loss.

Any ideas?
 

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With the amps input disconnected and shorted, there is no hum with the ground lifted. The power connection is coming in at the back of the amp, the signal input is at the front of the amp.

Yes, the earth ground is connected to the chassis as soon as it comes in.

When I first got these amps, the original audio input was grounded (the second G). The original input was three screws, marked (I)(G)(G) with the second (G) running to earth ground. I connected positive to (I) and signal negative to (G) that was chassis ground. However, when this was connected, it killed the signal.I cut this, and it came up. The result was the same on both amps.

I moved the audio ground over the (C) which is common earth ground, and both amps are passing audio. It makes no sense, but that's what I had to do. Now keep in mind this is WITHOUT an actual three prong plug. The hum comes up when I actually try to ground it with a three prong plug.
 

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RCA amps have a standard method of busing the signal 0V separately from the chassis that minimises hum and stability problems. They run a thick copper wire under the chassis to which all internal 0V connections are made. At the input end this is connected to the chassis. At the opposite end of the bus it is connected to the PSU HT 0V. From the schematic, it appears the first G is connected to the bus at the input end and the second on it connected at the output end of the bus. So you should use the first G for the input signal ground. You should no alter the connections to either G terminal.

Cheers

Ian
 
Ian,

Could I get you to look this over please and tell me how it relates to the problem? I THINK this might be where my entire problem coming from, especially on the amps without the matching transformer. How would this be set up PROPERLY with these RCA? This discusses how to setup XLR connections, how they are grounded. What I can tell you is that the RCA with the matching transformer, work on anything, the ones without are picky.

How to Connect Balanced and Unbalanced Audio - Avid Community
 
A ground loop is forming between the earth (power plug) and amp's chassis ground. Either use input isolation transformers, like your home system or lift the input ground off of chassis (use an iinsulated RCA or 1/4" jack). Make chassis ground of amp at Power supply ground.
I have never had luck with grounding an amp at the input and using a grounded power plug. A ground loop always formed. Chances are your preamp/mixer (whatever your amps are getting their input signal from) is also using a grounded power plug and this is where your ground loop forms.
 
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Ian,

Could I get you to look this over please and tell me how it relates to the problem? I THINK this might be where my entire problem coming from, especially on the amps without the matching transformer. How would this be set up PROPERLY with these RCA? This discusses how to setup XLR connections, how they are grounded. What I can tell you is that the RCA with the matching transformer, work on anything, the ones without are picky.

How to Connect Balanced and Unbalanced Audio - Avid Community

For safety you must have the safety earth of the mains lead connected to the chassis. You should also retain the internal grounding scheme of the RCA amp as manufactured.

Solving earth loops then becomes a matter of lifting the ground of signal cables in order to break loops.

Here is a link to an article I wrote recently about grounding in audio mixers. It does not cover unbalanced connections but the introduction should give you a better idea of what you are trying to achieve. The RCA amps sort of break one of the rules by making the HT 0V connection to chassis at the input end
but thta's because it is a power amplifier. Notice the 0V bus bar on the picture you posted.

Cheers

Ian
 
Ian, I need the link :)

Actually that's what I was doing, I had lifted the signal ground, moving the signal ground to chassis. This cleared up hum and restored audio when it was on the two prong plug.

But for whatever reason, this setup produces hum when using three prong grounded setup.

This is also what Dan was pointing out.
 
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Ian, I need the link :)

Actually that's what I was doing, I had lifted the signal ground, moving the signal ground to chassis. This cleared up hum and restored audio when it was on the two prong plug.

But for whatever reason, this setup produces hum when using three prong grounded setup.

This is also what Dan was pointing out.


Oops!!

http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/SimpleMixer/grounding101v2.pdf

Exactly here did you lift the signal ground? You should keep the RCA grounding scheme intact, connect mains safety earth to the chassis and then tackle hum loop problems by lifting the screens on the interconnecting cables, not by making changes to the amp.

Cheers

Ian
 
Thanks Ian,

I lifted the signal ground from the grounding bar that runs through the center of the amp. What was happening was that I was coming out of a non balanced audio mixer, through an unbalanced cable to an EQ, and through a XLR-XLR to the amp. In this configuration, it hummed and killed audio. I lifted the signal ground from the bus bar, the hum disappeared and the sound came back.

That's what I want to do, is get to the way it should be, and ground it properly.
 
Thanks Ian,

I lifted the signal ground from the grounding bar that runs through the center of the amp. What was happening was that I was coming out of a non balanced audio mixer, through an unbalanced cable to an EQ, and through a XLR-XLR to the amp. In this configuration, it hummed and killed audio. I lifted the signal ground from the bus bar, the hum disappeared and the sound came back.

That's what I want to do, is get to the way it should be, and ground it properly.

If it hummed and killed audio then there is definitely something wrong outside the RCA amp. I think you need to set the RCA amp back to what it was and look for the problem elsewhere.

Cheers

ian
 
Underside of amp.

pin 1 of XLR goes the "C" screw, this is also ground for the speakers.

Pin 2 is also going to "C" screw.

Pin 3 is Signal.

The cut wire went from the first "G" next the "I" screw, to the the ground rail just outside picture frame.
 

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Underside of amp.

pin 1 of XLR goes the "C" screw, this is also ground for the speakers.

Pin 2 is also going to "C" screw.

Pin 3 is Signal.

The cut wire went from the first "G" next the "I" screw, to the the ground rail just outside picture frame.

Pin 1 of the XLR should be connected directly to the chassis at the XLR. This is the #1 rule for XLRs. You should not be connecting any input wiring to the 'C' terminal. This is the common for the loudspeaker output. Pin 2 of the XLR should go the the first G terminal and you should reinstate the connection of this G terminal to the bus bar. Pin 3 is signal.

Cheers
 
What a pretty amplifier diagram.

Amazing what the "good old boys" were doing with pencil, drafting paper and then a "final finish" with pen.

Unfortunately, they use the old theory that "only dots should be on intersections when there is a 4-way intersection that is actually connected". The rest of the intersections either are 3-way (which are obviously connected), or having no 'dot', should be clear what the intent is.

I personally prefer the "little white gaps" for lines flowing over each other, to the sides of the lines. Don't need the semicircle bridge. Just little transparent gaps.
 
Okay, everything corrected according to the directions, and got hum on home stereo and DJ setup. It's passing signal though.

As long as you have the safety earth connected from the mains cable to the chassis, then now you need to start lifting the screens one at a time on the interconnecting cables. Start with the input cable to the RCA amp.

Cheers

Ian
 
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