60hz hum in DG 9300 rebuild

[markmcmuz]

I had solved some issues with this project thanks to folks advice here. I now have correct b+ voltages at all tubes, heater is 6.1 volts ac, I'm able to bias the power tubes to 20 mv (one tube is a little higher- 7 mv). Now I have a loud and I do mean loud hum when I connected speakers. I connected my oscilloscope and was able to identify a 60 hz signal that is amplified to over 5-6 volts with no input connected. Some of the things I have done in this build is connect the PT center tap ground to the can cap ground, use shielded cable for the inputs connecting the ground wires together at the RCA end (but not grounding there) and grounding the other end near the AF cathode cap grounds. Not sure where to go from here as I've scrutinized the build and can't find any glaring problems. However I am a greenhorn. Any help would be appreciated.

 

[Mitch Wright]

Not sure if I'm the right person to weigh in, as I am struggling with my own hum issue (though not as pronounced).

Can you add a pic that shows the complete underside of the chassis? That'll make it a little easier to follow where all the wires go.

Can you also add a pic of what you are seeing on the oscilloscope?

Is the noise more pronounced on one channel or the same on both?

If you trace back through the circuit or forward from the RCA jacks, are you able to determine where the hum starts/is being introduced? If you are use your oscilloscope, you'll want to be careful where you connect the probe, unless the probe/scope can be capacitively decoupled and can handle the high voltage. You can also use a voltmeter for this. It would be helpful if the meter shows frequency when on the AC setting.

The RCA jacks appear to be isolated, but the ground tabs don't seem to be connected to anything. If this is the case, what happens when you connect a test lead between the tab and chassis and see if there is any difference?

60 Hz generally implies heater hum or a ground loop. I see you have heater leads running from the 2nd 6BQ5 from the left to the 6EU7s. Those wires are running directly under the grid of the 1st 6BQ5. You may be getting some induced hum from there, though I'd expect to predominately be on the "right" channel (when viewed from the top of the chassis).

As an observation, it looks like you have a *lot* of individual connections to chassis. This could be causing a ground loop.

Note: I'm assuming you are familiar with and practice proper safety procedures. These will help avoid nasty and potentially fatal surprises when working on the underside of the amp when it is powered up.

 

[markmcmuz]

Not sure if I'm the right person to weigh in, as I am struggling with my own hum issue (though not as pronounced).

Can you add a pic that shows the complete underside of the chassis? That'll make it a little easier to follow where all the wires go.

Can you also add a pic of what you are seeing on the oscilloscope?

Is the noise more pronounced on one channel or the same on both?

If you trace back through the circuit or forward from the RCA jacks, are you able to determine where the hum starts/is being introduced? If you are use your oscilloscope, you'll want to be careful where you connect the probe, unless the probe/scope can be capacitively decoupled and can handle the high voltage. You can also use a voltmeter for this. It would be helpful if the meter shows frequency when on the AC setting.

The RCA jacks appear to be isolated, but the ground tabs don't seem to be connected to anything. If this is the case, what happens when you connect a test lead between the tab and chassis and see if there is any difference?

60 Hz generally implies heater hum or a ground loop. I see you have heater leads running from the 2nd 6BQ5 from the left to the 6EU7s. Those wires are running directly under the grid of the 1st 6BQ5. You may be getting some induced hum from there, though I'd expect to predominately be on the "right" channel (when viewed from the top of the chassis).

As an observation, it looks like you have a *lot* of individual connections to chassis. This could be causing a ground loop.

Note: I'm assuming you are familiar with and practice proper safety procedures. These will help avoid nasty and potentially fatal surprises when working on the underside of the amp when it is powered up.

I will post another pic of the entire chassis. First time user of an oscilloscope but I do practice the proper precautions (dim bulb tester for typical amp rebuild use, isolation transformer for using the oscilloscope ect). I've rebuilt several of these Magnavox and have never had this problem. The heater wires are pretty much running where the original ones ran and the RCA leads are shielded wires-they are tied together at the jack end and grounded on the other end as per a post from gadget73. This was to address a possible oscillation problem. I could try running those heater leads around the other side of the 6bq5 and see. Yes you are correct that there are many ground points. I can consolidate some of them. Although these Maggies do have a lot of ground points on the chassis. Thanks for your input. I will try to get more info ie: both channels, where it is in the chain ect

 

[JohnW]

I would try de-soldering the feedback loop ( off the output transformer) as a first step.
Its easy to get this switched around by accident.

 

[nerdorama]

Is your can capacitor well soldered to the chassis? The can case solder connections look a bit ragged and not well melted. The AC high voltage center tap should really run directly to the can cap case. It is hard to tell in the pics how it is routed.
John

 

[markmcmuz]

I would try de-soldering the feedback loop ( off the output transformer) as a first step.
Its easy to get this switched around by accident.

Are you thinking that I might have the feedback circuit going into the phase inverter side by mistake? The orange leads out of the OPT are the 8ohm taps.

 

[markmcmuz]

Is your can capacitor well soldered to the chassis? The can case solder connections look a bit ragged and not well melted. The AC high voltage center tap should really run directly to the can cap case. It is hard to tell in the pics how it is routed.
John

Yes I lengthened the CT lead so I could ground it to the can. It is a little ragged looking. I can give it another shot of heat to try and flow it better. thx

 

[JohnW]

Are you thinking that I might have the feedback circuit going into the phase inverter side by mistake? The orange leads out of the OPT are the 8ohm taps.

Not the output taps, but, if the primary leads to the transformer are connected to the output tubes backwards you get positive feedback rather than negative and lots of noise.
Removing feedback altogether is a quick way to see if this is the problem. If it is, you need to swap which tubes the output transformer leads are connected to which can be more of an effort.

 

[JohnW]

The light blue wires may need swapped, if disconnecting the yellow and green wire removes the noise.

 

[markmcmuz]

okay gotcha I will unhook the feedback circuit and see what we get when I get home from work! Thanks!

 

[gadget73]

feedback wouldn't make for 60hz hum but it would make for a high frequency horrible noise.

also just so we're all on the same page, can you post which schematic you're using?

 

[JohnW]

Maybe it is hidden in the picture or you have the input RCA jacks in a different place, but I don't see the input going anywhere.

 

[JohnW]

Maybe it is hidden in the picture or you have the input RCA jacks in a different place, but I don't see the input going anywhere.

Never mind, I see them.

 

[Mr Boochie]

I had solved some issues with this project thanks to folks advice here. I now have correct b+ voltages at all tubes, heater is 6.1 volts ac, I'm able to bias the power tubes to 20 mv (one tube is a little higher- 7 mv). Now I have a loud and I do mean loud hum when I connected speakers. I connected my oscilloscope and was able to identify a 60 hz signal that is amplified to over 5-6 volts with no input connected. Some of the things I have done in this build is connect the PT center tap ground to the can cap ground, use shielded cable for the inputs connecting the ground wires together at the RCA end (but not grounding there) and grounding the other end near the AF cathode cap grounds. Not sure where to go from here as I've scrutinized the build and can't find any glaring problems. However I am a greenhorn. Any help would be appreciated.

Did you build this with a star ground? At least keeping the grounding points of the input and gain stages far from the power supply's ground?

 

[StepheK]

"use shielded cable for the inputs connecting the ground wires together at the RCA end (but not grounding there) and grounding the other end near the AF cathode cap ground"

Not sure why you would connect the grounds together at the RCA end. From the picture, it doesn't look like the RCA jack negatives/body is connected to anything? If there is no connection there, the input signal is making the return electrical connection through your house wiring/fuse panel or I'm not sure where the path is! Attached is a crude Photoshop drawing of how I connect the RCA jacks using a twisted pair shielded cable. You don't want the body of the RCA jack grounded to the chassis there, but it has to be connected to ground, preferably using some sort of star ground system. I collect the grounds from each input tube and the RCA jack, and then run a larger wire from each individual input tube to the main star grounding point.

 

[bberkom]

"use shielded cable for the inputs connecting the ground wires together at the RCA end (but not grounding there) and grounding the other end near the AF cathode cap ground"

Not sure why you would connect the grounds together at the RCA end. From the picture, it doesn't look like the RCA jack negatives/body is connected to anything? If there is no connection there, the input signal is making the return electrical connection through your house wiring/fuse panel or I'm not sure where the path is! Attached is a crude Photoshop drawing of how I connect the RCA jacks using a twisted pair shielded cable. You don't want the body of the RCA jack grounded to the chassis there, but it has to be connected to ground, preferably using some sort of star ground system. I collect the grounds from each input tube and the RCA jack, and then run a larger wire from each individual input tube to the main star grounding point.

View attachment 2149388

I think this is your issue. Sure, you can connect the grounds for the input jacks together, but they still need to be connected to the ground tabs on the input jacks. Otherwise, there is no ground present for the RCA cable ground to connect to.

 

[gadget73]

yeah you need the input jacks tied to circuit ground, but they should only be tied to ground in one place. If you're using shielded cable, use the shield and connect the shield end to ground at the same point the input tube's cathode resistor goes to ground.

 

[markmcmuz]

It's connected at the other end, by the tubes, to ground. I read it here to do it that way-only ground one end nearest to tubes. per gadget73 (in a previous thread )

 

[bberkom]

The input ground should be connected to chassis near the input tube with the rest of the input tube components that go to ground, but the RCA jacks still need a ground connection to that point. The red and white washers insulate the jacks from the chassis.

 

[Lavane]

The input ground should be connected to chassis near the input tube with the rest of the input tube components that go to ground, but the RCA jacks still need a ground connection to that point. The red and white washers insulate the jacks from the chassis.