Weber 6M18 Hum Again!

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eljerman
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Weber 6M18 Hum Again!

Postby eljerman » Fri 05/10/19 10:13 am

I'm back again.....and so is my hummmm! Definitely sounds like 60 cycle. Thought I had it tamed somewhat with lead dress and shielded cable plus the different ground spots but when I put the amp to higher playing volumes that hum is annoyingly present. Normal hiss is ok and the tone is exactly what I wanted with all voltages stable. When the amp is on with no input you hear the hum, As you raise the volume upward it gets louder. Add the bass and mid controls and its way too much. I have read all the posts about hum and I'm getting nowhere. Should I be suspecting a faulty transformer(s)?The cap can?......Help!
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geoff 1965
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Re: Weber 6M18 Hum Again!

Postby geoff 1965 » Fri 05/10/19 2:38 pm

i can't see your can cap wiring very well from the pics you added but it looks like you've got your mains voltage earth grounding to the same point as the can cap/el84 bias.you need to seperate these and the mains in should ground to it's own point.try that and let us know if there is any improvement.
good luck
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Re: Weber 6M18 Hum Again!

Postby eljerman » Sat 05/11/19 10:20 am

Thannks for the info I will try that. Also I am NOT running 100 ohm resistors from the heater to ground at the pilot. Should I ?
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Re: Weber 6M18 Hum Again!

Postby geoff 1965 » Sat 05/11/19 10:43 am

the 2 100 ohm resistors create an artificial centre tap but your PT has a 6.3V centre tap "green/yellow wire" so you should'nt need to do this.
you could twist the 2 blue wires together that go from the pilot bulb to the 1st el84 and see if that helps.
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Re: Weber 6M18 Hum Again!

Postby eljerman » Sat 05/11/19 2:28 pm

A couple of observations. I disconnected the wire going from the ground tabs to the a/c ground lug. The noise increase was incredibly loud. Now I jumped the lead from the cap can to chassis and it went back to the original hum. My questions: should the red/yellow wire from the transformer be grounded at the same lug as the green/yellow? Also shouldn't the 4 ground tabs on the cap can that are soldered to the chassis be enough grounding for the can, and should the metal casing of the cap can be in direct contact with the chassis? I currently have a gasket provided with the can in between the casing and the chassis.
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geoff 1965
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Re: Weber 6M18 Hum Again!

Postby geoff 1965 » Sat 05/11/19 3:28 pm

yes your red/yellow and green/yellow are the centre taps for your HT and 6.3V heater supply and can ground at the same lug,where is your el84 bias ground?
the mains AC in earth should ground to it's own point with nothing else grounding at that lug.
the solder points of the can look cold! you need a high wattage iron to get good heat into the chassis for those.can you add some close up pics of the can inside & out?
have a look at the "modern ground scheme's" Josh posted and check over your amp in comparison for grounding point's.
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Re: Weber 6M18 Hum Again!

Postby eljerman » Mon 05/13/19 10:39 am

I happen to have the bias supply to the el-84's grounded to the same ground as the control pots and inputs. I'm assuming this ground should be independent also. Yes?
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Re: Weber 6M18 Hum Again!

Postby JMPGuitars » Mon 05/13/19 12:12 pm

eljerman wrote:
Mon 05/13/19 10:39 am
I happen to have the bias supply to the el-84's grounded to the same ground as the control pots and inputs. I'm assuming this ground should be independent also. Yes?
No. EL-84s and the rest of the power amp section go together to the star ground. This is the post and layout geoff was referring to: viewtopic.php?f=4&t=25372

Thanks,
Josh
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Re: Weber 6M18 Hum Again!

Postby eljerman » Mon 05/13/19 12:42 pm

Checked out the grounding scheme. So,,,,Input=Separate lug. Bias,PT,and cap can= same lug.. A/C=separate lug. Got it. Will fix and post. Another note is that all my pots and input jacks are grounded to the brass plate that mounts on the chassis between the pots +inputs. Will that be ok?
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Re: Weber 6M18 Hum Again!

Postby JMPGuitars » Mon 05/13/19 3:07 pm

eljerman wrote:
Mon 05/13/19 12:42 pm
Checked out the grounding scheme. So,,,,Input=Separate lug. Bias,PT,and cap can= same lug.. A/C=separate lug. Got it. Will fix and post. Another note is that all my pots and input jacks are grounded to the brass plate that mounts on the chassis between the pots +inputs. Will that be ok?
Not really. Using a brass plate like that means that your ground points to the chassis are random. All of the preamp section, including the pots, should be grounded at 1 single point with isolated input jacks, near as possible to the input jacks of the highest gain channel's input jack.

With your current brass plate setup, that could very likely cause hum or other unwanted noise.

I would suggest one of two options: either get rid of it altogether; or remove all ground connections from it and create a proper bus wire coming off the board (not on the back of the pots).

I looked at your photos in your old thread, and I can't tell for sure, but it looks like there's an extra ground lead coming off the brass plate to the chassis. I could be wrong, since I can't see the other end...assuming that's correct, I don't know how that wouldn't be creating a ground loop.

Please post a better photo showing the whole chassis in 1 image.

Thanks,
Josh
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Re: Weber 6M18 Hum Again!

Postby eljerman » Mon 05/13/19 3:19 pm

Thank's for your time and helpful response. I'm going to Not use the brass plate as a ground and set up a ground buss. I will try this as it can only help and the post.
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Re: Weber 6M18 Hum Again!

Postby eljerman » Tue 05/14/19 9:00 am

I set up a ground buss for the inputs, pots and pre-amp section. Still hums.
Could it be the jacks? They do work as far as input signal being amplified. The tone of this beast is great(sans hum) and my voltages steady at 12v and 345v. This is crazy but I wont give up. So, with all the grounds done correctly at least that's out of the way. A couple of added notes. The hum is present in both channels but way louder in the TMB side. The hum increases with volume and more so with the Bass and Middle turned up. I will take some photos and post. Thank's once again.
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Re: Weber 6M18 Hum Again!

Postby eljerman » Tue 05/14/19 9:23 am

Just noticed something. Don't know if it is a factor but......The schematic/layout shows the brown or blue wire from the PT(I'm using the brown) going to the a/c connector and the black wire from the PT going to the on/off switch. My amp has the brown wire going to the on/off switch and the black wire going to the a/c connector. Should I swap these?
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Re: Weber 6M18 Hum Again!

Postby JMPGuitars » Tue 05/14/19 10:56 am

Yes, the PT should be connected as per the layout/schematic. ;)

If there's still some hum, then you might need to do some chopsticking and/or use shielded wire for the inputs/gain pot.

There will always be a little hum, but it should be very little if the amp is clean and well dressed.
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Re: Weber 6M18 Hum Again!

Postby eljerman » Sun 05/19/19 4:32 pm

Question..... should my shielded connections from the inputs and gain pots run under or over the turret board, or is it not a factor?
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