# Silverface Bassman - Hum in one channel only - Stumped



## Michael Baker (Jan 23, 2019)

After 30 some odd years of working on tube guitar amps I finally have one that I cannot seem to resolve.
1970 AA371 Original circuit - a real "Hanger Queen" - had "questionable" power transformer.
After the usual - Caps, AA165 Output rebuild, new jacks and controls, and a replacement Classic Tone Power Transformer, I put a new set of tubes in and fired it up. HUMmmmm.
Pull V1 (Bass channel) no hum and little noise. V1 put in - Hummmmm until I turned to volume above 8 then it goes away.
"Chopsticked" the wires - no change. Read voltages within 5%, Did get some noise when I checked at the tube pins - to be expected. Double checked heater connections for phase - heater winding has central tap to ground.
Voltages and ohm readings for both channels identical
No idea why one channel hums and the other doesn't
Amp sounds great with the BASS channel tube pulled (V1)
Any ideas why one, and only the one, channel continues to hum. No apparent voltage on any grounds and ground plate isn't "floating" and, did I say, only the one channel hums.
Amp sound too nice to not resolve.
Thanks
Michael


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## greco (Jul 15, 2007)

Following this thread with interest from an "academic" standpoint only.
I enjoy others troubleshooting and discussing electronics. Wish I could help.

@jb welder, @dtsaudio, @WCGill , @cboutilier, @mhammer, will find this interesting and will hopefully be able to help. Apologies for those electronics gurus/amp techs that I missed. It wasn't intentional.

Welcome to the forum. I am sure you will enjoy the amp related discussions and that you have much to offer given your extensive experience.

Cheers

Dave


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## mhammer (Nov 30, 2007)

This is a query that should be directed to the Music Electronics Forum, perhaps specifically at member Enzo.


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## greco (Jul 15, 2007)

mhammer said:


> This is a query that should be directed to the Music Electronics Forum, perhaps specifically at member Enzo.


Why?


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## WCGill (Mar 27, 2009)

With the mods could it possibly be a grounding issue?


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## dtsaudio (Apr 15, 2009)

> Pull V1 (Bass channel) no hum and little noise. V1 put in - Hummmmm until I turned to volume above 8 then it goes away.


Nothing worse than hum problems. Last time I saw something like this, it was the ground leg of the volume pot. Try moving that connection to either where the following tubes cathode grounds or connected right to the input jack.
It certainly does sound like a grounding issue.


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## jb welder (Sep 14, 2010)

Sorry to ask this, but just in case, you didn't mention trying a different tube in V1. 
'New' does not always equal good.


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## nonreverb (Sep 19, 2006)

Definitely sounds like a grounding issue. Check the strap ground for channel one to the chassis. They sometimes come"unsoldered" due to stress on the wire from the board. Check the input jack shorting blade. Also, I've seen where the jack actually has resistance between it and the chassis due to loose or dirty contact. Check for any bad solder on the components to ground. As great as point to point is on these old Fenders, stress from the board flexing over the years can sometimes break the solder joints.


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## dtsaudio (Apr 15, 2009)

I noticed in the old layout drawing, the volume pot is grounded on the pot body. I've seen the solder not make good contact even though it looked solid. Check that whole assembly.


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## gtrguy (Jul 6, 2006)

jb welder said:


> Sorry to ask this, but just in case, you didn't mention trying a different tube in V1.
> 'New' does not always equal good.


This, had the same problem with an AA864 Bassman, banged my head against the wall for a while then swapped the tube. Problem solved. A tube swap is worth trying for sure


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## WCGill (Mar 27, 2009)

dtsaudio said:


> I noticed in the old layout drawing, the volume pot is grounded on the pot body. I've seen the solder not make good contact even though it looked solid. Check that whole assembly.


It's surprising how quiet Fenders are as the grounding scheme is definitely not "best practice" these days.


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## greco (Jul 15, 2007)

@Michael Baker ... oh brother where art thou


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## cboutilier (Jan 12, 2016)

It seems to me like a ground loop that is completed by some part of the V1 circuit


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## nonreverb (Sep 19, 2006)

dtsaudio said:


> I noticed in the old layout drawing, the volume pot is grounded on the pot body. I've seen the solder not make good contact even though it looked solid. Check that whole assembly.


I'm reminded of a Fender I had some time ago with similar problems. It turned out the volume pot was not tight to the chassis causing an intermittent ground. Also, the clasps that hold the pot together can be troublesome as well if not tight. Particularly where the plastic shaft pots from the '70's are concerned. Unlike the metal shaft ones they can be pushed in and dislodge the back cover. I've seen that problem a few times.


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## cboutilier (Jan 12, 2016)

nonreverb said:


> I'm reminded of a Fender I had some time ago with similar problems. It turned out the volume pot was not tight to the chassis causing an intermittent ground. Also, the clasps that hold the pot together can be troublesome as well if not tight. Particularly where the plastic shaft pots from the '70's are concerned. Unlike the metal shaft ones they can be pushed in and dislodge the back cover. I've seen that problem a few times.


I've also had an old amp with a wild hum and buzz issue that was entirely caused by corrosion inside the reverb pot.


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## Michael Baker (Jan 23, 2019)

After consulting the Talmud, and the local Fender tech, I removed all the pots and jacks from the front panel and had a look. The local tech, who I've known for years, told me that he believed the metal in Fender chassis changed when the switch was made to "Silverface," the steel is less conductive. He has started polishing the brass grounding plate and the inside of the chassis and then uses a heavy duty gun to spot weld the two together. For anything but original restorations to remain vintage, he goes for a modified "star" ground which is what I did.
I suspect the root cause was an overall poor connection for the ground of the Channel one input jacks - when I added isolation washers and "hard wired" the jack grounds to a "preamp" ground point the hum went way almost completely. So I removed all the ground points from the brass ground plate, created two ground busses and grounded everything to the the busses, one for the initial B+ rectification, the other for the preamp/PI ground.


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## WCGill (Mar 27, 2009)

Thanks for the update.


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## greco (Jul 15, 2007)

VERY pleased to hear that you solved the frustrating issue. Thanks for the detailed update. I always learn do many interesting electronics 'bits and pieces' from these types of threads.


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## cboutilier (Jan 12, 2016)

Never underestimate the disruptive power of a bad ground.


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## J-75 (Jul 29, 2010)

mhammer said:


> This is a query that should be directed to the Music Electronics Forum, perhaps specifically at member Enzo.


Yes, Enzo helped me out once with a Peavey Triumph 120. He's Good!


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