Univox U1011 Hum

[g1]

I tried grounding out signal traces around/connected to the trace leading to V3 - no effect.

Not sure what you mean here as you had said grounding V3 grid kills the hum?
Are you saying grounding at the board doesn't help but grounding at the tube socket does?

P.S. how do I change my picture? I can't seem to find it in the settings.

For me (firefox), I click on my avatar at the upper right corner of the page, then select 'my profile'. Then in the profile settings there is an 'edit avatar' button.

[tubemonster]

Not sure what you mean here as you had said grounding V3 grid kills the hum?
Are you saying grounding at the board doesn't help but grounding at the tube socket does?

Sorry about that - if you ground out the V3 P7 grid or where the wire from the V3 grid connects to the board the hum goes away.

[tubemonster]

For me (firefox), I click on my avatar at the upper right corner of the page, then select 'my profile'. Then in the profile settings there is an 'edit avatar' button.

Well, I can't find the hum or the Avatar Button - even bugged the wife for some help finding the avatar button - lol...
Anyways, this is what I was going to set it to:

[glebert]

For me (firefox), I click on my avatar at the upper right corner of the page, then select 'my profile'. Then in the profile settings there is an 'edit avatar' button.

In Chrome you click on your username in the upper right corner, click "My Profile" and then on the left side is the avatar pic placeholder with an "Edit Avatar" blue button. Click that and browse to find and upload picture or put in URL.

[Helmholtz]

Still waiting for a scope pic of the hum at the speaker output.

[tubemonster]

Still waiting for a scope pic of the hum at the speaker output.

Well, I got some pictures of the scope, but I'm getting this error when trying to upload them:

"Upload failed due to your usergroup's upload quota. This file will require 116.9 KB but you only have 55.2 KB of 976.6 KB remaining."

In the meantime, I was having a hard time getting the scope to show anything useful hum wise on the speaker output without turning the volumes up, even though you can clearly hear the hum come in and out when turning the bass up and down with the volumes down. It's easy to view the hum with the volumes down on the V6 pin7 12au7 - the half just before the phase inverter.

I'm thinking I need to sit this amp off to the side for a day or two and then come back to it and start at one end and work my way to the other with fresher eyes type thing

[Helmholtz]

In the meantime, I was having a hard time getting the scope to show anything useful hum wise on the speaker output without turning the volumes up, even though you can clearly hear the hum come in and out when turning the bass up and down with the volumes down. It's easy to view the hum with the volumes down on the V6 pin7 12au7 - the half just before the phase inverter.

Use DC coupling with the scope and turn up volumes if necessary.

[tubemonster]

Use DC coupling with the scope and turn up volumes if necessary.

OK - I'll try again, it's all still set up, I should be able to escape for a couple minutes to do it - lol...

[tubemonster]

Use DC coupling with the scope and turn up volumes if necessary.

I did it again on the speaker output and the stage just before the phase inverter and I get the same scope results whether the inputs were set to DC or AC.
I tried to upload pictures again, but got the same type upload error.

I don't have an online file storage site I can upload pictures and create links to.

[glebert]

There were some website updates being done today, maybe tboy hasn't gotten all the bugs worked out for the file uploading.

[glebert]

Couple extra questions, forgive me if you already gave this info and I missed it. Is there anything plugged in when the hum is happening? With nothing plugged in does the hum change if you touch the chassis/faceplate or get something near the first gain stage components? The reason I ask is that I am having an almost identical problem on a completely different amp (Ward Airlines tube bass amp). I realized that the hum went away when I touched the chassis, and gets much worse if anything approaches the components of the first gain stage. Then I realized that there was a foil covered cardboard sheet that had been on the bottom that tore off and I hadn't replaced. Additionally the channel that I have the noise on has unshielded lines from the jacks to the turret boards, but the quiet channel has shielded cables.

[tubemonster]

I knew I had a Homer Simpson Doh moment coming and it's a whopper of a DOH moment too.
It's pretty embarrassing really

So, today I got up and around and figured I’d take a look at the amp and I spotted something pretty much first thing.
How I missed it before this, or even did it in the first place, I don’t know, but you guys are going to kick me of the forum with my first thread or at least put me in the corner with a dunce hat on.

Here's what led to it.

So, when I got this amp it had a two prong power cord and like norm I switched the amp over to a three prong power cord.

The filter caps appeared to be original, where way out of tolerance, and the amp hummed, etc, so they got changed.

Also, noticing that the filaments for the 6l6gc tubes (which have their own ac feed separate from the preamp section tubes) were wired with one side to ground, I decided it'd be best to use a couple of 100 ohm resistors to do the whole artificial center tap thing, again, pretty much standard quo/norm.

And, I figured while I was at it why not raise/elevate the center tap for the heaters feeding the preamp section, maybe not the standard quo/norm, but certainly not unheard of, common, and not that difficult to do (especially if it's not your first time behind wheel doing so). To do this I used HT B+ and a voltage divider because the amp is fixed biased.

Well, here's where I found my Doh moment and it's probably the biggest I've made I think.

(see picture below)

For the HT B+ I tapped into the same node the Choke uses. I used the correct resistor values, etc - I even connected the preamp center tap to the correct position.
But, you know what I didn't do, BIG DOH HERE, is ground the voltage divider network to the correct place.

I was pulling the HT B+ from the positive of one multi-section cap. For the voltage divider ground I was using the negative of the other multi-section cap (by mistake of course, and this is the doh moment).
Moving the ground of the voltage divider to an actual ground has the amp running very quiet and the bass control no longer brings in a lot of hum. Go Figure…

I also completely forgot to even mention I was raising/elevating, or I guess I should say attempting, to raise/elevate the heaters in the preamp section.
I'm sure it would have brought about questions about it.
My bad, sorry about that.

Here’s a picture of what I’m talking about.



I do appreciate the help and time about this.
Once I get out of the corner and take my dunce cap off, if you haven't changed my password, I'll be back - lol...

I won't forget this mistake anytime soon - wow...

[tubemonster]

Couple extra questions, forgive me if you already gave this info and I missed it. Is there anything plugged in when the hum is happening? With nothing plugged in does the hum change if you touch the chassis/faceplate or get something near the first gain stage components? The reason I ask is that I am having an almost identical problem on a completely different amp (Ward Airlines tube bass amp). I realized that the hum went away when I touched the chassis, and gets much worse if anything approaches the components of the first gain stage. Then I realized that there was a foil covered cardboard sheet that had been on the bottom that tore off and I hadn't replaced. Additionally the channel that I have the noise on has unshielded lines from the jacks to the turret boards, but the quiet channel has shielded cables.

I really appreciate the help, but I found the problem and it was a mistake I made.
See my post prior to this one.