Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TubeGeek on September 23, 2012, 06:36:56 pm
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The amp is complete. I found the 120Hz hum coming from a faulty 12ax7.
I have just finished building a Marshall 2204 clone for my own collection. I am in the process of voicing the amp right now. It was not built from a kit. I followed the original Marshall schematic, ceriatone layout and another layout that I think is from Metro. Between the three I think I got it right considering the layout I went with.
The voltages are within spec and compare closely to a bunch of actual 2204's I have had come across my bench.
The tubes I have installed right now are: reissue tung-sol, mullard, and TAD 12ax7, Winged C EL34's. Hammond transformers. 4x12 cabinet with Celestion greenbacks.
The amp sounds really nice at this point but there is some 120Hz hum that I am not sure about. I'd like to see if I reduce it further. It is dead quiet otherwise. I wonder if the 120Hz hum I have is acceptable for this circuit or if there is something else I can do in grounding or something I am missing???
Here is what I have done to troubleshoot:
1. Verified that it is indeed 120Hz.
2. Since it is 120Hz it is post rectification. Suspected not enough filtering in power supply. Temporarily connected a 100+100uF cap in place of the first cap after the rectifier diodes. The hum did not decrease. Tried moving the grounds of filter caps between preamp and power section ground points with no reduction.
3. I moved the bias filter cap ground over to the preamp buss ground. No reduction in hum.
4. Initially in this amp I had the filament wiring different than pictured. I did not twist the wires. I knew that I was going to have to twist them to absolutely rule that out.
I have run out of fresh ideas. Do you see anything obvious? :help: :dontknow:
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Did you DC-lift the filaments?
Jaz
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One of your fuse holders is right on top of an output tube socket with the screen grid stopper really close to the B+.
Also 1 of the speaker jacks is right on top of an output socket and is almost kissing the heater wires. The other speaker jack is right on top of the PI socket.
Brad :think1:
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Did you DC-lift the filaments?
Jaz
No the filaments are not raised. Wouldn't this be 60Hz hum though?
These filaments are 6.3VAC
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One of your fuse holders is right on top of an output tube socket with the screen grid stopper really close to the B+.
Also 1 of the speaker jacks is right on top of an output socket and is almost kissing the heater wires. The other speaker jack is right on top of the PI socket.
Brad :think1:
Yeah these thoughts have crossed my mind also. I wonder if the proximity of the power, output and choke on the top of the chassis may be the cause too.
In a perfect world I'd have more space to spread things out. I don't in this box, I will have to rebuild if it gets to that point.
I have seen other 2204's in small boxes like this, I didn't realize when I bought the chassis and cabinet that space would be this tight.
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I wonder if the proximity of the power, output and choke on the top of the chassis may be the cause too.
Got a picture of the outside top of the chassis to post?
Brad :icon_biggrin:
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Yeah. I'm doing this from an iPhone, hopefully it works.
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Looks real nice! :icon_biggrin:
PT and OT might be a little close but you do have the 3 cores at 180 degrees from each other and all 3 have end bells.
Brad :think1:
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Thanks. I'm hesitant to rebuild. Possibly it's due to the way I have the laminations??? May be a stretch.
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I'm hesitant to rebuild. Possibly it's due to the way I have the laminations???
KOC in his TUT3 recommends the same orientation for the PT and OT as you have.
Wait and see what some of the other guys have to say.
Brad :think1:
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Other thoughts I'm having:
More bias supply capacitance... Maybe increasing the 8uF caps to 20uF, recommended or not?
Possibly bad filter cap? I have had the JJ multi caps sitting on the bench for 5 years or so.
Tubes... Should have done this already but I'll try a new set of preamp and power tubes tonight.
That's all I got for now. Thanks for chiming in.
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I've built 2204's in smaller chassis with no issues.You have done something wrong and it's not immediately obvious,but easy to overlook.
I bet it's the WAY you wired your filter caps.
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I can't completely tell from the pictures, but it looks like you're using a single point star ground? My quietest builds and in amps I've reduced the hum in have been acheived by following the hoffman grounding scheme. i.e.:
-Green wire from the AC wire is grounded by itself 2-4 inches away from the power supply nodes at one end of the chassis.
-Power supply nodes grounded by themselves with PT center taps 2-3 inches away from the AC cord ground
-Preamp nodes grounded at the other end of the chassis.
I've always had hum when the AC cord and power supply nodes are grounded at the same point.
And.... if you're already following this scheme, please ignore. Good luck figuring it out... nice build!
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I concur.I never ground the AC cord with other wires.
Where is the third filter cap?
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Yeah just noticed that I did this, I've never done it this way before either. I'll move the ac earth ground to its own tonight.
The third cap is located under the turret board. It connects to the 2 10k resistors on the board.
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I've built 2204's in smaller chassis with no issues.You have done something wrong and it's not immediately obvious,but easy to overlook.
I bet it's the WAY you wired your filter caps.
I am trying to figure out where you are headed... The WAY I wired them?
Should I use a different color wire? Lol
I have looked over the layouts and schematics, things look good to me.
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Hmmm....looking a little closer,your presence control is wired wrong.Not sure if it will make noise but it's wrong nonetheless.One mistake leads to other's,so get a good layout like the one at Ceriatone's website and look again.
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Hmmm....looking a little closer,your presence control is wired wrong.Not sure if it will make noise but it's wrong nonetheless.One mistake leads to other's,so get a good layout like the one at Ceriatone's website and look again.
Good eye! I see I need to jumper the pot lugs on the 0.1uf side. It works as it is, adding the jumper will officially make it a variable resistor as it should be.
I was originally going to copy the Marshall layout but after drilling holes in the chassis I realized I did things differently. I had to flip things backwards and inverted. If you look my inputs are on the left side of the chassis and Marshall's are on the right side. This also caused me to wire all pots backwards, I have fixed the pots though. It got a little confusing. I missed that presence jumper though.
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Look again.The jumper is missing but the wire to the board goes to the empty lug,not the cap side with the jumper.
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Moved the IEC earth ground to its own bolt. Did not reduce the hum.
Verified presence is working correctly. It is.
I'm still stumped. I'm beginning to think the hum is just inherent in this amp and I am just being too picky.
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Ok so i found the problem. It was a preamp tube giving me the 120hz hum. :BangHead:
I should have checked this very first but of course i assumed because the tubes were new, it wasnt the issue.
The amp is exactly how i want it now.
Thanks for the help guys.
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Glad you found it!
It's a beautiful amp build inside and out.
Brad :bravo1:
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Here is one more pic of her all buttoned up and ready for rock :headbang: :guitar1 :m8 :m7
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Nice work TG, way to stick to troubleshooting and figure it out.....sometimes nothing feels better than the satisfaction of that.....and its all for the sake of education...looks great!
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Nice work TG, way to stick to troubleshooting and figure it out.....sometimes nothing feels better than the satisfaction of that.....and its all for the sake of education...looks great!
This hobby would be boring if everything always worked right off the bat!
I enjoy a challenge every now and then. The education is invaluable too and the feeling one gets when it's complete is priceless.
Now that this amp is done my bench feels empty. I'll start a new project tonight!
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Now that this amp is done my bench feels empty. I'll start a new project tonight!
:laugh:
Brad :thumbsup:
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Beautifully crafted build there TubeGeek. I bet it sounds great too. I like the easy to read labeling and the use of gold paint. Pretty amazing collection you have there at casa de la Glacier. Is this going to be a head or combo?
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Look in reply #22. Pics of it are posted.
Brad :icon_biggrin:
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errrr, da. :rolleyes:
Nice!
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Nice work Mr. Geek, You got the right idea, don't let the soldering iron go cold!!! :icon_biggrin: