Can't get the hum out of Champ

[NotSure]

I know, another hummy Champ post. I've checked as many posts that I can find on this subject. I've checked all the usual heater wiring issues, double checked everything I can think of. I'm not "star grounded", but my first champ build, from a kit, had 4 different ground points to lugs soldered to the chassis. I built this one from scratch the same way. There are no cold solder issues to the chassis. I was a pipe welder for many years and I know a bad weld when I see one. The lugs are soldered directly to the chassis, not a brass plate, and ground leads soldered to the lugs.


My symptoms: The hum, I believe, is 60 cycle. The amp is quiet with no volume and no guitar plugged in. The hum is affected by volume. That is, it becomes audible at about the 3 setting and gets louder turning up the volume. It is really loud at full volume and changes pitch a little. When I plug in a guitar the hum is reduced significantly. I played the amp for a while when I first finished it and didn't really notice anything extraordinary. I just happened to turn it on with no guitar plugged in one day and heard the really loud hum, with volume. Now that I've heard it I believe the tone is seriously compromised by the hum when being played through.


If any of you all have seen the Uncle Doug video where he turns on an amp and its dead quiet, and then he checks to see if its working by plugging in a guitar cable and touching the end to make a noise through the speaker. My amp is the opposite situation. Its noisy with nothing plugged in and volume up, and becomes quieter with a cable plugged in and my fingers on the end, or plugged into my guitar.


Thanks for any tips to cure the hum.

[shooter]

I believe, is 60 cycle.

typically indicates filament interference with signal

The hum is affected by volume

typically indicates problem is being injected to the "left" of the pot.

try these;
disconnect -FB if you have it, test
pull V1

make sure the grounds for the pre amp tap are not directly tied to the power amp tap

lead dress, wrong grounding, bad grounding are really important in a SE design because you don't get the normal "cancelation" from a PP design

[sluckey]

The lugs are soldered directly to the chassis, not a brass plate, and ground leads soldered to the lugs.

So, do you have a steel chassis? I ask because you can't solder to an aluminum chassis.

If any of you all have seen the Uncle Doug video where he turns on an amp and its dead quiet, and then he checks to see if its working by plugging in a guitar cable and touching the end to make a noise through the speaker. My amp is the opposite situation. Its noisy with nothing plugged in and volume up, and becomes quieter with a cable plugged in and my fingers on the end, or plugged into my guitar.

That makes me think you don't have a switching jack on the input, or if you do, it isn't wired properly.

[Willabe]

What Sluckey and Shooter wrote.

And, did you hook up the heater center tap (CT) to ground? Or if no CT, did you hook up the 2 @ 100ohm R's for a faux CT?

[mresistor]

also might try a different 6V6 and 12AX7

[Platefire]

The original power supply filtering is an issue 16/8/8 uF. I simply changed mine to 22/10/8. That don't seem like much of a difference but made a world of difference in my operating hum. Now don't tell me I'm just getting hard of hearing Platefire

[sluckey]

You can also add a 20µF cap and 100Ω resistor between the rectifier and the first filter cap.

And probably the best way to reduce hum in a champ is to use a cheap 8" speaker like Leo did. 

[NotSure]

I want to thank all for the replies to my post. There are several points that you all have made that need to be checked out. I'll definitely post a reply if there is a fix that addresses the hum, and may help others.


I also want to use this post to point out that 92 members have read this subject. Noise and hum remain an issue even with the most simple guitar amp circuit there is. So don't be embarrassed to ask questions about your first build.


Lastly, I want to take this opportunity to say how much I appreciate this forum, the moderators, and the high level contributors, hell, all the contributors, that provide their knowledge and expertise for FREE. I recognize how encouraging their contributions are, never casting judgement on us folks learning how to do this craft, and clearly promoting the pure joy of bettering the amp building community that they can reach. Thank you again. This forum is truly a treasure. This last paragraph doesn't come close to expressing how much I personally appreciate this resource.


Happy Holidays to all,


David

[shooter]

want to use this post to point out that 92 members have read this

we're just bored 
You have found an amazing place, you do have to carry your own chassis though, read ALOT, if 'yer dyslexic like me, there are lots of graphs n pics also, If you do put in the time, be warned, you WILL become an addict 

[NotSure]

I've attached my power transformer schematic. I guess the inconsistency with my build and drawing is my Hammond contact said "DO NOT attach the Grn/Yel wire to ground." I guess he's right. I've already burnt up one of these 290AX's so now I'm a little paranoid.


But, I don't see why running Grn/Yel to ground would cook my transformer. Seems like this is the heater center tap that yall have been recommending to make sure 1. that I have one, and 2. ground it if I do. I do, and I didn't. Its coiled, capped, and tied off.


If Hammond is right then do I have to build in the pseudo tap thing? And where does that go in my circuit? And is this the most likely cause of what I'm hearing?


Thanks all

[sluckey]

I guess the inconsistency with my build and drawing is my Hammond contact said "DO NOT attach the Grn/Yel wire to ground."

That's nonsense! Connect that grn/yel to ground if you want to reduce filament hum.

[dude]

Ha, no wonder you have hum, didn't ground the filaments or use 100 ohm resistors.
I think the original champ PT didn't have a CT on the filaments and they grounded one tap, so all champs hum. Usually the filament wiring is changed and two 100 ohm R's to ground solve the hum, it did for me.


Ground that wire to your PT CT ground spot, bet the hum leaves.

[NotSure]

Dude you're exactly right.


And thanks Willabe for the original suggestion regarding the pseudo tap. I created the CT with the parallel 100 ohm resistors, even though I do have a CT in my PT. I've left my CT lead capped and not grounded. I know Sluckey, by the schematic it makes no sense to make a pseudo CT, but I'm afraid I'll burn up another transformer. I've built a dim bulb tester since the last fire and smoke afternoon. Would that save the tranny if I go ahead and ground grn/yel to my chassis lug? The way I did the grounding last time was with red/yel and grn/yel soldered to the same chassis lug together. Did that burn my transformer? Should they have been grounded in separate places on the chassis?


Now the amp is as quiet as one could hope for with this circuit. Now only when I really drive it at max or near max volume does the hum become pronounced. And that gets pushed away as soon as I play. Just like any good tube amp.


But, I don't like solving the problem with a less than elegant or non-standard method. I would much prefer to use the provided CT in my PT. For now the amp sounds like it should.  And, at least, this has been an excellent teaching experience.


Appreciate any more thoughts on this. The whole grounding issue, star grounding, grounding buss bars, grounding to the PT bolt mounts, brass plates, Gerald Weber does the ground lug thing on his Champ kits, etc, has got me confused. Seems like its a different grounding scheme for every type of amp as well.

[sluckey]

The way I did the grounding last time was with red/yel and grn/yel soldered to the same chassis lug together. Did that burn my transformer?

ABSOLUTELY NOT!