I need some help getting rid of a hum in my new build.

[mpayne]

I’ve been trying to eliminate this hum from my new build and can’t seem to find the source.  The hum is not as loud as the guitar but seems excessive to me.  I’m new to amp building so I could use some advice on what to try next.

Here is a link to my schematics...

http://www.el34world.com/Forum/index.php?action=dlattach;topic=11423.0;attach=22861

The hum occurs when nothing is plugged into the amp.  I've verified that both high and low inputs are grounded when nothing is plugged in.
I’m assuming the problem is with poor grounding but the grounds seem good.  My board is a PCB with the bottom layer being a ground plane, mounted onto a metal plate that is also grounded.  I've grounded the backs of all the pots.
If I disconnect the “Preamp Out” connection to the effects loop the hum goes away.  So this tells me the problem is in the pre-amp.
Adjusting R24 “Preamp 1” does not affect the volume of the hum.
Adjusting R12 “Preamp 2” does affect the volume of the hum but does not eliminate it when turned down.  Since nothing is plugged in, and I know the inputs are grounded, I assume the problem is around pin 6 of V1 going into C3, R13, C8.  I’ve measured and poked around those components but have not found any problems.

Any suggestions on what I should try next?

Thanks,
Mike

[kagliostro]

You have a PCB, ok, but heaters are on the PCB traces or twisted wires ?

Have you tried with DC heater voltage as to have a test ?

Kagliostro

[HotBluePlates]

If you pull V1 from its socket, does the hum go away?

If yes, use an alligator jumper from V1, pin 7 to a certain ground (power supply ground, 3rd prong ground, etc). Does the hum go away?

If yes, move the same jumper from pin 7 to pin 2 of V1. Does the hum go away?

[mpayne]

Kragliostro,

The heaters are traces on the PCB.  I've not tried DC heater voltages.  Is that just a matter of running them at 6 VDC?

HotBluePlates,

"If you pull V1 from its socket, does the hum go away?"
  - YES

"If yes, use an alligator jumper from V1, pin 7 to a certain ground (power supply ground, 3rd prong ground, etc). Does the hum go away?"
 - NO

I really appreciate your help.  Thanks,
Mike

[HotBluePlates]

Okay, move the jumper to V2, pin 2. Does the hum go away?

[kagliostro]

Follow the suggestion by HotBluePlates, he is a very expert builder

about the use of 6v instead of 6.3v, yes, you can, the use of less voltage, within a reduction of max 10%, is well known to lower humm

but also if the use of 6v instead of 6.3v is fine, in general, go down with the heater voltage to eliminate humm isn't an "elegant" solution  

as a better solution you can try DC or elevated heaters, but before, go on with the suggestions by HBP

Kagliostro

[mpayne]

"Okay, move the jumper to V2, pin 2. Does the hum go away?"
- YES

What should I check next?

Thanks,
- Mike

[Colas LeGrippa]

do you use elevated heaters ? ( center tap with a dc+ reference )

do you use signal shielded wires, shield connected to one side only ?

has every preamp tube it's own ground ?

tubes good ? .....................lots of things may induce hum........bad solder , improper layout.......

good luck

Colas

 

[mpayne]

Hi Colas,

I'm not using elevated heaters.  I use shielded wires for connections not on the PCB.  I tried swapping V1 with another 12AX7.  The hum was still there.  Thanks for your help.

I examined my layout and noticed that one side of R2 was close to the heater trace going to V1 Pin 9.  I lifted that side and made the connection with a shielded wire.  That did not make any difference.  The hum was the same.    I've checked to see if any of the heater pins are shorted with another connection.  I can't find any bad connections, but like I've read here "If it was wired correctly it would work".

Thanks again for the help,
Mike

[HotBluePlates]

"Okay, move the jumper to V2, pin 2. Does the hum go away?"
- YES

What should I check next?

If you can, CAREFULLY move the jumper to the R13 side of C3 (the coupling cap out of the second stage of V1. Does the hum stop?

If it does, I'd look hard at the proximity of the heater traces to all the components between V1b and V2a. We might have a placement problem, where high impedance components are near a large hum current (the heater traces), which will induce hum into the high impedance circuit.

A second approach is to get a 6v lantern battery from someplace like Home Depot/Lowe's. I assume you have headers for your heater supply wires from the PT (or can unsolder those wires). Temporarily tack in the battery to supply your heaters. You'll know immediately if the hum is coming from the heater traces.

If the heater traces are the problem, you probably can relocate parts to get away from them. Consider using a twisted pair of wires instead of the heater traces. I am guessing you have board-mounted sockets; if so, you can cut the traces running to the filament pins of the tubes. Fly the twisted wire well above the board (to stay away from high impedance circuits), and have the wires drop down right to the sockets. You might want to leave small tabs of the cut traces, so you can scrape away the solder mask with a razor blade, and have a convenient point for soldering.

Please verify that the heaters are actually the problem before cutting/changing anything.

[kagliostro]

May be this can help

Kagliostro

[archaos]

May be this can help

Kagliostro

I like Merlin pics. 

[HotBluePlates]

Well, we might not want to dress the heater wires flat, because that would put them close to the circuits we're trying to get away from.

I suggest (depending on the p.c. board layout, which we haven't seen yet) that the wires be dressed like a blackface Fender amp. See the green wires along the lower-left portion of the chassis below.

[Ed_Chambley]

My board is a PCB with the bottom layer being a ground plane, mounted onto a metal plate that is also grounded.  I've grounded the backs of all the pots.


Thanks,
Mike

[/quote]
Take a look at this pot grounding method.
http://acapella.harmony-central.com/showthread.php?2291424-Jtm-45-build-instructions./page4#
It probably will not work with your build, but since seeing this I have not had to add to my pots.  I think is is a great idea and looks very nice as well.  The reason I mention this as I have had noise problems from soldering to the back of post in the past.

[mpayne]

I tried connecting a 6v lamp battery but that was not able to supply the heater voltage.  The tubes did not glow, the amp made no sound and the voltage across the battery dropped to about 3.5 vdc.  I also tried a 6V wall transformer with the same results.  I guess I need to connect it to V1, V2 heaters only?  I just replaced the two 3.15vac wires from the PT with wires from the battery.  Is that not the correct way?  I also made sure the battery ground was common with the amp ground, but that made no difference.

Thanks again for all the suggestions,
Mike

[kagliostro]

Your amp sucks more current than that the battery and your wall wart are able to give

try with only preamp tubes as you supposed

Kagliostro

[sluckey]

I also made sure the battery ground was common with the amp ground,

Don't do that. Connect one battery lead to "heater A" and connect the other battery lead to "Heater B". Disconnect the PT Blk and Org leads from the heater string.

[mpayne]

I tried re-wiring the heaters for V1, V2.  I cut the traces on the bottom of the PCB at the PT plug and at the tube sockets.  I used an ohm meter to make sure the traces were cut.  I twisted some shielded wires together and made the runs about as clean as I can.  It made no difference.  The hum was the same.  I've attached a picture.  Should that heater wiring be okay? 

Thanks again,
Mike
   

[HotBluePlates]

Well, I didn't want you to do anything drastic like that unless it was proven the heaters were the cause of the hum. So replacing the heater string would only help if using the lantern battery eliminated the hum. Further, if replacing the heater string with wiring, you'd have to replace all the heater traces with wiring; not doing it all wouldn't eliminate the cause of the hum, again if we confrimed the heaters to be causing the hum.

Can you take a photo of the part of your board that houses the components I pointed out between V1 and V2? We might need 2 photos, one of the traces and a second of the parts.

As for the battery not working, review Sluckey's comments and see if something was not hooked up properly. Obviously, you need a lantern battery with some heft, but we're also not expecting it to work more than a few minutes (you only need a ~40 second test to figure if it kills the hum).

[mpayne]

Here's a photo of V1, V2 and a  screen capture of the PCB layout.  On the layout, the red traces are on top and blue are on the bottom.  The vacant tube socket in the photo is the effects tube.  The hum is present without it so I've removed it.

I did try Sluckey's suggestion and connected one battery lead to "heater A" and the other to "heater B" on the PT org and blk leads.  The battery could not heat all of the tubes.  I'll try just hooking the battery to V1, V2 heaters next.

Thanks,
Mike

[sluckey]

I did try Sluckey's suggestion and connected one battery lead to "heater A" and the other to "heater B" on the PT org and blk leads

You did disconnect the PT filament leads whild doing the battery test, right?

[mpayne]

"You did disconnect the PT filament leads whild doing the battery test, right?"
- Yes.  The plug uses screw terminals.  So it was easy to remove the two PT 3.15vac (org/blk) wires and replace them with leads from the battery.

[tubeswell]

If it hums with nothing plugged in, it could either be a bad tube (try swapping them), or grounding hum.

How have you got the ground returns arranged?

[HotBluePlates]

I think I've found your issue.

Note the signal output trace for V1b (pin 6 to coupling cap). It is in parallel with the heater trace on the underside of the board. I'd bet that will account for your hum, why it is not killed by ground V1b's input, but is killed by grounding V2a's input.

[mpayne]

HotBluePlates,

Thanks for looking over my photos.  I see the heater/audio trace in parallel you mentioned, but I cut that part out of the heater loop when I replaced the traces with twisted wire.  I cut the traces in both places, at the plug and the socket, so that heater trace should be dead copper.  I checked again with an ohm meter to make sure.

Tubeswell,

Thanks for the reply.  I have swapped out the tubes.  The ground is a fill on the bottom of the PCB.

I got an adjustable DC supply and connected it to just the V1/V2 heaters.  I verified that it was not connected to the PT 3.15vac pins and set it to 6.0 volts DC.  I then powered up the rest of the amp (with the heaters for V3, V4 & V5 coming from the PT).  The hum was still present, maybe a little quieter, but still there.  I listened for about 1 minute then turned off the DC supply (while leaving the amp on) and after about 5 seconds the hum faded away. 

One thing I did notice is that when a guitar was plugged in and I turned off the amp the hum dies immediately, while the guitar could still be heard for several seconds (as usual).   Could that be an indication of poor filtering of the power?

I’ll keep looking around.  Thanks again for all the help,
Mike

[HotBluePlates]

HotBluePlates,

Thanks for looking over my photos.  I see the heater/audio trace in parallel you mentioned, but I cut that part out of the heater loop when I replaced the traces with twisted wire.  I cut the traces in both places, at the plug and the socket, so that heater trace should be dead copper.  I checked again with an ohm meter to make sure.

But then you dressed your new heaters wires against the board; that's the same as not cutting those traces.

Regardless, the point is high-current a.c. near high impedance circuits equals hum. Move the twisted pair away from the board, and see if the hum drops.

[mpayne]

Okay, that make sense.  I'll move the wires away from the PCB and see if that helps.

- Mike

[HotBluePlates]

The other alternative (if you don't have room under the board to get the heater wires away from the circuit) is to work from the top.

Unsolder the leg of C3 furthest from the pots (and closest to V1). Stand that cap up vertically. Run a jumper wire from the uppermost leg of C3 back over to the 100k resistor for the second half of V1.

Question: Is this a kit? If so, you oughta contact the maker, and tell them of the design flaw. I'm pretty confident this change will at least reduce the hum, if not cure it. They made a big error in their trace layout running the heaters and the signal path in parallel, which ensures hum.

[mpayne]

It's not a kit, it was my mistake.  Earlier in the week I tried lifting that leg of C3 and using a shielded wire to connect it to the 100k.  I did not work but I don't think I accomplished what you are suggesting.  Instead of removing that trace from the circuit I just added another wire parallel to it.  I'll correct that problem.

Thanks,
Mike

[Willabe]

It's not a kit, it was my mistake.

So you made the PCB Mike?

Man nice, even if you have a flaw, you'll get it worked out for this amp and the next build you do.


                   Brad     

[HotBluePlates]

Well, the upside is I know I can't design a pc board, much less one that looks as nice as yours. I just know some of the problems that crop up with them.

[mpayne]

HotBluePlates,

I corrected the parallel trace problem you found with my layout and the hum has gone away.  Thank you!

I cut the trace between C3 and R2 (at both ends) and soldered a wire, looping above the PCB, between the two.  I cut the V1 heater traces on the bottom of the PCB and used a twisted pair from the V2 for the V1 heater supply.  This removed both the top and bottom parallel traces from the circuits.

Thanks again for all the help.
-Mike

[HotBluePlates]

You're welcome! I'm glad we found and fixed the problem!

I really felt like I swallowed my foot after asking if the build was a kit. The overall quality looked so great, I just assumed it was a kit designed by an amp company. Like saying how ugly some kid is, then find out you were talking to their mother! 

But hopefully we all learned a lesson that will only make our future projects better.

[mpayne]

I was not offended the slightest bit by the kit comment.  I knew I had made a mistake somewhere and was anxious to find it.  I'm just glad you found it.

- Mike