Hum only while playing

[fossilshark]

Hi all, I recently rebuilt my LA50BL clone (think 50w plexi circuit) and it sounds amazing. Only thing I noticed is when no signal is present there is little to no hum but when I start playing I can hear a distinct (maybe 60hz?) hum in the background. I am kind of thinking grounding issues but I am not sure.

Any advice is greatly appreciated!

[tubeswell]

Need more information

[fossilshark]

Alright, Ill post pics of the layout and schematic when I get home.

The hum is not affected by the gain knob but goes up in proportion to the volume.

[Merlin]

Does it hum when you touch the strings but don't play?

[Dave]

Yesterday a guy called me all the way from England to ask me about an amp problem. He said it's making a really weird noise. I said, "turn the lights off." He was shocked. As soon as he turned the lights off (fluorescent), the noise stopped. Sometimes you have to look outside the box.


Dave

[fossilshark]

Does it hum when you touch the strings but don't play?

No it does not, only while playing

[fossilshark]

Yesterday a guy called me all the way from England to ask me about an amp problem. He said it's making a really weird noise. I said, "turn the lights off." He was shocked. As soon as he turned the lights off (fluorescent), the noise stopped. Sometimes you have to look outside the box.


Dave

I thought about that but I was home alone and I didn't even have as much as a light on while playing.

[fossilshark]

OK finally got around to taking the chassis out of the cabinet. I am 100% sure this layout has some inherent problems, I am definitely hearing a hum no matter if im playing or not, I probably just notice it more while im playing.

this was one of the first amps I ever built and I only recently "rebuilt it" because I have no other amp to playthrough at the moment.

I am unsure if I should be sharing this schematic as Laney sent it to me about a year before they announced the re-released supergroups and I don't think they would be so kind about sharing it these days, but ill post it anyways.

Any advice on making the layout better is greatly appreciated.

[fossilshark]

Schematic

[Esquirefreak]

Cool build 👍

Could you draw your grounding scheme?

Also, voltages will help others help you.

/Max

[fossilshark]

I hope this makes sense. A is for the master vol pot not in the schematic and B is where those lines are connected to the chassis.

[Latole]

IMO, the reds wire with tye-rap are a mistake. Never un wire in parallel so close.
The can do some noise

All yellow wires going to the pots must run close to the chassis .
Not in the air or raised where they will act as an antenna for noise

[AlNewman]

if it's humming when you're playing or not, try pulling your tubes and see where it starts.

[Latole]

if it's humming when you're playing or not, try pulling your tubes and see where it starts.

OP wrote : " ..only while playing.."

Bad wires dress ?

[Ronquest]

OP wrote : " ..only while playing.."

OP then also, "I am definitely hearing a hum no matter if im playing or not, I probably just notice it more while im playing. "

[Latole]

OP wrote : " ..only while playing.."

OP then also, "I am definitely hearing a hum no matter if im playing or not, I probably just notice it more while im playing. "

Poor wire / lead dress

[Ronquest]

Post a recording of this hum. With the guitar plugged in and with nothing plugged in to the amp.

Without getting into your actual wiring/build.
Does the amp hum with power tubes removed (if voltages are safe to power up without tubes) ?
What's your bias for each power tube?
What frequency, 50/60 or 100/120 Hz?
Does the hum change when you turn your guitar in a different direction, volume up down, and unplugged ? 

 

[Latole]

Does the amp hum with power tubes removed (if voltages are safe to power up without tubes) ?

You will never hear a hum , or hiss or sound with power tube removed.

[Esquirefreak]

If the OT is picking up the magnetic field from the PT you can sometimes hear it through the speaker.

Here's how I usually ground my amps. Crude drawing, but hopefully you'll get the idea. Point C connected to chassis ground near input jack, point B connected to C via a bus bar. Point A connected to chassis as far away from C as possible.

/Max

[Latole]

Looking at the (highly questionable) wire assembly, it's a problem that needs to be corrected first and foremost.

And I doubt it's coming from the transformer, as you suggest.

[Ronquest]

Does the amp hum with power tubes removed (if voltages are safe to power up without tubes) ?

You will never hear a hum , or hiss or sound with power tube removed.

Transformer coupling. 

[Ronquest]

Looking at the (highly questionable) wire assembly, it's a problem that needs to be corrected first and foremost.

And I doubt it's coming from the transformer, as you suggest.

Agree about the wiring and not doubting that there could be improvement in the wiring, just trying to source out other quick checks.

Have you ever heard a Marshall Origin 50?  I may be a bit sensitive to amp noise, but I've moved brand new amp transformers.
Preamp tubes can hum too, maybe a few things going on here.  People running speaker cables for instrument cables and just plain bad cables.

[AlNewman]

Looks like it could be a grounding issue?

http://www.valvewizard.co.uk/Grounding.pdf

It's amazing that the people who share these resources are actually in these very forums.

[bmccowan]

It could be a grounding issue, but I agree with Latole that the lead dress should be corrected.
A clue is

OK finally got around to taking the chassis out of the cabinet. I am 100% sure this layout has some inherent problems, I am definitely hearing a hum no matter if im playing or not, I probably just notice it more while im playing

If I understand, the hum increased when the chassis was removed - that is not unusual, but could point to a lead dress/shielding problem.
If you look at a Marshall 50 watt layout - I believe there are some on Rob Robinette's site and also some on Ceriatone - likely others too - you will see some successful lead dress and grounding schemes.
That amp has a ton of filtering so should sing rather than hum.
Edit - forgot to add that I agree with Latole that those leads should be unbundled and then you can move them around to get the lowest noise floor.

[tdvt]

Looking at your last pic, L4, it looks as though your PT copper shielding band is running front-to-back. I have found (the hard way) that the banding needs to be on-axis with the chassis length to put the circuit in a protected zone.


The band doesn't offer any shielding to the circuit in that position. I know it is a bit of work, but try spinning the PT 90°.


I have found it is a losing battle fighting the hum with the PT in that orientation. 

[Willabe]

Edit - forgot to add that I agree with Latole that those leads should be unbundled and then you can move them around to get the lowest noise floor.

Yes, agree.

[Willabe]

Looking at your last pic, L4, it looks as though your PT copper shielding band is running front-to-back. I have found (the hard way) that the banding needs to be on-axis with the chassis length to put the circuit in a protected zone.

The band doesn't offer any shielding to the circuit in that position. I know it is a bit of work, but try spinning the PT 90°.

I have found it is a losing battle fighting the hum with the PT in that orientation.

Huummm, never heard that before. 

[Dave]

Maybe I'm not seeing it, but is there an artificial center tap on the heater secondaries? I don't see a real one, so it would need to have an artificial one. If it has nothing, then that would be a big hum problem.


Dave

[Willabe]

Maybe I'm not seeing it, but is there an artificial center tap on the heater secondaries? I don't see a real one, so it would need to have an artificial one. If it has nothing, then that would be a big hum problem.

Dave

I think you got it Dave.   

I don't see it either. PT says 6.3 @ 6.5A  VIO-VIO, no CT listed. And I don't see any artificial CT R's either.

[Latole]

Maybe I'm not seeing it, but is there an artificial center tap on the heater secondaries? I don't see a real one, so it would need to have an artificial one. If it has nothing, then that would be a big hum problem.


Dave

Good point :  Schematic show a center tap to the ground .  Amp show no ground ; look screen shot

[Willabe]

Good point :  Schematic show a center tap to the ground .  Amp show no ground ; look screen shot

That orange wire is the 28v CT. Look at the PT's label.

[sluckey]

Good point :  Schematic show a center tap to the ground .  Amp show no ground ; look screen shot

That schematic does not match the actual amp! Best to just ignore the schematic.

[tubeswell]

Good point :  Schematic show a center tap to the ground .  Amp show no ground ; look screen shot

That schematic does not match the actual amp! Best to just ignore the schematic.

Yes the green-orange-green is a 28v winding. Bias I’d say

[fossilshark]

Yes guys this amp is a hodgpodge of parts and mods sorry. That being said now that I think about it after what I learned building the wendigo I am pretty sure the heater lines don't have a CT. This is most likely the issue. That being said I will be taking all of your advice into consideration and doing some experiments to find the lowest noise. Thank you all for taking a look at my ghetto blaster.

[tdvt]

Looking at your last pic, L4, it looks as though your PT copper shielding band is running front-to-back. I have found (the hard way) that the banding needs to be on-axis with the chassis length to put the circuit in a protected zone.

The band doesn't offer any shielding to the circuit in that position. I know it is a bit of work, but try spinning the PT 90°.

I have found it is a losing battle fighting the hum with the PT in that orientation.

Huummm, never heard that before. 

I only discovered this by dumb luck trying to eliminate hum in an amp a friend had built.

It had a hum that would not go away no matter what was tried, & by chance, while doing the "headphone trick" for transformer placement, I found that lining up the PT shielding the length of the chassis finally eliminated the issue.

The cause & effect was totally repeatable.  Then looking at multiple vintage Fender amps confirmed that the PT orientation was a thing.   

[sluckey]

Looking at your last pic, L4, it looks as though your PT copper shielding band is running front-to-back. I have found (the hard way) that the banding needs to be on-axis with the chassis length to put the circuit in a protected zone.


The band doesn't offer any shielding to the circuit in that position. I know it is a bit of work, but try spinning the PT 90°.


I have found it is a losing battle fighting the hum with the PT in that orientation.

I've also never heard of that. I have several amps that have the copper band facing front to back and never hand any issues. I think the Fender PT orientation is all about layout, being able to line up tubes behind the PT and bias board in front of the PT in that narrow chassis. If the PT were rotated 90° then you could not put a rectifier tube between the PT and the back edge of the chassis and put the bias board between the PT and front edge.

[tdvt]

I've also never heard of that. I have several amps that have the copper band facing front to back and never hand any issues. I think the Fender PT orientation is all about layout, being able to line up tubes behind the PT and bias board in front of the PT in that narrow chassis. If the PT were rotated 90° then you could not put a rectifier tube between the PT and the back edge of the chassis and put the bias board between the PT and front edge.

I am just putting that out there as a possible factor, based only on my own (limited) observations, nothing hard & fast about it.

I'm sure there are more factors at play than just the copper band orientation, starting with overall layout, lead dress, etc. & also sure that there are many amps that work fine with the band off-axis on the chassis.

But I have made a point of checking the last few transformer sets for hum before mounting anything (again, the headphone trick) & there was a consistently quieter area with the band facing the OT  than with the band off-axis to the OT.

[AlNewman]

I've also tested transformer orientation with headphones, and it certainly makes a difference.  Whether that works with rectifier or layout placement, I'm not sure.
Since there isn't one already, and it is likely the easiest solution, I would think an artificial center tap is the best place to start.

[tubeswell]

consistently quieter area with the band facing the OT  than with the band off-axis to the OT.

The way the Faraday shield is wrapped around the PT core, is a result of the orientation of the PT winding/core - and it is the latter that can cause unwanted EMF coupling.