Question on Bassman 5F6A hum

[pompeiisneaks]

I've got someone that took my Bassman off my hands but they're not fond of the hum at max volume that I attribute to the transformer in this thing being like 1" from one of the speakers in the bassman cabinet, but they've sent it back with me for now to see if I can figure it out. I'm pretty comfortable that the heaters are good, I'm going to swap out the two 100ohm to ground for a bias balance pot to see if I can dial it in a bit better.  Any other tips to getting that hum down?  It is only at really high volume, maybe 70% or higher that it's obvious. 

Or is it just bad heater setup somehow?

~Phil

[shooter]

Or is it just bad heater setup somehow?

do you have a scope?  You should be able to "see" the hum, whether heater, or other.

[robrob]

Lead dress can cause hum, especially when grid and plate wires are too close to one another.

[pompeiisneaks]

Or is it just bad heater setup somehow?

do you have a scope?  You should be able to "see" the hum, whether heater, or other.

I do, I'll give that a go, so just trace it with the hum only and see where it appears in the signal chain?  Sounds like a plan thanks!

(robrob I'll check the lead dress as well, but I thought I've seen tons of amps with all three wires right next to one another (grid/plate/ground).  Is that what you mean?

~Phil

[shooter]

I do, I'll give that a go

At V1a it will probably be in the "weeds" of the scope, play with the time-base, stare at the "noise" , once you think you've identified the culprit, then look to the plate for "that" signature signal.

[pompeiisneaks]

Okay I'm pretty sure I've isolated it to V1.  That tube even seems to make a lot of noise when tapped with a chopstick. I swapped the 12AY7 with a 12AT7 I had here just to see if that 'microphonic' effect would go away, but it doesn't.  The noise is very visible by the time it gets to the tone stack and is big by the PI.  This ONLY happens past volume of say 8-9 of 12.  I tap around with the chopstick and it seemed to make a lot of noise around one of the two 100k resistors to the anode of V1, so I swapped that with a spare to no avail, I swapped the original back in and it's a lot quieter tapping but the hum is still there.  Could this just need a hum  balance pot?  (Although I'm pretty sure the frequency of hte hum is 120Hz.  (the freq counter on the scope seems to bounce around between 120 and maybe 180 and even over a bit, it's not 'consistent'. 

I'm definitely at wit's end at this point, because I know where the hum comes from, but not why.  (You can still even hear the hum a tiny bit at 0 volume on both pots.)

Ideas?  I'm going to try a hum balance pot and see if that helps any.

[pompeiisneaks]

Hum balance pot seems to be able to take the 'edge' off a bit, change it some so it's a bit 'cleaner' sounding but not lower the sound out completely, it really seems like something at the input is screwy.  Maybe the coaxial cable isn't doing a very good job of shielding?  Maybe the jacks themselves are being weird (They're brand new with this build that's only maybe 4 or 5 months old).

Ugh! trying to get this fixed for the guy by tomorrow, he's got a chance to A/B it with an original 59 bassman, and I want it clean and silent!

~Phil

[shooter]

Maybe the jacks themselves are being weird

I'm pretty sure I've isolated it to V1

Sluckey had some try just a unshielded wire for a reverb noise issue, maybe try that.  If you have it at V1 and the amp works, it's gonna keep getting bigger.
I just swapped a tube that was microphonic, it took 2 tubes to fix mine

[pompeiisneaks]

I found the issue, but not yet a fix.  It's the input jacks grounding to chassis, causing a grounding loop.  I unscrewed them and pulled them out so I could touch things with the chopstick better in the inputs and boom, sound gone.  I need to see what would be a good spot to ground them, because when I cut the ground to the main bus, the noise got 40 times worse.  Just a grounding issue /sigh.

~Phil

[pompeiisneaks]

ok did a split of the ground, tied off the PI out to the inputs up at that end by the inputs at a bolt on the chassis, then tied the Power section ground to the bolt on  the transformer.  That made the hum go away for good,  But now with all my monkeying with the inputs I seem to have horrid buzz on the inputs.  IS that just bad ground for them only somehow?  It's basically gone on a humbucker but the single coils sound horrid.  Or is that just my shitty power at the house?

~Phil

[pompeiisneaks]

Man this is so frustrating, that hum is now gone and I just get that damn buzz.  I did try the noisiest input as just a straight wire from the resistors to the grid, but it doesn't seem any better. 

~Phil

[pompeiisneaks]

I moved it to a different location, thinking it could just be the typical crap 'hum' you get from really dirty power.  I'm upstairs where I tend to have less noise, and into a Furman power conditioner and it's still horrible.  I tap V1 with the chopstick and it truly sings, even tapping the wires sings. Do I have a bad socket?  swapping the 12AY7 with a 12AU7 or 12AX7 doesn't help any, so multiple different tubes haven't caused it to stop, but of course htey're also different gain so they seem a bit noisier.  (I don't have another 12AU7 here). 

What does it mean when I'm tapping the input wires and it even makes noise? I even suspected that one of my wires was bad (normal channel is by far the worst sounding) and put a brand new shielded wire in it. 

Again, this is way different noise than what I was getting with the other ground loop, that was a low heavy hum, this is that buzz you get when near neon lights, but yesterday it seemed to go away with humbuckers, but now my SG is doing it.

~Phil

[shooter]

What does it mean when I'm tapping the input wires and it even makes noise?

I was working with another builder and had a similar problem, EVERYTHING was microphonic.

I came back the next day and that problem was solved, I slipped a decimal on bias calc's and when bias was gone correctly, walla, microphonics gone.  Don't know if that helps you, just a tidbit from my toolbox to check.

[pompeiisneaks]

I realized the bias was pretty cold, on the power tubes right?  I had it like 38mA and the 6L6GC at that voltage should be more like 40 to 42, but that didn't help.  I swapped the socket, no more of the microphonic effect but the buzz is still just as ugly.

~Phil

[pompeiisneaks]

well 6 plus hours yesterday, 10+ today and I've replaced almost all of the carbon comp in the path, thinking it's what's causing the tone issues, and still no love.  I do think the 12AY7 may be part of the problem, I swap it out with a 12AU7 I have here, and it is cleaner sounding, less hum/buzz, but I can't get it to go away.

I had some old transformer low level hum until I split the grounding from a single bus to ground (Except the input jacks, which was causing the hum) and the ugly deep hum is gone. Of course the buzz has appeared now and won't dissipate, except swapping tubes.  I can almost get it silent on humbuckers but the second I tap it or put in single coil bam it's back.  It's barely audible below 9 or so but from 9 to 12 it's horrid. 

Could it still be a grounding loop of some kind?  the scope seems to show this hum closer to 60Hz now instead of the 120 of the ground loop I was getting before I switched the main ground out. 

I'm at a loss at this point, but I need to figure it out and get it fixed. 

~Phil

[sluckey]

Didn't this buzz problem just appear in the last couple days while you were chasing hum issues? What has changed since then? Can you roll it back a couple days?

[pompeiisneaks]

Yes the buzz problem showed up the last couple days, it seems to have showed up after I split the grounding scheme (that removed the massive hum, but now seems to have added the buzz. Is it just a grounding loop issue of a different kind?)  How do I go back?  Restore the grounding loop?

~Phil

[NewYorker]

Phil:

Is it possible that the "buzz" was always there, but hidden or masked by the "hum"?  And now, with the hum gone, you can hear the buzz?

What happens if the amp is powered up and not on Standby, with nothing plugged in?  is any hum or buzz audible?

How close is your guitar to the amp when you hear the buzz?  The power transformer can emit enough 60 Hz EMI for your guitar to pick up.  Obviously this is more likely with single-coil pickups, But I've found that even humbuckers (or the wiring in the guitar) can pick up hum from the PT if it's close enough to the amp.  What happens if you move the guitar ten or more feet away from the amp?

Ed

[pompeiisneaks]

There's no buzz when the amp is at full volume but no guitars are plugged in.  The buzz happens only at about 8-9 volume with the guitar plugged in.  (even with a humbucking guitar).  The buzz seems a bit worse on the normal channel than the bright, but it exists in both.  It originates in what appears to be the normal channel.  If I hook up a scope to the cap after the normal channel output I can see it, even with the volume at 0. 

~Phil

[shooter]

There's no buzz when the amp is at full volume but no guitars are plugged in.

That kinda sorta indicates it's coming in with the signal apposed to being created by internal gremlins.
try with a long chop-stick to "open" the tip-ground of the input jack, like would happen when you plug in.  Buzz there?

[sluckey]

There's no buzz when the amp is at full volume but no guitars are plugged in.

I would be reluctant to blame the amp.

Put the correct tube in it. Insure the input jacks, 1M, and 2x68Ks are wired correctly. But look closely at your instrument cable, guitars, and surroundings, ie, lighting or other noise generating appliances, etc.

[pompeiisneaks]

You're both right , it is something in the environment!  I push the jack with a chopstick at max volume and 0 buzz.  I've got the damn amp into a furman power supply, I thought it would reduce if not remove that!  Any suggestions on  how to clean up my power so I'm no longer chasing gremlins?

~Phil

[sluckey]

It's not coming in through the power. It's coming in through the air.

[pompeiisneaks]

So does that mean my guitar's aren't well shielded?  Or that my cables have bad shielding? Or is it just normal?

~Phil

[shooter]

So does that mean my guitar's aren't well shielded?  Or that my cables have bad shielding?

Any or all the above plus some

take a piece of copper wire, wrap it around a coffee cup, clip the Neg lead of your scope to one end, the tip to another, play with the amplitude n frequency knobs, you will probably "find" all sort of cool waves.  Wander around the house n "find" that quiet place

[PRR]

> wrap it around a coffee cup

A single-wound (not humbucker) pickup is the same thing already mounted in a handle (guitar) with a jack. Probably simpler in this world.

[pompeiisneaks]

OK cool, thanks everyone, I already knew about this phenomenon, I just didn't realize how noisy that part of my house was, I normally work in the garage, but it's not heated and a bit cold, so I guess the kitchen's a lot noisier :)  (The garage is down the basement, probably all that earth and concrete blocks a bunch of noise.

~Phil

[shooter]

Probably simpler in this world.

I can see myself back in the day, strat slung over the shoulder, "hey boss, check out this noise"

[NewYorker]

Kitchen?

All sorts of things can throw off noise there - dimmer switches, CFL's, fluorescent lights, etc.

Another thing regarding your guitars - make sure everything is grounded properly, including the ground wire coming from the bridge.

Ed

[pompeiisneaks]

Ed,

thanks, yeah I'm planning on doing some checks on my guitars.  one of them is a fairly new gibson, and I'm shocked at how noisy it is.  A new design by them that I do really like but I'm amazed at the buzz it puts off.  (it's the new N-225, I dig the tone, its got a P90 neck and a coil tapped humbucker at the bridge, and is pretty cool looking, but man this seems like poor work by gibson: http://www.gibson.com/Products/Electric-Guitars/Designer/Gibson-USA/N-225.aspx)

The kitchen just wasn't 30degF like the garage, so I opted for there :)

~Phil

[HotBluePlates]

Kitchen?

All sorts of things can throw off noise there - dimmer switches, CFL's, fluorescent lights, etc. ...

Fridge compressor motor...

[NewYorker]

One more time, then I'll let it go ... 

Keep in mind that the P90 is single-coil, of course, and that the bridge pickup is effectively single-coil when coil-tapped.

Ed

[pompeiisneaks]

Yup understood, but I've got another guitar with two P90's (Epiphone ES-339) and it sounds way less noisy that this thing... sorry tangent from the original, but yeah.  anyway :)  I know that, I was using that guitar because Ihad both humbucker and single coil on the same guitar so I could tell the difference.  (Even in humbucker non coil tapped it was noisy,) I ended up testing the humbucker setup with my SG.  Dual Humbuckers, no tap.

~Phil