Help troubleshoot hum in 100w build

[amplifiednation]

I figured before pulling the power supply board in this amp I would post a help thread.

This amp was functioning fine and was dead quiet and then was shipped...twice!

After it was shipped the second time, the plates on one of the 6L6 shorted to the heaters, blew out the ground reference resistors.

The amp was humming like a bastard!!

I ended up replacing the PT for other reasons, this one has a center tap, so I used that as a ground reference.

Still hummed!

I rewired the entire heater string, thinking there may have been more damage to it.

Still hums!

I just reflowed all of the ground solder joints (I actually did find a bad one).

The amp still hums

I'm trying to figure out where the hum is coming from.

• Higher pitched
  Hum volume changes with the master volume
  Goes away when the PI is pulled - but no other tube makes a difference

I tried to measure the frequency of the output with my Fluke. I got 180hz...does that even make sense?

[jelle]

Bummer... Please post pics.

[M Fowler]

Another source for hum is the bias circuit either bad cap, bad trimmer or ground lifted.

[alvarezh]

What brand/model amp? Does it have an EFX loop so you can split the amp in it's two sections?

It will be fixed, Jelle it's already here and others will show up!

Best wishes.

[amplifiednation]

why would only the PI tube make the hum disappear? i found that odd.

combined with the hum increase with the master volume?

If it was the bias supply wouldn't the amp continue to hum with the PI pulled?

I think it's 120 cycle hum, thats what I was trying to check with my Fluke by measuring the frequency of the speaker out. I have a scope now but don't have enough experience to use it!

i'll split the preamp into another amp and see what happens.

there's not a lot left from a visual perspective...

[Cliff Schecht]

Have you checked for loose hardware (speaker jack, chassis ground connections, etc)?

Also you can try to trace out the noise with a scope. If it's in the power supply it'll be all over the amp, if it's in just a particular stage you'll find it by working forwards from the first preamp stage.

[amplifiednation]

Have you checked for loose hardware (speaker jack, chassis ground connections, etc)?

Also you can try to trace out the noise with a scope. If it's in the power supply it'll be all over the amp, if it's in just a particular stage you'll find it by working forwards from the first preamp stage.

yeah i reflowed all the grounds and there was a loose ground from the preamp board, that got rid of some other ugly noises.

if i trace the noise with my scope do i need any kind of signal or can i just view the hum?

[Firestorm]

It's pretty hard for a tube to short plate to heater unless the welded leads at the base of the glass come loose. Have you looked really hard for carbon tracks on the sockets? Arcing between pins is easier for the culprit tube to do. PI socket, too.

[mhartman]

Quit using the PT's heater center tap. Use an artifiial CT (ie, two 100ohm resistors to ground from the heater wires) instead.

[amplifiednation]

It's pretty hard for a tube to short plate to heater unless the welded leads at the base of the glass come loose. Have you looked really hard for carbon tracks on the sockets? Arcing between pins is easier for the culprit tube to do. PI socket, too.

Yes there were carbon tracks between the plate and heater pins on one of the sockets.

I looked at the hum on the scope, when i scope the amps output it has the jagged shark teeth at the bottom.

I can't find any visible hum in the preamp, but when I looked at the coupling caps coming off the PI feeding the power section, both sides are jagged shark teeth out of phase with each other (obviously).

This is with no signal going into the amp.

[Firestorm]

Could the coupling caps from the PI have gotten pinholed?

[martin manning]

Sounds (looks) like it is getting in at the entrance or the tail of the PI- it is there on both phases, and it stops when you pull the PI tube. Since it's 120 Hz (use your scope to confirm that) it is coming from the HT supply.

[amplifiednation]

Sounds (looks) like it is getting in at the entrance or the tail of the PI- it is there on both phases, and it stops when you pull the PI tube. Since it's 120 Hz (use your scope to confirm that) it is coming from the HT supply.

I have limited experience with the scope, but it appears that 120 Hz hum is jagged waves, and 60 Hz is more of half a sine wave.

So that being said, believe this to be 120hz hum because of the jagged, uneven wave, and yes it is on both sides of my PI.

I can't find it on the grids or the cathodes of the PI, but it is definitely on the plates.

Could it be the PI power supply? Also the bias feed is there - the 120HZ is still considered AC signal right? So it could pass through the coupling caps?

Also, the hum remains visible on the output section when I pull the tube (not audible though)

[martin manning]

How much AC is on the B+ nodes?

[amplifiednation]

No AC on the power node, but there is noticeable AC on the bias node! So would it be safe to assume that the bias circuit is what is inducing hum into the amp? Maybe the bias cap isn't grounded properly or is bad?

This is with the PI tube pulled.

I'm very surprised how easy the scope is to use. other than hunting for signal this is the first time I've used it to troubleshoot.