60hz hum 1st stage FIXED IT with elevated heaters

[ElectronAvalanche]

Hi gang,

I ripped apart my 2nd HRM 100W ODS and brought it to #183 specs. Very nice sound I must say

Then I swapped all my shielded cable to RG178 coax and the sound improved still more

BUT

I do not know why, but I get 60hz hum in the first stage.

Grounding the Volume pot on clean gets rid of the hum, same with the 220k/500pf combo. Now grounding the Bass decoupling cap reduces the hum as well, same thing when turning down the Bass all the way.

Grounding the 22k grid resistor does not change a thing.

Before I had the heaters elevated to 60V, I changed that to a hum balance all AC setup, using a 200 Ohm pot and two 100 ohm resistors. Heater supply - 100 Ohm - lug of hum balance pot, wiper to ground (same on the other Heater supply wire.

I can dial out most 120Hz heater wire using this pot (especially in the OD channel), but I do get some 60Hz hum, I can even make the hum worse by unbalancing the heaters more.

I am reluctant to go back to the elevated heaters since in my other ODS amps I do not have this problem using AC heaters. The PT is an old Twin PT salvaged from a PA135 head. It works flawless.

Before changing the shielded cable to coax I had the shield of the input grid shielded wire attached to a chassis ground close to V1a. Now I have it on the Volume pot (putting it on the bus ground did not help unfortunately). I need to check this.

Any ideas? I am chasing my tail here.

The amp sounds fantastic, but this hum (most notably at settings of the MV higher than 4) drives me crazy.

Thanks in advance for any help!

Electron

[Structo]

Using the 200R hum balance pot is fine but I don't understand why you would have two 100R resistors feeding that.?

The 200R pot should replace the 100R resistors.
Just take a line from each half of the heater supply and connect those to the outside lugs of the pot and take the wiper to ground.
Then rotate the pot for least amount of hum.

Fender did that a lot.

As far as the grounding of shielded cable goes, I always ground the shield at the source of the signal.
So for the V1a grid, I would ground it to the ground buss that the input jack is grounded to.

For any of the other shielded cables, ground the shield where the signal originates.

Of course you only ground the shield at one end not both, or else you get a ground loop.

Make sure all low level signal carrying wires are away from AC wires, like the heater wires.
Do you have your heaters ran overhead?

If you follow the grounding scheme of #124 exactly, you shouldn't have any hum.
ground the power tube filters at the point by the power transformer and the PI at point #2.
The preamp cathodes and buss get anchored by the input jack. (point #1)

On my amps, the only time I get any hum is on the OD channel and if I'm standing too close to the amp.

In clean mode, I can't even tell the amp is on until I strike a note.


So review the ground points shown on the #124 layout and you should be good.

I'm sure you have tried different preamp tubes right?
Some tubes will hum like a mutha.

You probably have this schematic but it shows the ground points real well.

[Chris333]

Could be wrong, but I always thought 60hz pretty much has to be heaters or pre-rectifier. I'd try going back to the elevated heaters just to see if it helps. I have an old Ampeg V2 with 60hz hum, and I've tried everything to fix it. It's driving me up a f**king wall. I feel your pain!

[ElectronAvalanche]

Hi Tom and Chris,

thanks for the input.

Of course the shielded wire is ground only on one side.

I use 2 100Ohms since with the hum balance pot alone the hum was louder than with using the two 100 Ohms inline. Don't know why though.

I tried the #124 grounding scheme but in this particular amp it did not get me hum reduction.

Btw: I of course have a dedicated relay supply fed by a 12V transformer.

It must be in the first stage. Or to be precise in the input stage. I will recheck the grounding scheme again.

Oh and btw, yes different tubes yielded different amounts of hum, but I think that has more to do with the fact that the tubes have different gains. Thus a "hotter" tube will sound like it has more inherent hum, when it acutally just amplifies the hum more.

The struggle continues.....


Thanks again,

Electron

[talbany]

Here is a cool test:
Does the hum get loudest part-way up on the treble control? In other words if the treble control is maxed, the hum is slightly less than it is with the control maybe 3/4 up?

In any case, connect a clip wire to ground, set the treble control at mid point, and now clip the groudn wire to the top of the pot - . If the hum stops, it is coming from treble cap or stack or something before it. If the hum remains, it is coming from the pot itself or something else connected to the circuit.

Now, ground the wiper of teh treble pot. If that kills the hum, then indeed it is coming in from the pot or the following grid.

Tony

[heisthl]

make sure the t,m,b wiring from the main board to the controls is not the culprit. Easy to pick up relay hum there. Also make sure there is no signal wires running near the input jack.

[ElectronAvalanche]

Hi Tony,

Here is a cool test:
Does the hum get loudest part-way up on the treble control? In other words if the treble control is maxed, the hum is slightly less than it is with the control maybe 3/4 up?

In any case, connect a clip wire to ground, set the treble control at mid point, and now clip the groudn wire to the top of the pot - . If the hum stops, it is coming from treble cap or stack or something before it. If the hum remains, it is coming from the pot itself or something else connected to the circuit.

Now, ground the wiper of teh treble pot. If that kills the hum, then indeed it is coming in from the pot or the following grid.

Tony

The hum is somehow influenced by the setting of the Treble pot. Albeit grounding the Treble wiper kills the hum (input to the Volume pot).

The Mid Boost has an influence on the hum, making it louder in the boosted setting. BUT: with the Mid boost off, the hum gets a bit louder when turning the Treble pot all the way up, with the Mid boost on turning the Treble pot all the way down results in the most hum.

I changed the Treble cap and the Bass cap, no change in hum.

Grounding the Bass decoupling cap leads to the biggest reduction of hum btw.

By subsequent grounding the stages beginning from the PI input I think I can isolate the problem to the very first stage or the TMB network. As said grounding the 220k/500pF combo on V1b nulls out the hum, same when grounding the Volume wiper or the Treble wiper.

There are no signal wires next to the inputs. The relais are positioned as in every of my amps, thus this is not the problem.

One other idea might be that it is indeed the PT.

The PT in itself has some mechanical 60hz noise, more so than other PTs. If there is an isolation problem between the B+ windings and the heater windings this could be a problem? I think Andy LeBlanc has pinpointed to this in a different thread concerning hum somewhere else on this forum.

Maybe I should go back to the 40-60V floating heaters?

I would hate to buy a new PT.

Thanks again for all the help.

I will report back with my findings concerning the floating heaters. Grateful for any other ideas!

Cheers,

Electron

[markusw]

Stupid question: have you tried a different tube in V1?

Peace,

Markus

[markusw]

Stupid question: have you tried a different tube in V1?

Peace,

Markus

Edit: didn't see you mentioned that you tested different tubes. Sorry

[ElectronAvalanche]

Hi gang,

first off, thanks for all the help!

I fixed it. It was indeed needed to float the heaters at 40V above ground.

Btw when I derived the DC from the B+ that serves as the supply for the inbuilt Loop I got some crosstalk of the OD channel.

Moving to the PI B+ got rid of this problem as well.

I am happy as of now, but still wonder if the PT is indeed not in top condition. It gets a bit hotter than my other PTs.

Anyways thanks again!

Lesson learned:

60Hz hum in the first stage might come from the heaters. Using elevated heaters might help here. Make sure to derive the B+ for the elevated heaters before the Loop supply or subsequent stages.

HAPPY EASTERN!

Electron

[heisthl]

Make sure the bias tap and center tap leads on the PT were not mismarked and reversed - IF so it will result in a warmer than normal transformer and extra hum - the amp will still work though.
I've also seen a case of forgetting to tie the rectifier diode strings together - the amp worked but had too much hum (amazingly the voltages were only off by 40 volts???).