HUMMMMMM

[trobbins]

If you can google a few grounding references you find that a key issue is to take the rectifier negative lead to the first filter cap negative terminal directly. The rectifier neg in your case is from the bridge diodes, whereas often it is shown from the secondary CT.

How do you determine hum level, and what contribution is 60Hz, and what is 120Hz?

[Malcolm Irving]

... . I have yet to add the other primary in parallel. Does phase matter when doing so? Or should both black primaries and both red primaries be tied together? Also, why is this important to do? ... .

Yes, red and red tied together, and black and black together. It's important because otherwise the two primary windings are out-of-phase and produce equal and opposite magnetization of the core - net result no magnetization and no magnetizing inductance - the primaries then only have their very small resistance and the mains fuse blows.

[lowell]

Ok moved the 12ax7 grounds to the input jack. Killed most of the 120hz. I mostly get 60hz when the master volume is towards the upper end. The master volume is post-6v6. Remember this is a preamp so no OT. I connected the first filter and rectifier ground together and to chassis ground...no change in hum.

So I guess NOW, if I retry the DC filaments I may notice a difference. It's pretty quiet, except for when the master is up... regardless of the preamp volume level. 60hz means it's heater hum right? Since it's not rectified?

[lowell]

So RG says this: http://www.geofex.com/article_folder...nd/stargnd.htm

I forget WHERE I read, Kevin O'Connor's book/s? I thought each stage was supposed to be star grounded WITH it's respective filter cap, THEN that star grounded thru one wire to chassis. I guess I'll try it this way as I certainly trust RGs experience.

[lowell]

Here's a video of the amp. Nothing plugged in (input shorting jack to ground). It's running into a LM3886 kit, and into a EVM12.

Knobs are: Gain - Tone - Volume

NOTES:

Gain down and volume up = 60hz
Tone seems to affect the hum
12ax7 pulled - no hum
post-stage 1 coupling cap shorted to ground kills hum, however only if Gain is up all the way. (This is a 5e3 volume and tone stack) 5E3 SCHEMATIChttps://robrobinette.com/images/Guit...atic_Clean.gif
This preamp is the same as this 5E3 schematic, except the power amp is like a Champ, sans OT, and substitute the PI with a typical gain stage. I have added a 470k grid-stopper to stage 2. I'm wondering if this is the problem. Total grid leak is 1.47M. ??
Hum has to be in 12ax7 stage two. If the foremention coupling cap is shorted to ground, and the Gain is down, the hum remains.
60hz gets much louder when Volume is at 3 0'clock and higher
Quietest setting is Gain at 11 and Volume at 3 o'clock or lower ()
I uploaded this using the 8mm filter for fun... but it caused weird noises in the right speaker. Ignore the crackling and cutting out in the right speaker. Just listen to the hum specifically. Whoops.
Finally, pink nails, must be a Dad

https://youtu.be/Ln9-KoiAG0U

[Enzo]

If hum is least at non-zero control settings, that means to me you have more than one source of hum, and they are out of phase. Those settings balance the two sources so they cancel.

[lowell]

Ok so I was thinking the same thing. Makes sense, humbucking.

I tried the DC filaments, again, and NO change in the hum at all. I'm at a loss here. I'm convinced it's in stage 2 for sure.

[Enzo]

Isolate the problem. You are using this as a preamp. You MIGHT have a ground loop with the power amp in use. So do you have any sort of battery powered amp? Plug this preamp into that so there can't possibly be a ground loop. Any change?


You have some sort of resistor and cap off the power tube plate, instead of a OT? Have you tried clipping in an OT and eliminating those resistor and cap, just to see if it makes a different driving a speaker.

Do you have a signal tracer? If so, pull the power tube, and no connection to a power amp. Use the tracer to check for hum in the remaining triode circuits.



Does pulling the dual triode kill all hum, or just part, or no effect on hum?

[lowell]

Isolate the problem. You are using this as a preamp. You MIGHT have a ground loop with the power amp in use. So do you have any sort of battery powered amp? Plug this preamp into that so there can't possibly be a ground loop. Any change?

I have a ground lift on the preamp plug. Tried it the other way too, with the lift on the power amp. No change.

You have some sort of resistor and cap off the power tube plate, instead of a OT? Have you tried clipping in an OT and eliminating those resistor and cap, just to see if it makes a different driving a speaker.

I haven't...yet

Do you have a signal tracer? If so, pull the power tube, and no connection to a power amp. Use the tracer to check for hum in the remaining triode circuits.

Will do, although I've tried this alot with hum, and it's hard to hear the interstage subtleties. I'll try it though.

Does pulling the dual triode kill all hum, or just part, or no effect on hum?

It really kills 99% of the hum.

Also, had all the signal wires shielded except 2. Was lazy. Shielded those last 2 and it did help alot. Still humming at higher volumes, but definitely gigable. But want the next one to be dead quiet. This was #2. Guess it'll take a few builds to get it right.

So star grounding. Do all the stages get grouped, then everything run to a MAIN star ground (which is not grounded to chassis), and run that MAIN star ground to chassis with ONE wire? Is that right? I want to apply this to the next more thoroughly. I will add a ground switch that lifts that main star ground from chassis ground so as to eliminate the need for a ground lift plug. Ofcourse the chassis will always remain earthed at the IEC.

[lowell]

OK so i plugged this into my interface and recorded a bit. It's really not humming bad at all this way. No more than your typical amp, if not even less so. I tried scoping the hum too and couldn't see any hum.

So I'm convinced it is indeed a ground loop or some interaction between the preamp current and the power amp current is going on. The thing that's confusing me at this point, is that I tried a earth-lift adapter on my power cord and it didn't help at all.

This being the case. How do I isolate the output jack of the preamp from the power amp? And, is this different from using a earth lift adapter on my power cord? Do I simply leave the output jack isolated from the chassis and NOT connect anything to its ground lug?

[lowell]

Furthermore, with my strat plugged in, and in 2-pickup humbucking pos 2, the hum is LESS when the guitar volume is at 10, and the hum increases tenfold when the guitar volume is backed off a bit.

[Enzo]

Ok, but does turning the guitar volume to zero kill the hum?

[d95err]

Havet you tried different guitars? Could the hot and ground on the guitar output jack be swapped?

[lowell]

A friend has it at the moment and is gonna take it on some gigs so I can't see if my Les Paul has the same volume/hum issue.

I just modded a Valve Jr similarly to my preamp. It in fact had the same hum issues that my preamp does. I ended up following this in regards to grounding: http://www.valvewizard.co.uk/Grounding.pdf.

I also added 110uf in parallel to the stock 20uf, helped pretty completely rid of the 120hz hum. Ended with a pretty darn quiet preamp with that. There was still some 60hz hum in the background, but deal-withable. I couldn't see how there could be any 60hz with the DC filament supply. Maybe adding more filtering to the 4700uf woulda helped. I'm on to building #3 and will follow the valve wizard document to the T. Will report back.