50Hz hum

[txstrat]

Hello fellow solder fume breathers,

in my latest build I have a 50Hz hum which I can assign to the power amp.
The test run of the amp turned out fine - soundwise. The hum is annoying. I'm wondering why this one hums because the build before turned out to be the most quiet amp I've built so far and the grounding is the same almost to the spot. Only difference is the size of the chassis which is rather small in this build (350mm = 13.7 inches wide)
This amp is a clone of a mix in between a BF Princeton and a ChampII - with a 1 tube reverb (thanks to tubenit). 3 x 12AX7, 2 x 6V6GT which run together at 60mA - over one 1ohms resistor (see pic).
Preamp is standard Fender BF, PI is split load.
Pulling preamp and reverb tubes don't stop the hum.
Turning the amp off with the power switch - while stand by in play mode stops the hum immediately.
Disconnecting the heater winding with the amp running - so the tubes run with no heater current (as long as they are warm) doesn't stop the hum.
I've pulled everything out of it again and only have the power stage - including PI - running (input grid grounded) - still hum.
PT is x-mounted.
Have tried to move the OT around to no avail.
Have turned the PT as far as the wires would allow to no avail.
Attached a sketch of my layout. The yellow lines on the filter caps are grounding wires. The gray bar on the pots is a solid wire leading to the input jacks.
The pics are from my mobile phone with the preamp still wired. Sorry for the poor quality.

Any ideas?

Thanks

Matt

[Enzo]

So pull the PI tube and see if the hum remains.

If it is 50Hz instead of 100Hz, then it is not power supply ripple, EXCEPT for the bias supply. Most Fender amps had a half wave rectified bias supply. FUll wave ripple is twice mains frequency, but half wave ripple is at mains freq. SO ther is that possibility, but also any grounding issues would be 50Hz.

The thing to do is find out where the hum is coming from. Forget thinking about how the other one doesn;t hum. That won't help.

Did you try differnt power tubes? And a different PI tube?

[txstrat]

Oh, I forgot, when I pull the PI tube the hum disappears.

I have a half wave rectifier for the bias supply.
And yes, I did try different power tubes and PI tube.

Thanks for your thoughts.

[Robert M. Martinelli]

Hi Matt, Hi Enzo!

According to what I see, I would start rewiring/rerouting the heaters' wires, they should be twisted as tightly as possible, run as close as possible to the chassis' corner and then reach the sockets. The way they're wired now could lead to unwanted capacitive coupling.

Hope this helps

Best regards

Bob

[txstrat]

Hi Matt, Hi Enzo!
The way they're wired now could lead to unwanted capacitive coupling.

Bob, do you mean this could lead to hum even when the heaters secondary is disconnected?
I used a clip lead for the heaters secondary and disconnected it while the amp was running. No change in hum.

Thank you for contributing

[Robert M. Martinelli]

Hi Matt...
sorry, I missed that line in your opening post....my apologies!

I still think you should try to rewire the heaters' wires, though, as the hum seems to radiate from the PT, and the heaters are by construction very close to cathodes, so those wires could act as a "service door" for the hum to get into the valves.

JM2CW

Best regards

Bob

[txstrat]

Please have a look at the pic with the different wiring of the bias ground. Do you guys think this could be the culprit?
In fact this is the difference in wiring between my two builds.

[txstrat]

No, that wasn't it.
Actually I thought so before, but you know how it is, sometimes you clutch at straws...
I think in the next step I'm gonna disconnect all the heater wires except for the PI and power tubes AND will rerouting them. Gimme some time.

[pdf64]

How about the geofex idea of running the heaters from a battery as a test, to absolutely rule them out?
Plus, what makes you certain that the hum is 50Hz, not 100Hz?

[txstrat]

How about the geofex idea of running the heaters from a battery as a test, to absolutely rule them out?
Plus, what makes you certain that the hum is 50Hz, not 100Hz?

Can I run the heaters simply by tying them to a 9v block battery?

I believe the hum is 50Hz because it's kinda low (frequency wise) I think 100Hz is more like a buzz. Besides where would the buzz come from? I have grounded the PI input grid with a wire. When I disconnect the grounding wire from the grid I get a bad buzz - which I consider to be 100Hz.

[txstrat]

Well, running the heaters on a 9v block battery didn't change the amount of hum.

Let me summarize. I have a secondary winding of 275 vac which is filtered by 22uF after the rectifier. Followed by a 1K/2w resistor for the grids, which is also filtered by 22uF (btw. I had clipped another 47uF parallel top the first in case there was too less of filtering, to no avail). Followed by a third cap of 22uF which feeds the PI and the preamp stage before.
The caps negative sides run to a PT bolt on a buss wire. The cathodes of the PI and preamp stage are tied to a buss wire (on the back sie of the pots) running to the input jacks.
Speacker jacks and input jacks are not insulated. Tried the amp with the speaker jacks floating over the chassis to no avail.
I think about using insulated input jacks and taking the buss wire off the pots.

[pdf64]

6V lantern battery, or motorcycle battery.
But use a scope or frequency meter to confirm the 100 or 50Hz thing first, it's key to the subsequent path.

[pdf64]

The caps negative sides run to a PT bolt on a buss wire

Reservior and screen grid node caps are best with seperate ground return to the PI etc.
The grounding arrangement of reservoir cap and rectifier is the main thing.

[txstrat]

Reservior and screen grid node caps are best with seperate ground return to the PI etc.
The grounding arrangement of reservoir cap and rectifier is the main thing.

Rectifier ground and reservoir cap negative are tied together - see pic 1 in my initial post. I've tried both to connect the second caps ground to them or to the buss wire on the back of the pots. No change in hum. I even tied the PI ground to the wire of the reservoir cap although I was assuming this would lead to the worse. Nothing changed.

edit:
I could try to build a star ground with separate wires from every cap and recto to see if it helps.

[Enzo]

The key observation is in post #3. He reports that pulling the PI tube kills the hum. That takes suspicion off the bias supply and the power tubes, in my view.

My first reaction then is looking at WHERE the PI stage is grounded.

50 and 100Hz are the same note, just 100 is an octave higher. Both are bassy, but 50 will be deeper.