Tweed 5E3: 50 Hz buzz out of the output transformer (magnetic coupling with PT)

[Lambertini]

Hi there!

I recently built a 5E3 from a TAD kit. It sounds as it should except one thing: a nasty 50hz (and harmonics) buzz is getting out of the output transformer.

I did a bunch of tests to locate the problem (heaters 6.3VAC, rectifier 5VAC, tubes, lead dress, grounding...) with not success.

In the last test, I removed all tubes, unsoldered all primary wires from the OT, so it is completely isolated from the rest of the circuit, just connected to the output jack. Powered the amp on, and the hum was still there.

Then I unsoldered the OT secondary signal wire and the hum was gone. I concluded the OT was humming by himself, independantly of any signal, even when not electrified.
Suspecting an electromagnetic coupling with the PT, I've tried to move the OT away from it. The hum varies with the distance, but never really stop, as far as the wires let me move the OT away.

Photos and specs of the OT.
Photos and specs of the PT.

Could that be a bad arrangement of the transformers on the chassis?
Could that be caused by bad transformers, ie PT is emitting too much EMF or OT is not enough shielded?

Is there any other thing I can try?

[Williamblake]

So your heaters have a DC connection to the circuit? Have you checked it?

[Lambertini]

So your heaters have a DC connection to the circuit?

Thank you for taking the time to answer. Unfortunately I don't understand what you mean here.

Anyway, heaters are not involved, the buzz is there even when they're disconnected.

[sluckey]

Are you hearing the hum through the speaker or is the OT producing the hum sound even with speaker disconnected?

[Lambertini]

Are you hearing the hum through the speaker or is the OT producing the hum sound even with speaker disconnected?

Hi sluckey, no hum when speaker is disconnected. The current / audio signal is generated inside the OT and gets out by the yellow secondary wire.

[acheld]

Looking at the specs and wiring diagram for the PT, the 6.3VAC winding does have a center tap (green/yellow) which should be connected to ground.   

It's a bit confusing, because the secondary 325-0-325 winding also has it's center tap colored green/yellow.

In your photo, it's hard to make out -- but it looks like one of the two green/yellow wires is taped off and not connected to ground.   If true, that would be my bet as the culprit.

[Lambertini]

Thanks acheld. The HT center tap (first yellow/green wire) is grounded to the chassis, and the heaters CT (the second one) is "elevated grounded" at the cathode of the 6V6 (around +18V) to limit hum. I've connected it to 0V real ground, does not change anything.

As said earlier, even with heaters disconnected, all OT primary disconnected, tubes removed - ie nothing is powered at all except the PT - the OT generates a current "by itself" that goes through the secondary out to the speaker. I suspect the PT EMF inducts some current in the OT windings and I don't know how to fix that :/

[shooter]

As said earlier, even with heaters diconnected all OT primary disconnected, tubes removed - ie nothing is powered at all except the PT - the OT generates a current "by itself" that goes through the secondary of the OT to the speaker.

can you "repeat" that test, except SHORT the primary side together, that should kill any strays on the secondary.


are you using the speaker jack as a "ground post"?  what's the Resistor n switch used for?

[Lambertini]

what's the Resistor n switch used for?

NFB, V1 cathode cap bypass or standard 5E3, I've disconnected it during test. Nothing to do with the hum.

can you "repeat" that test, except SHORT the primary side together, that should kill any strays on the secondary.

I don't see any rationale behind shorting disconnected wires.

[shooter]

the OT is a step-down transformer, shorting the "HV" side stops any inductive coupling through "open wires" acting as antenna.
if you kill the hum, the 50hz is most likely sneaking in the front door n coupling out the speaker

[Lambertini]

At the end of the day, the primary wires have to be connected. And when they are the hum is there, as well as when they're not.
I don't believe in your antenna theory. I still don't see any value in redoing this test except messing up the solder joint, the lugs and the wires a bit more.
What would be the next step if shorting them would do anything?

[Ronquest]

You're going to have to move your OT transformer to a better spot.  The quietest spot will be as far from each other as possible.  Next is have them in-line and clock the OT till it's the quietest, like dialing in a station or no station in your case.  I will not mount a transformer till I've found the quietest position, within reason.  If I have a coupling transformer set, it will drive me nuts.

I will add, some transformers are just bad.

[JB]

I had similar hum coupling issue with my 5E3, despite following the standard Fender layout. I tried two different power transformers but only had the one Mercury Magnetics output transformer to play with. I ended up angling the OT and also putting a mu-metal plate in between them which helped a lot. In a quiet room it’s still audible but not an issue when used in anger at gigs or rehearsals

[tdvt]

I have used the "headphone trick" any number of times to find the quietest position for the transformers.

In one instance I used it, I was rebuilding someone's unfinished project & couldn't eliminate hum until I remounted the PT 90°. I probably wouldn't have found the issue otherwise.


There are many descriptions of the procedure, this one is as good as any. Post #4

https://el34world.com/Forum/index.php?topic=22613.0

[sluckey]

This may or may not help but is easy to do and worth the effort...

Remove the four PT mounting nuts from the inside of the chassis. Now lift the PT up enough to gain access to the four nuts that hold the PT together. Tighten those four nuts then bolt the PT back onto the chassis.

[Lambertini]

I've tighten the PT nuts and set the OT at an angle, it's a bit better. Thank you.

[Lambertini]

DELETED

[Lambertini]

It bugged me so much I decided to completely rewire the OT. I did both the headphone trick and the AC measurement at speaker.
Extending the cut-too-short wires and moving the OT almost at the opposite side of the chassis until I bearly hear the hum. Reuse one of the eyeletboard screw for one side of the transformer and drill a new hole for the other.

And the buzz is gone!

That's great, thank you all.

[tdvt]

Now that I can see your PT from the top, I see that the copper foil shielding is facing the sides of the chassis & not down the length of the chassis (& towards the the OT)

This was the exact issue I encountered & mentioned above on a rebuild for a friend.

He had the PT mounted in the same orientation as yours & I could not get rid of all the hum.


By turning the PT so that the copper foil shielding was facing the OT, I was then able to mount the two transformers close together in a typical fashion with almost no hum.

I have not really seen the point of foil orientation made anywhere, but seems like it should be included in basic layout info.

[Lambertini]

I have not really seen the point of foil orientation made anywhere, but seems like it should be included in basic layout info.

I didn't think about that at all. Make sense. However neither it's documented on the kit schematics, rather the opposite as I respected the documented orientation of the wires ; nor the PT can be screwed to the chassis the other way using the pre-drilled holes.

[Lambertini]

I see that the copper foil shielding is facing the sides of the chassis & not down the length of the chassis (& towards the the OT)

Does that also mean I can simply add a copper foil sticker to the side of the OT to obliterate any kind of hum?

[shooter]

obliterate

no


build a copper box, sealed seams, that covers the OT, well grounded, maybe
steel works also
if it was coupled PT - OT, because of windings, the above works, if it was coupled through the leads that were all moved, shielding the windings won't help.
building xSE amps in small boxes, the best solution I've found for coil coupling is take one out of the plane, so mounted under, back.
lead dress, length, proximity between noisy and sensitive signals, should all be though out before you commit to the build.

[sluckey]

This is how I oriented the PT in my 5E3. No weird hum issues. Just saying...

[tdvt]

This is how I oriented the PT in my 5E3. No weird hum issues. Just saying...

Assuming the foil is a band, yours is oriented the length of the chassis, Lambertini's is across the chassis. I don't know enough about how they are made to say, but they look oriented 90° different from each other to me.

Sounds like Lambetini's was installed as intended, given the chassis cut-out & PT width/length.

It might be possible to unbolt/spin it in place to test, but might not be worth the trouble if you have the issue solved.   

[Lambertini]

It might be possible to unbolt/spin it in place to test, but might not be worth the trouble if you have the issue solved.

The amp already looks less nice than it should due to the wires extension and the weird OT angle. Messing around with the PT, where wires are cut short calling for extensions, and mounting holes would not be aligned requiring to drill new ones, would make the amp even ugglier. I'll leave it as is as the hum is gone, regretting not having known all this from the beginning.

This thread were a great learning to me, PT orientation and OT magnetic coupling tuning are major things that I'll take care before cutting transformers wires and mounting the board in my next build.

I'll get in touch with TAD to report the issue and suggest them to update the design (there's plenty of room to rotate the PT and cut the chassis accordingly) and the documentation.

[tdvt]

I am no expert & I can only offer my own experience & observations, so take that for what it is worth. But I can relate to the frustration of trying to solve that issue & it was definitively resolved by spinning the PT.


It was a bit of dumb luck that I tried re-orienting the PT on that rebuild in the first place, but I had noticed the PT orientation one of the Fenders I am slowly rebuilding & when I checked more of them it firmed up my theory.

Pics below; 61 Showman, 66 Bassman, 79 Pro Reverb, all showing the shield orientation.



 

[pdf64]

I don't think that copper screening will affect the magnetic field that's being radiated from the mains transformer.
Rather a ferromagnetic material is needed to do that.

I think the main purpose of the copper band around the windings is to shield electric fields, eg noise on the mains, to prevent it contaminating stuff in the chassis.

My understanding is that with an EI style transformer, the strongest magnetic field radiation will be from either end of the E section's centre bar. So it's best for the layout to have that at 90° to the OT, so that the magnetic field projects out from the front and back, or up and down, in relation to the chassis. Not along the length of the chassis.
The centre bar is usually the transformer stack's longest dimension, so with laydown mains transformer chassis cut outs, the longer side of the cut out is at 90° to the chassis length.

The coil winding go around thd centre bar, and then the copper band, if fitted, goes around the windings.

So the copper band should be in line with the chassis length.

That TAD mains transformer looks like the centre bar (and hence coil windings and copper band) may be in line with the shorter side. So the preferred mounting might be with its longer side in line with the chassis.
But the copper band will still be at the side, in line with the chassis length.

[tdvt]

So the copper band should be in line with the chassis length.

This is exactly my take-away, as well.

I am about to assemble a Princeton-ish amp & the transformer set are both the stand-up type & don't have any foil at all. But the fields will be oriented yet another direction, so...


No idea how that will unfold, but I will definitely be checking the transformer interaction before drilling any holes (blank chassis)