5e3 build - PT inducing hum in OT/ speaker even with no valves connected

[CirrusBand]

Just finished my first amp, a 5e3 that basically followed the classic circuit other than 47nf coupling caps off V1, and 3x 22uf filter caps.

It's all working great and I loved the whole process from start to end, the only fly in the ointment is that the power transformer is coupling with the Output transformer - so even with no valves installed, the instant the amp is turned on, 50hz (I'm in the UK) hum starts coming out the speaker. It's not loud but it's noticeable, and I'd like to fix it, presumably by re-orienting the OT. The only thing I'm confused about is that the OT is already 90 degrees out of alignment with the PT in terms of both the iron stack and the bobbin wind, so I wanted to make sure moving the OT is worth trying, or if there are any other areas (could a grounding issue do this?) that are worth looking at first.

This pic is from halfway through the build but shows the spacing and orientation of the transformers;

 

[D'tar]

Can ya flip 'er over and lets have a peek inside?

 

[CirrusBand]

Sure can!

 

[schmee]

Get any preamp grounds away from the power Transformer end, keep them tight together at the input jacks.
I would get that buss wire I see away from the filter cap and run the stuff on that PT end to the PT stud or thereabouts.

 

[CirrusBand]

Just to add, the pot in the 2nd speaker jack hole is the "Sweet mod" as per robrobinette's page - a pot in the place of a P.I. grid stopper, and the 2nd Fuse holder where the death cap ground switch would be is a H.T. Fuse.

 

[CirrusBand]

Get any preamp grounds away from the power Transformer end, keep them tight together at the input jacks.
I would get that buss wire I see away from the filter cap and run the stuff on that PT end to the PT stud or thereabouts.

Which one you talking about in particular? The short one on the PT end goes to a lug on a PT bolt, the long one grounds to chassis through one of the Jacks.

 

[schmee]

Which one you talking about in particular? The short one on the PT end goes to a lug on a PT bolt, the long one grounds to chassis through one of the Jacks.

The long one on the board. I tried that same thing on a 5E3 build and it hummed bad. I just ran all the pre end grounds to a bolt near the input jacks. (grounded to the jack with a wire also for insurance) Run that last filter separate to the PT end. It was tons quieter after I did that. It's strange because everything goes to the chassis eventually right?, but it does help.

 

[D'tar]

nice build there. Have you tried chicken head knobs? You cant have a 5e3 without chicken head knobs. The rock n roll gods are likely upset!

Joking aside.

If you are humming without any tubes installed (rectifier?) Have you tried chopsticking the wires around the power section.

 

[CirrusBand]

Yeah, I've had a poke around with a chopstick. I think the circuit inside the chassis is fine, tbh. In actual use, with the volumes turned down if it wasn't for the induced transformer hum you wouldn't know it was powered up, and as the volumes go up the frying pan hiss from the first gain stage is at a level that I consider normal - my AC30s have a similar signal to noise ratio there.

I've just removed all valves (including rectifier), unscrewed the OT and moved it round a little with the primary and secondary wires still connected & speaker plugged in.

The hum disappears almost entirely when the OT is oriented like this;

Which feels a bit counter intuitive since the laminations are now oriented in the same plane, but I can't argue with the results - the induced hum has vanished.

So, the next mission is to work out an elegant way to hold the OT there.

 

[D'tar]

Do the laminations have holes in The corners to mount same as the pt? Long screws and some standoffs? Or make a bracket or box to mount it to? Sukks

??? Try rotating the PT???

 

[dogmeat]

the negative terminal on the speaker needs to also connect to the chassis. that usually happens if there is a jack in the chassis and a cord to the speaker

 

[Lowerleftcoast]

Distance between the transformers will also quell the hum. Will the hum be acceptable with the OT further away and still using the original mounting bracket?

 

[2L man]

Yeah, I've had a poke around with a chopstick. I think the circuit inside the chassis is fine, tbh. In actual use, with the volumes turned down if it wasn't for the induced transformer hum you wouldn't know it was powered up, and as the volumes go up the frying pan hiss from the first gain stage is at a level that I consider normal - my AC30s have a similar signal to noise ratio there.

I've just removed all valves (including rectifier), unscrewed the OT and moved it round a little with the primary and secondary wires still connected & speaker plugged in.

The hum disappears almost entirely when the OT is oriented like this;

View attachment 1055832


Which feels a bit counter intuitive since the laminations are now oriented in the same plane, but I can't argue with the results - the induced hum has vanished.

So, the next mission is to work out an elegant way to hold the OT there.

If you want that installation look very neat there possibly is large enough cap between coil and iron to push wires to the other side. Then install an end bell to top side. You can carefully pry that mounting band away Then you can use long screws or thread bars thru corner holes which come open when band is removed. Swcrews should be isolated from the iron. Masking tape fitting is fine. Also end bell should be isolated from the core and screws and washers made of cardboard are often used. You can put enough nuts to botton side against the chassis. Or buy a "riser bed" which seem to be made of copper.

 

[CirrusBand]

I still don't totally understand what's caused it in mine, tbh. I mean, whatever the specifics of the individual transformers, my layout's the same as 90% of 5e3 clones, right? So why do most not have this issue, despite having the same transformer orientations as me? What would make two transformers couple *less* when the iron laminations are oriented the same, like I just found?

My main thought is that the PT I have is oversize - it's taller/ heavier than a standard 5e3 PT, and is designed to run 6L6s too with better current regulation on the HT and a higher max current on the heaters. But if that was the only variable, making it smaller would surely just reduce the interference a little and still leave the orientation related effect the same? Baffling. I wonder if anyone else has used a bigger Power transformer and had this issue?

I'm almost inclined to try a different PT rather than start jury rigging/ hacking up brackets on top of the chassis, but it'd be a shame because the voltages are bang on.

 

[Powdog]

Remount the OT with the bolt only on the tube side of the chassis, leave the bolt above the pilot light off. Then move the OT away from the PT like this. At some point the hum will go away. Sometimes just a few degrees will do the job. If things line up like they should you should be able to use that other bolt at 45 degrees.

Lowery organ amps did this with some of their power amps.

 

[dan40]

My main thought is that the PT I have is oversize - it's taller/ heavier than a standard 5e3 PT, and is designed to run 6V6s too with better current regulation on the HT and a higher max current on the heaters.

I was gonna ask why the PT was so large. I suppose it's possible that the extra mass of the PT is creating a much stronger magnetic field that is coupling with the OT. Mounting the OT at an angle like Powdog showed above has worked well for me before. Moving the OT further away from the PT has also works well.

 

[CirrusBand]

Remount the OT with the bolt only on the tube side of the chassis, leave the bolt above the pilot light off. Then move the OT away from the PT like this. At some point the hum will go away. Sometimes just a few degrees will do the job. If things line up like they should you should be able to use that other bolt at 45 degrees.

Lowery organ amps did this with some of their power amps.

I did give that a go while I was moving the OT about - the hum only faded a little when I rotated it in that direction. I'm just so confused what would make the OT have less interference when the laminations are aligned with the PT - there's so much I don't understand about it!

 

[Powdog]

I did give that a go while I was moving the OT about - the hum only faded a little when I rotated it in that direction. I'm just so confused what would make the OT have less interference when the laminations are aligned with the PT - there's so much I don't understand about it!

They’re in the same plane but at 90 degrees from each other.

That is a freaky large PT for a 5E3 Deluxe.

 

[chas.wahl]

Caveat mutator. My suspicion is also that the over-large PT, and/or the addition of a HT fuse (odd; why?) are the source of the problem. The 5E3 standard chassis does have the OT and PT mounted very close to each other. As noted previously; have you tried simply increasing the distance between them?

I believe that the optimum topology for mounting a lay-down PT (not observed in many Fender amps, for the obvious reason that it puts the PT closer to other stuff) is to have the short side of the stack facing the other transformers, not the long side.

The other possibility is a significant ground loop, but your reduction in hum when moving the OT probably rules that out.

If you prefer to proceed with the OT in "lay-down" position, then a couple simple 90-degree bent angles (or a longer single angle) could easily solve the mounting issue. I'd make it so there's a bit of clearance between the OT windings and the chassis surface.

 

[CirrusBand]

Caveat mutator. My suspicion is also that the over-large PT, and/or the addition of a HT fuse (odd; why?) are the source of the problem. The 5E3 standard chassis does have the OT and PT mounted very close to each other. As noted previously; have you tried simply increasing the distance between them?

Re; HT fuse, I had no particular plans for the unused ground switch hole and figured an extra fuse wouldn't hurt; as you can see in the pics it doesn't really lengthen or change the path of the HT centre tap wire much, and people put Standby switches in exactly that place on their builds without causing problems.

A couple of times over the years I've been using valve amps, the HT fuse has blown as a power valve spectacularly died under load so it felt like it wouldn't be a bad choice. It's also a way to be able to run the amp without B+ for troubleshooting etc.


Yes, tried increasing the distance without changing orientation. The noise drops off slowly, I think I'd need to mount eh OT over on the input end to minimise the noise that way.



At any rate... used it at practice last night and after the first note I didn't hear the annoying hum again, it was enough orders of magnitude quieter compared to the noise of the band that it just didn't matter.

I've got a couple more things to try and the maker of the PT has very kindly offered to make me another to try.