Strange HUM.....

[don280z]

rooster:
Yes, 1/2" because of space requirements.
I removed the Heater center-tap-to-ground and installed 2 100ohm resistors (to ground) and it didn't have any impact on the hum.
I truly believe it is inductive coupling because when I installed a shield between the transformers the hum was reduced by about 60%.
Plus, while I was testing where to place the shield, you could hear the hum increase and decrease while I moved the shield around.

[rooster]

don280z - OK, still no pix. Are you sure you don't want to let us take a look at things?

[M Fowler]

It's the PT not the OT.

[T Wilcox]

Well I desoldered the OT, took it out and redrilled the holes so it rotated 90 degrees. Resoldered all the wires and powered up. The Hum on standby is completely gone. I cannot beleive how much of a difference that made. Guess I made the mistake in the first place because the PT is so beefy and wide I was not paying attention to the bell ends when I originally mounted it. Lesson learned
Thanks Rooster and thanks Don280Z as well for opening this thread.
Usually when someone says their amp is humming I say its because it doesn't know the lyrics, not in this case!

Todd

[rooster]

Todd - I'm glad you posted this last reply. Good on you for taking resposibility for the bad alignment. And that's the positive side of a mistake, sharing it with others so that they might benefit.

Take care.

[T Wilcox]

Well unfortunately I dont have anyone else to blame
I do hope someone else may learn from my mistake though!
Thanks again

Todd

[don280z]

Well, I re-drilled holes and moved my OT 2". I still have the same hum.
I think I need to bring my other TW home and listen to it. It may have the same problem and I just haven't noticed it.....???

[RJ Guitars]

Several of the trainwreck style builds I've messed with have this phenomenon to some degree, even with the orientation right. The majority have it minimally if at all but a few will really annoy you. I've seen it with Heyboer, Edcor and Pacific iron. IMHO - It's primarily because in the Wreck chassis the transformers are just so close the OT is just begging to pick up some signal from the PT. For some reason it's not entirely consistent from amp to amp... most of them work fine. The very best way I've found to sort this out is to headphone the OT as you move it around. If you get hum in the headphones your gonna have hum in the amp and something has to move. I've tried mu-metal shields and had no luck.

It's also worth reiterating what some folks have mentioned about a mechanical vibration in a power tranny. I've seen it twice in a Heyboer and once in a Pacific power tranny. This is a different phenomenon than the coupling issue mentioned above and I don't think there is any fix for this except to send it back.

rj

[rooster]

don280z - Well, I'm glad you moved the tranny, and I know that was hard. So now stop looking at the trannies as a source of your hum. Believe me, this is not your problem.

The only page left to turn is your layout. Post a cell phone pic if that's all you have, and don't be shy. We've seen pics from a Ukraine builder that used the top of a washing machine for his chassis. Really.

[don280z]

OK..OK...Here is a bunch of photos....

http://abe.midco.net/dons/Trainwreck/

[RJ Guitars]

... So now stop looking at the trannies as a source of your hum. Believe me, this is not your problem....

I'm not entirely sure I can buy that yet but see if my logic makes sense to you.

Is the hum in the speaker? If it stops humming when you unplug the speaker then it's not a mechanical vibration but rather a real AC signal coupled into your OT.

If the amp has a hum in the speaker immediately when you turn it on, and the HV (standby) switch is off you have eliminated the preamp, poweramp, and most of the power supply filtering as the source of the problem. If it hums when the tubes are cold (or better yet removed) then it's likely coupling between the two transformers.

If the hum shows up after the tubes are warm then it's down stream somewhere and probably not transformer coupling.

With no tubes in the amp the only hot component in the amplifier is the PT but keep an eye on the location of the heater wiring. Without tubes, their is no current flowing in the heater wiring but pay attention to the routing and proximity to the OT wiring just in case. If you want to be double sure then disconnect the heater wiring from the tube sockets and move the wires out of the way (with the ends of them insulated of course).

Looking at your current OT location it may be the same or even could be worse than the previous one but you won't know unless you actually move the tranny around a bit and listen for the hum to change. Using headphones makes this really obvious.

I have one great sounding Wreck that has a very little bit of this but it's literally in the noise unless you are in a dead quiet room. I have chosen to go with this as a gigging amp and leave it alone rather than mess with the great tone.

Hope this helps,

rj

[rooster]

rj - Sure, and I think in a similar way believe me.

Don does have his trannies directions reversed here and offset however, and this is worth some thought. Not that I am a purist by any means but I do follow the typical layout of the trannies and have not run into any problems. Hm. What do you think about this?

I have built Marshall style amps, a few of them, and like an early Marshall with the PT layed down, I place the PT right next to the OT, and then follow with the choke on the outside. The first time I did this I thought it would be a problem but it wasn't. Those amps are so quiet you can't tell if they are actually on, and I am serious here. Which is to say at that point I stopped thinking about all the hoops people were jumping through on TAG when they tried to solve the Express hum issue. True, the Express trannies are 'standup', but when you visualize the coils, the PT has it's coil lying face down so it's just like the early Marshalls.

So don280z, you did good here getting these pics up. I do see why you offset the trannies, it was that iec connecter. We all deal with that when we opt to use them, but - with some effort - most of us get them an inch or so away from the outside edge of the chassis and still manage to bolt the OT in a straight line from the PT. I mean, it may not be obvious, but the eddy currents of all tranny coils are nulled at the center line, which means if you first position them on the center line, and then position them at 90 degrees to each other, and then stand one up, you can avoid a lot of eddy current interaction. And this is how the TWs were built, Ken knew all this. Which is to say it was not a random decision to mount the trannies EXACTLY as he mounted them - or have them built the way he had them built.

So, now looking at your pics, you can see that both your PT and OT have their internal coils standing on end - like two donuts standing up at 90 degrees to each other. If you look at the pic I am posting, you can see that the PT of this 'official' TW has a PT with it's 'donut' of wire lying on it's face. This is what you want to see 'ideally' when the trannies are this close.

I will still suggest you could quiet things down a bit if they were in a straight line. I have used this exact pair that you have here, for the record, and the amp was the quietest Express I've built. It's all about hitting that null point in the center of the tranny. FWIW, I do have to add that on the one I built, the tranny POSITIONS were different (i.e., turn the PT 90 degrees AND the OT 90 degrees) - and this will make a difference. And this would be because the PT has a larger core than the OT which would increase it's null point's width. You should try to take advantage of that when you can.

OK, I will look at the gut shots later tomorrow. I also realize what I am saying comes as news to you. Well, believe me, I am not here to make you angry. Take care.

(Note: I edited one sentence here to clear up surfsup question that follows. In simple terms, looking at the chassis, looking at the control knobs with the amp set up for play, the PT should have the core laminations facing forward. OK?)

[surfsup]

don, I looked at the pics until I got dizzy...

Regarding hum, there is a parallel run of the plate, signal and cathode from stage two right next to the heaters (yellow) running to stage three (second socket). That might need a little TLC.

So, now looking at your pics, you can see that both your PT and OT have their internal coils standing on end - like two donuts standing up at 90 degrees to each other. If you look at the pic I am posting, you can see that the PT of this 'official' TW has a PT with it's 'donut' of wire lying on it's face.

The "donut" is oriented in which way on the PT and OT? looking at it as a die with the 1 and 6 sides with the end bells, the 3 and 4 as the laminations and the 2 and 5 as the top(copper) and bottom(bolts), the donut hole axis is parallel to a line going through 1 and 6? or 2 and 5?

FWIW, I do have to add that on the one I built, the tranny directions were reversed by 90 degrees - and this will make a difference. And this would be because the PT has a larger core than the OT which would increase it's null point's width. You should try to take advantage of that when you can.

I think I understand this, You're saying rotate the PT so the end bells are not pointing toward the OT? Could you comment on this orientation?

[img1500]http://chicagocadcam.com/ChrisHahn/xen10/Xen10_7.JPG[/img]

[don280z]

roster.....my tranformers WERE lined up just like the photo "DSC00705.jpg".
You can see the mounting holes in my photos before I moved the PT.
I have the same humm with the OT in either location.

[surfsup]

Innards of a standup transformer is shown here:

[img:250:226]http://upload.wikimedia.org/wikipedia/c ... maller.jpg[/img]

I aligned mine with the end bell of the PT toward the OT. I headphoned it and had a barely audible hum. Moving the OT around the hum did increase. I am unsure, in the future should the laminations be oriented toward the OT? or the endbells, like I have it on that amp.