Headphone trick: Wow! How do I eliminate xfrmr crosstalk?

[Aaron Smith]

For about a month, I've been working on a vintage 5B3 that came to me in parts. It had a lot of ugly when I got it, including transformers that were replaced sometime around 1960 or so. They are not the original spec transformers, but would have gone into a Brown Princeton at that time. Pretty nice iron. Everything on the board was original, and when I started working on it I intended to keep it as original as possible.

Recently I realized what a fool's errand that was. Every single capacitor leaked like crazy, and every resistor had drifted at least 30%. The amp was noisy as heck, and sounded bad. So, component by component, I replaced pretty much everything on the board.

Now it sounded better, but still had a ton of line noise. Rewired heaters with twisted pair and artificial center tap; noise no better. I started to realize that the design of the amp just isn't that good. Everything is locally grounded to the chassis, including the input jacks, volume pots, heaters at each socket, etc. There's no way to even troubleshoot the grounding. I figure at this point, I might as well do a total rebuild and have a great- sounding and reliable amp in a cool vintage chassis and cab.

I have it stripped down to the bare chassis. i'm going to overdrill the jack and pot holes to accomodate iso washers. Started the process of rebuilding today, and figured I would try the "headphone trick" to see if the OT was picking up noise from the PT. HOLY COW- mystery solved. In the current mounting locations, with the OT right next to the PT, there is a ton of line noise coming through the OT.

Here's my issue... obviously moving the OT away from the PT lessens the noise. Changing the orientation helps a bit too. Problem is, even at the far end of the chassis, in the best orientation, there is noticeable line noise. Is there something else I can do to isolate them?

[xtian]

On all smaller chassis, I cannot move the two transformers far enough apart to get zero noise out of the OT. Only on 24" chassis can I move it far enough. Not to worry, though. A very low amount of hum is present in all builds, and disappears below the noise floor on decent builds.

Great observations! It took me two years to learn all the stuff you mentioned in your post.

[Aaron Smith]

On all smaller chassis, I cannot move the two transformers far enough apart to get zero noise out of the OT. Only on 24" chassis can I move it far enough. Not to worry, though. A very low amount of hum is present in all builds, and disappears below the noise floor on decent builds.

Great observations! It took me two years to learn all the stuff you mentioned in your post.

Thanks xtian. So do you feel that it's better to put the OT at the far end of the chassis, even if that puts it close to the input jacks and means long transformer leads? Or would you shoot for a happy medium, somewhere in the middle of the chassis?

[Firestorm]

If it's truly the coupling of the transformer fields, only distance and orientation (plus, maybe shielding) can help. But noise may be radiating from the connections on the line side and the HT side. Marshall put 0.22s to ground on the HT side; Fender used only a single "death cap" on the line side. It's not just to establish a ground reference, but to shut up the winding. Nowadays, we rip 'em out, but there are valid reasons for it to be there even in three-pronged cord builds. Still looking for the article. It's actually supposed to be from both sides and it MUST be of the fail open type.

Just pointing it out (I will find the article eventually).

[mandopicker]

Have you considered trying new 6SC7's? These tubs can be quite noisy all by themselves.

My guess is that if you separated the transformers - from end to end - and the noise is still there, the tubes might be the culprit.

Just food for thought.

[Ken Moon]

You'll see a lot of amps built by really good builders that have the PT right at one end, and the OT in about the middle of the chassis, so that's a pretty tried and true setup.

Sometimes I end up with the OT being at a 45 degree angle, and I just go with that, even though keeping it at a 90 degree angle looks neater.

Hiwatt and some others have the trannies at the far ends, but as you've noticed, that requires some creative wiring and can increase line noise coupled through the OT.

Laydown or toroidal PTs can help because they are in the Z plane to the OT.

When I want super low noise, like in a recording amp, the final step for me is to use a separate heater transformer.

Here's a pic of the tranny layout of my latest amp, using a Hammond heater tranny:

[img:931:469]http://i514.photobucket.com/albums/t346 ... eck_sm.jpg[/img]

[Aaron Smith]

If it's truly the coupling of the transformer fields, only distance and orientation (plus, maybe shielding) can help. But noise may be radiating from the connections on the line side and the HT side. Marshall put 0.22s to ground on the HT side; Fender used only a single "death cap" on the line side. It's not just to establish a ground reference, but to shut up the winding. Nowadays, we rip 'em out, but there are valid reasons for it to be there even in three-pronged cord builds. Still looking for the article. It's actually supposed to be from both sides and it MUST be of the fail open type.

Just pointing it out (I will find the article eventually).

Thanks Firestorm. Just to clarify, the noise is definitely coming from coupling of the fields. The HT side isn't wired to anything right now, and the noise varies in relation to distance. The OT isn't connected to anything at all (except the headphones of course).
Would love to see that article if you can find it though.

[Aaron Smith]

Have you considered trying new 6SC7's? These tubs can be quite noisy all by themselves.

My guess is that if you separated the transformers - from end to end - and the noise is still there, the tubes might be the culprit.

Just food for thought.

In this case, not the tubes. There are no tubes installed. It is a bare chassis; the only connections made are between the mains and the PT primaries, and the OT secondaries and the headphones.
I agree those tubes can be noisy though. I have three different sets, and some are definitely better than others.

[Aaron Smith]

You'll see a lot of amps built by really good builders that have the PT right at one end, and the OT in about the middle of the chassis, so that's a pretty tried and true setup.

Sometimes I end up with the OT being at a 45 degree angle, and I just go with that, even though keeping it at a 90 degree angle looks neater.

Hiwatt and some others have the trannies at the far ends, but as you've noticed, that requires some creative wiring and can increase line noise coupled through the OT.

Laydown or toroidal PTs can help because they are in the Z plane to the OT.

When I want super low noise, like in a recording amp, the final step for me is to use a separate heater transformer.

Here's a pic of the tranny layout of my latest amp, using a Hammond heater tranny:

[img:931:469]http://i514.photobucket.com/albums/t346 ... eck_sm.jpg[/img]

Good feedback Ken. I'm now thinking... the original PT in this amp was a top-mounted transformer... i wonder if it was a lay down? When they replaced the original transformer with the 125P1A that's in there now, they sawed a big knarly hole through the chassis for the bottom bell to drop through. I wonder if going back to a period-correct PT would help here?

[Firestorm]

Fender were also masters of killing one hum by inducing another (out of phase).

[Aaron Smith]

Fender were also masters of killing one hum by inducing another (out of phase).

Humbucking amp... I like it.

[Tillydog]

Here's my issue... obviously moving the OT away from the PT lessens the noise. Changing the orientation helps a bit too. Problem is, even at the far end of the chassis, in the best orientation, there is noticeable line noise. Is there something else I can do to isolate them?

Can you not find a different orientation where the coupling is minimised? Most every time I've done this, I've found that even with the transformers touching, there's a particular orientation & relative position (not always centre to centre) where there is pretty much zero hum. Position can be very critical, though. If you can't find that orientation, it's difficult to move the OT far enough away.

You may need to turn the power transformer as well as the output transformer (in case you aren't).

Bear in mind as well, that the level of hum from transformer coupling will be constant - it won't increase with amp volume, so getting rid of it completely is more a matter of pride than necessity (IMHO).

[xtian]

+1, Tillydog.

[Aaron Smith]

Here's my issue... obviously moving the OT away from the PT lessens the noise. Changing the orientation helps a bit too. Problem is, even at the far end of the chassis, in the best orientation, there is noticeable line noise. Is there something else I can do to isolate them?

Can you not find a different orientation where the coupling is minimised? Most every time I've done this, I've found that even with the transformers touching, there's a particular orientation & relative position (not always centre to centre) where there is pretty much zero hum. Position can be very critical, though. If you can't find that orientation, it's difficult to move the OT far enough away.

You may need to turn the power transformer as well as the output transformer (in case you aren't).

Bear in mind as well, that the level of hum from transformer coupling will be constant - it won't increase with amp volume, so getting rid of it completely is more a matter of pride than necessity (IMHO).

With the PT in the current mounting position, it didn't matter how I rotated the OT, the hum was still strong. However, rotating the PT 90 degrees reduced the hum but at least 50 percent.

This created quite a challenge in a chassis that had already been creatively modified to fit the current non-original PT. Let me tell you, I felt a bit guilty taking a jigsaw to a 60 year old amp, but you do what you have to do. There was a lot of creative engineering involved to cut enough metal to fit the bell through in the new orientation, but leave enough so I had something to put the mounting screws through. Had to mount it about 10 degrees off of square.

It looks ugly as heck, but the hum problem is greatly reduced. I can now mount the OT mid-chassis and have minimal hum. I'll never think twice about it once the amp is mounted in the cabinet. Thanks for the suggestions!

[Aaron Smith]

One other thing I should note. When I was about to mount the PT in the new location, I noticed that the mounting screws were designed to sandwich the laminations with a nut/star washer, and that all the nuts were loose. So I tightened all the nuts, and tried the headphone trick again. Jeez, it's amazing how much more quiet the transformer was!

So... I guess the looseness of those bolts, and perhaps the laminations, was also contributing to the noise. Learning new things every time I work on this amp...