How Important is the physical OT Placement, hum question

[frischmann]

Hi Guys,
I finaly got my hands on a new OT for my D'lite. I added a choke a while ago so now it's more like a d'medium..

I put the choke between the PT and the old tremolux style OT and all was fine.

I had to move the new OT over about 1-3/8" towards the input side of the amp (to the left if the transformers are on top) to ensure I'd still have access to all of the screws for the board mounts.

I did a quick check soldering everything in place (but not cutting to length, yet) to see if anything weird would happen.

It seems the noise floor has come up considerably....lot's of 60 cycle hum most noticeable on the gain channel, even worse with the PAB on...

According to my measurements, the OT is 1-1/4" to the right of PI and 1-1/2" behind the PI which should be cool.

Any thoughts?

Could my OT placement cause me problems like hum or the dreaded PO?
Is there a rule of thumb about placement of the OT, Choke etc?

thanks
Paul

[frischmann]

Hmmmm,
I wonder if I just bumped a ground. My hands were in all kinds of places in this amp and dainty they ain't...

[heisthl]

Long leads are probably your problem, especially if they run near the PI grid wires or parallel with the output tube grid wires.

[talbany]

Sorry wrong Post

Tony VVT

[drz400]

Hi Guys,
I finaly got my hands on a new OT for my D'lite. I added a choke a while ago so now it's more like a d'medium..

I put the choke between the PT and the old tremolux style OT and all was fine.

I had to move the new OT over about 1-3/8" towards the input side of the amp (to the left if the transformers are on top) to ensure I'd still have access to all of the screws for the board mounts.

I did a quick check soldering everything in place (but not cutting to length, yet) to see if anything weird would happen.

It seems the noise floor has come up considerably....lot's of 60 cycle hum most noticeable on the gain channel, even worse with the PAB on...

According to my measurements, the OT is 1-1/4" to the right of PI and 1-1/2" behind the PI which should be cool.

Any thoughts?

Could my OT placement cause me problems like hum or the dreaded PO?
Is there a rule of thumb about placement of the OT, Choke etc?

thanks
Paul

Your output and power should be at a T and have some distance between them or you will inject hum into the secondary of the output transformer. This is evident by a hum even when the amp is in standby and you shouldn't hear anything.

[frischmann]

I understand the OP and PT being oriented 90 degrees from each other.

But how close can they be to each other and and things like the choke or tubes before you will have problems?

[soje]

***Newbie warning***

I read a describtion about this somewhere, I don't remember if it was Paul Ruby, and I don't know if it's the right thing to do with a dumble clone but anyway:

Desolder the OT, and connect a pair of headphones to the OT secondaries. Power on the amp, and listen while moving the OT around, until you find a decent humfree spot.

...or am I off on a limb here?

***ducks and runs for cover***

Regards
soje

[jelle]

I guess you can come back now... Great approach.

The best orientation is not always 90 degrees.

[JimiB]

I read a describtion about this somewhere, I don't remember if it was Paul Ruby, and I don't know if it's the right thing to do with a dumble clone but anyway:

Desolder the OT, and connect a pair of headphones to the OT secondaries. Power on the amp, and listen while moving the OT around, until you find a decent humfree spot.

Is this with the amp fully assembled and tubes in?

[frischmann]

I guess you can come back now... Great approach.

The best orientation is not always 90 degrees.

Really?
I always thought that was transformer rule #1. OT and PT 90 degress from each other...

Are there other amps with different orientation.

[jelle]

The magnetism lines of the magnetic field are NOT straight lines they do look more like a 8 shaped pattern IIRC. Try sprinkling iron powder over a magnet and you'll get the idea. Most of the times 90 degrees works fine depending on relative orientation between the transformers.

jelle

[briane]

Yeah, jelle's right on. it is a figure 8.

The short answer is yes. Physical location is CRITICAL.

90 degrees is the cheap easy answer. A better one relates to the triangularity of the relationship of the OT to the PT. The funny thing about doing the headphone trick many times is you start to see a fundamental relationship in locations...always 90degrees, always OT off the 45 degree corner of the PT (well actually not exactly 45, more like 38 degrees). And the distance is always about the same. Oh, and the OT cant be over the pre-amp, or that causes another set of problems...Move it too far and the noise is replaced by oscillation.

I use the headphone trick on every build, and yes it does work. Doing that you can see how even 1/2 an inch in the wrong direction can double (or more) the noise floor. I see everyone posting how they mount in a place where they can get at the bolts. Not me, I mount where the noise floor is the lowest, then figure out how to access the bolts (layered build, needlenose pliers, whatever).

The headphone trick should be done after the PT is installed, I ussually do it then before I install the boards or anything else. Essentially a pre-power up. I have found this essential to a noise free amp.

[talbany]

In my experience the smaller power transformers seem to emit a larger field.. If you can feel the transformer vibrate it's most of the time giving off some serious eddy currents.. Obviously Ken fisher bought into the 90 degree theory.. He had plenty of room behind the power tubes to put the OPT.. instead he mounted them right next to each other and jam a piece of wood between them so they are actually touching.. There you go a 25k amp with the transformers touching.. I could see the grief I would catch if I did this Whew!!!

Tony VVT

[frischmann]

This is awesome stuff.

My concern was having the OT "over the preamp" as briane said. I'm not sure how far to follow this...does the PI count? etc...

So,
That being said. I have a D'lite. MY New Bassman OT is currently 1-1/4" to the right of the center of the PI tube and 1-1/2" back from the center of the PI tube. (transformers on top)

Am I cool or do I have to move it?
I don't want to cut all the leads until I'm sure.

If I move it over so that the OT is not over the preamp circuit board at all (I can likely do this) I will have to take off the OT every time I want to get at the bolts to take off the preamp board as I'm experimenting, this would be a real pain....

Tone comes first of course.

[heisthl]

the OT cant be over the pre-amp, or that causes another set of problems...Move it too far and the noise is replaced by oscillation.

Maybe that is do to using aluminum - I never have any hum or oscillation problems with the steel chassis I use and on this style build I'm right on top of the input stage (and really close to the tubes).