How Important is the physical OT Placement, hum question

talbany · Post by **talbany** » Thu Dec 04, 2008 11:46 pm

Dang Henry that's a gutsy build there I wouldn't have the kahoonas to do that..Nice Build!!!
The Pi is less susceptible to picking up outside noise issues.. on a Dlite it's V2 ... I think you will be fine with that layout .. If your concerned pick up some steel tube shields... not Aluminum..

Tony VVT

John_P_WI · Post by **John_P_WI** » Fri Dec 05, 2008 12:40 am

Beautiful build Henry.

I haven't read all of the posts thoroughly but I would guess:

1) Wrong choke orientation 2) not twisting appropriate wires 3) changing a ground point of a filter cap or caps with the installation of the choke etc.

In the old days Hammond chokes used to have a dot indicating the orientation (start IIRC) of the wind.

Edited to reflect Henry's post below.

John

heisthl · Post by **heisthl** » Fri Dec 05, 2008 1:27 am

That's not a choke - it's a reverb transformer

John_P_WI · Post by **John_P_WI** » Fri Dec 05, 2008 1:33 am

Ooops sorry Henry....

heisthl · Post by **heisthl** » Fri Dec 05, 2008 2:01 am

I think the key to hum free "forced" O.T. placement (when you don't have any good choices) is to run the O.T. leads outside the chassis as much as possible.

frischmann · Post by **frischmann** » Fri Dec 05, 2008 3:10 am

Outside the chasis?
I've never seen this. Do you have an example of an amp where this has been done?

I feel silly but I was under the impression fender chokes were omnidirectional.

Can anybody explain the process for using the headphones to test for noise?

Thanks again,
Paul

heisthl · Post by **heisthl** » Fri Dec 05, 2008 3:24 am

frischmann wrote:Outside the chasis?
I've never seen this. Do you have an example of an amp where this has been done?
Paul

Look closely at the picture I posted on page 1 of this thread. Both the primary and secondary wires run outside the chassis until they enter the chassis near the output tubes. This avoids running those leads close to anything inside that would pick up their noise.

frischmann · Post by **frischmann** » Fri Dec 05, 2008 1:59 pm

Got it,
Cool idea.

Fischerman · Post by **Fischerman** » Fri Dec 05, 2008 3:15 pm

FWIW the Soldano SLO has the OT over at the preamp end and it's a high gain amp. However the preamp tubes are at the front of the chassis right under the controls so no preamp wires go back there. However the OT primary wires do run underneath the preamp board (but they are toward the back...under those blue Sprague Atom filters). EDIT: actually Soldano runs the OT primaries 'through' the sockets...parallel with all those other wires just kind of tucked between pins and then the CT goes straight to the back panel and then goes to the fuse holder from there. Here's a pic of my clone and then a gut shot of the preamp end. I had to elevate the heaters to get rid of some low level hum (and it worked) but I don't think it was related to the OT placement...that amp has two cathode followers at pretty high-voltage and I think it had more to do with that (but I'm just speculating).
[img:800:600]http://i108.photobucket.com/albums/n4/m ... pFront.jpg[/img]
[img:800:600]http://i108.photobucket.com/albums/n4/m ... tGutRP.jpg[/img]

drz400 · Post by **drz400** » Fri Dec 05, 2008 4:24 pm

Sweet Looking SLO build !!!!

I have owned 2 original SLO's and they were also very noisy amps in the hum department. On the one I kept the longest I first elevated the heaters and twisted all the heater wires. This helped a lot ! They also have a tendency to blow the faux center tap resistors when you blow a power tube. I finally installed DC heaters and redid the ground path. This made it totally hum free at any volume.

briane · Post by **briane** » Fri Dec 05, 2008 7:58 pm

I'm not sure how far to follow this...does the PI count

no, over the PI is fine.

As heistl has mentioned the OT over the pre-amp can work....Though to me its a bit of a shot in the dark. I've seen it work, but also had horrible oscillation problems with high-gain amps like a marshall 1986. Its likely more a factor of wire being too long, running where they should not, etc...

I do tend to use alum.

I also like to run the OT wires outside the chassy, till near the back, then they go in near where they are connected to the jacks. There's also quite a bit of back and forth on whether they should be twisted.

My main point is while I prescribe to the 90 degree theory, I have found a slight offsett to that results in significant humm reduction. Probably better to do a 80 degree offsett, if that makes sense.

In regards to needing to remove the pre-amp board to make circuit mods....Thats a bit extreme in my view. Just pull the part from the board and replace. Unless theres something odd there, like all your components are under the board. Then the only major work is replacing an OT (and I did have a faulty one once, what a pain, but it can be done).

heisthl · Post by **heisthl** » Fri Dec 05, 2008 9:06 pm

briane wrote: Then the only major work is replacing an OT (and I did have a faulty one once, what a pain, but it can be done).

If you want to "build to replace" you can use longer screws with some glue on a kep nut at the head end that sticks to the chassis inside surface on initial assembly.
On an existing build you can use dental floss and masking tape to keep a sideways tension on the 4 screws to allow replacement (requires locktite and double nuts on reassembly because the head may spin).

briane · Post by **briane** » Fri Dec 05, 2008 9:48 pm

good thought heistl....

though, I dont plan on ever replacing the OT...This one was faulty from the manufacturer. Tested questionably...worked with some crackling. Installed worked fine till it had no output a few months later. Assume a loose wire inside moving around.

In that case it just took 1/2 hour rather than 10 minutes. I have 1 or 2 screws that fall somewhat under the board on my builds. Never really worried about it, cause once the build is stable, it will pretty much be good for my lifetime. It was a fluke.

That said I have many tricks for working underneath the board. None of them fun though.

frischmann · Post by **frischmann** » Fri Dec 05, 2008 11:50 pm

Well,
It looks like the place I had marked was the only place the OT could fit.
Everything is neatly routed and in place....

It sounds really good there is some hum on the lead channel and the PAB jumps up the hum like 500%.

Structo · Post by **Structo** » Sat Dec 06, 2008 1:01 am

frischmann wrote:Well,
It looks like the place I had marked was the only place the OT could fit.
Everything is neatly routed and in place....

It sounds really good there is some hum on the lead channel and the PAB jumps up the hum like 500%.

Bummer!

I have been following this thread because I am about to install a new OT on my D'Lite.

I was planning on installing it is the same general spot.

Not sure of the mounting hole locations yet or if the grommet holes I have will work for the wires.

frischmann, did you try the headphone trick or any other noise monitoring method when you oriented yours?

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