How to Mount Power Transformer on Chassis?

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The New Steve H
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How to Mount Power Transformer on Chassis?

Post by The New Steve H » Fri Jun 03, 2011 12:14 am

Today I machined a big hole in my homemade amp chassis and stuck a power transformer in it. This is a 5f6a clone.

I have a question.

The transformer has some kind of plastic shields on the screws that hold it together. These are the same screws that attach it to the chassis. There are four of them. One of them has no insulator on it, and it has a toothed washer on it. I assume it's for the ground.

Am I supposed to put the washer between the transformer and the chassis, to keep it level (to compensate for the lack of an insulator), or should I put it on the other side of the chassis, under the nut? If I do it the latter way, the transformer will not be level.
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M Fowler
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Re: How to Mount Power Transformer on Chassis?

Post by M Fowler » Fri Jun 03, 2011 12:24 am

The transformer bolts should have came tightened already and you run those mounting bolts through the chassis and use a washer with either nylock nut or keps nut.

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The New Steve H
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Re: How to Mount Power Transformer on Chassis?

Post by The New Steve H » Fri Jun 03, 2011 12:29 am

Um...are you saying I made a mistake by removing the nuts so I could use them to fasten the transformer to the chassis? I'm supposed to go get four more nuts?

I guess this would explain why they were so tight...
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Re: How to Mount Power Transformer on Chassis?

Post by Cliff Schecht » Fri Jun 03, 2011 12:32 am

No it's not a big deal that you removed those. Just crank em down tight again (don't strip them though!). I don't float the transformer above the chassis like you mention. I remove the bolts, drop the tranny flat against the chassis, put the bolts back in and crank em down..

Now quit thinking so much and build your amp already Steve! Maybe that physics degree did hurt more than it helped :P.
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The New Steve H
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Re: How to Mount Power Transformer on Chassis?

Post by The New Steve H » Fri Jun 03, 2011 12:34 am

Hey, I can't help it. Those insulators hold three corners of the transformer a millimeter off the chassis, and the groundy corner is flat against the metal. Tell me that wouldn't drive you nuts.
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Re: How to Mount Power Transformer on Chassis?

Post by Cliff Schecht » Fri Jun 03, 2011 12:40 am

Can you not get rid of the washers? I always get rid of anything insulating the transformer from the chassis. I do use washers transformers where the screw meets the top of the transformer and where the bolts hit the chassis but there shouldn't be washers in the way anywhere else. This is why I keep spare transformer-sized screws around.
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The New Steve H
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Re: How to Mount Power Transformer on Chassis?

Post by The New Steve H » Fri Jun 03, 2011 12:42 am

I want to keep the insulators because I read that a transformer should only be grounded to the chassis at one point. I guess I could put a conductive washer under the low corner.
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M Fowler
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Re: How to Mount Power Transformer on Chassis?

Post by M Fowler » Fri Jun 03, 2011 12:43 am

There are a few ways to mount the PT.

But yeah after I get a PT in the chassis I use four more nuts, washers and tighten it down.

Those plastic sleeves should be in further?

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David Root
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To Float or not to float

Post by David Root » Fri Jun 03, 2011 12:45 am

That's not something I ever thought about to be quite honest.

I have bought transformers that were fully bolted up and came with a set of locknuts for bolting up to the chassis ie "floating". (I assume we are talking about laydown arrangement here).

Is there a sound reason for not doing it that way?

Update. I checked some of my stock transformers.
Two Magnetic Components 40-18024 laydown BF Twin have nylon bolt insulators that DO insulate the PT body from the bolts.
One Mag Cpts Marshall 100W STANDUP OT 40-18026 also has what appear to be the same nylon bolt insulators but is NOT insulated from the mounting bolts.
One NOS Fender Bassman 70 laydown mounting bolts have washers on them and the mounting bolts are NOT insulated from the PT body.
Last edited by David Root on Fri Jun 03, 2011 1:05 am, edited 2 times in total.

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M Fowler
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Re: How to Mount Power Transformer on Chassis?

Post by M Fowler » Fri Jun 03, 2011 12:48 am

Some people don't want the amp floating on top of the nuts and others want to isolate the PT from the chassis.

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Re: How to Mount Power Transformer on Chassis?

Post by Cliff Schecht » Fri Jun 03, 2011 12:56 am

Floating on nuts with thin aluminum is asking for problems, so I don't do it on flimsier chassis. Aluminum doesn't help or hurt with shielding but I thought steel chassis couple the stray flux on a laydown PT (hence why Fender usually put the OT pretty far from the PT).
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David Root
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Re: How to Mount Power Transformer on Chassis?

Post by David Root » Fri Jun 03, 2011 1:07 am

So this is a mechanical rather than electrical issue?

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The New Steve H
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Re: How to Mount Power Transformer on Chassis?

Post by The New Steve H » Fri Jun 03, 2011 1:36 am

Wow, it sort of looks like this wasn't a totally stupid question.
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diagrammatiks
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Re: How to Mount Power Transformer on Chassis?

Post by diagrammatiks » Fri Jun 03, 2011 1:44 am

I always thought that the one place rule was about the grounding of the center tap or the reference of the rectifier.

The old fenders had the laydown drop through pt's. I don't think those were completely isolated since they sat a bit inside the chassis.

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David Root
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Re: How to Mount Power Transformer on Chassis?

Post by David Root » Fri Jun 03, 2011 3:50 am

I don't recall the original Fender PTs having insulating shoulder washers on the mounting bolts.

I can pull the chassis on a '67 AB165 Bassman & check but that is a PITA. Anyone happen to know "fer sure"?

But those two new Mag Cpts Twin PTs I have are "fer sure" insulated from the mounting bolts!

I can see if you had a winding short to the core you might not want it (the core) grounded to the chassis, OTOH isn't that why they have bells so that the electromagnetic emissions ARE grounded to the chassis?
Last edited by David Root on Fri Jun 03, 2011 3:59 am, edited 2 times in total.

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