Antek Toroid PT questions

[mirage_indigo]

For those of you that have used Antek PTs in builds before:

1) How do you mount it to the chassis? The wires all come out on what would be the bottom of the transformer assuming the lettering is right side up, so maybe this is in fact the top? What did you do with the rubber washers? Under the transformer? One up, one under?

2) What did you do with the "static shield" wire? I'm assuming you just ground this?

[bal704]

static shield goes to ground. I put the metal cup on top of the xfmr, followed by the rubber pad. The other rubber pad goes between the chassis and xfmr.

[T Wilcox]

Ive used these multiple times.
I drill 3/8 hole in chassis center of trannie. stick bolt through, add 1 rubber gasket, mount trannie wiring on chassis side. Make sure the wires are not staggered or crossing eachother. stick 2nd rubber gasket on top then the metal washer and nut. Tighten until secure but do not over tighten.
I agree with all the above post except I put the 2nd rubber gasket under the washer to insulate the trannie from the washer
PM me your email and I can send pics if helpful. Im at work so too much hassle to upload here atm

[lord preset]

Here's a pic of a previous build if that helps.

[darryl_h]

Just a couple of thoughts on mounting toroidal transformers - not specifically Antec related.

I usually attach the transformer mounting bolt to the chassis with "mudguard washers" ( "fender washers" in North America ). The washers are used above and below the chassis to reinforce the chassis and spread the mounting bolt load across a broader area. A nut and spring washer are then used to hold this assembly in place.







The second thought - if you need extra rubber gaskets for mounting toroidal transformers, they can be very easily cut from a discarded inner tube.

[R.G.]

Some additional information and cautions on toroids:

They usually mount as the pictures show, with a bolt through the middle. If that bolt is conductive, it amounts to a one-turn secondary. If the bolt is attached firmly to the chassis, and you accidentally short the top end of the bolt to the chassis, you have accidentally created a very high current short circuited secondary. This is how welder transformers are created. There will be a lot of heat - melting-things heat! - blowing fuses, all he issues you would expect from short circuiting a power transformer secondary. It's easy to do this accidentally - go ahead, ask me how I know...

It would be a very good idea to put some kind of non-conductive cover over the top of that bolt to keep you from creating a melt down.

Toroids are small, highly efficient, and razor-edged. They are almost totally intolerant of DC on their primaries. One fellow with toroidal power tranformers in his high-end stereo noticed that it suddenly was getting hot and making lots of mechanical hum. It took him some days to find the cause. The toroids were just fine, but one day he had put one of those "life extender" pellets in each of his garage exterior lights that he left on all night. Those are actually flat diodes, cutting power to the light bulbs by half wave rectifying the AC power line. This caused enough DC offset on his house's AC lines to let the toroids walk into the edge of saturation.

[mirage_indigo]

Thanks all! I'm aware of the shorted turn problem, I already cut down the center bolt to get it below the top of the toroid so I can mount some kind of wood disk on the top. Although now I'm thinking I should have left it a little long and used the excess as a screw into a threaded insert in the wood cap.

RG - is there a typical circuit solution accessible to regular people for this? I'm only using them for power; is there a DC servo style circuit for correcting this? Regular people here being me, who has a Master's in EE but learned all I know about circuits from reading ampage.org. I'm mainly after the lack of stray 60hz hum. The circuit I'm using them in has no center tapped windings, it all bridge rectified with the exception of the bias circuit which is capacitor coupled from only one side of the winding.

[R.G.]

I can only think of two ways to fix this eventuality - a series capacitor in the AC primary line, or an isolation transformer.

A capacitor in series with the AC line guarantees that any DC offset on the primary is self correcting. It has the disadvantage that the cap has to be non-polar, and has to be able to pass the entire AC current into the primary without dropping too much AC voltage from the cap's AC impedance, and has to not overheat from carrying that much current. So the capacitance needed is big, and the non-polar nature makes it non-electrolytic, and huge.

An isolation transformer of the E-I type ahead of the toroid would tolerate the offset and produce non-offset AC, but then why use a toroid?

Pretty much, one has to deal with it by just knowing that offsets are possible on the AC line and dealing with it when and if it happens. DC offsets are pretty rare, but sometimes they can happen.

As an asterisk-ish note, if you're gigging with your amp, you might consider buying a 1kW to 5kW constant voltage transformer. These beasts would weigh 30-80 pounds (Yikes! But still, less than some big speaker cabs.) and cost $50-$150 used, but they will guarantee >>quiet<<, isolated, safe, >>regulated<< AC power for an amp or most of the band. They offer a way around trusting that the place you're playing has properly wired and safe electrical supplies. It's not as barking-mad a concept as it sounds.

[Phil_S]

Would a nylon bolt like this elimination the problem with the bolt?
https://www.mcmaster.com/91244A638/

[R.G.]

Probably. I had to go look. I translated the question as "Will a 3/8" nylon bolt provide enough pressure to clamp the transformer in place without breaking, yielding or creeping over time?"

A loose interpretation of a few web sites on nylon fasteners is that you can count on a 3/8" nylon bolt to fail somewhere over 450-480 pounds in tension, and maybe 20% more in shear. Tension is what holds the toroid down to the chassis, shear is what the bolt takes when there is an abrupt sideways force on the toroid body - like you dropped it on the end of the chassis. Of course, the dropped-on-its-end case is also mitigated by the friction of the toroid and its pads holding against the chassis to resist sliding.

450 pounds directly down on a pad will probably work OK to resist both tension and shear. I think.

The company that provided us our power transformers for the Workhorse amp series used one steel M8 bolt and just warned us not to short the top of the bolt.

[darryl_h]

Many years ago an Australian manufacturer of rackmount solid state power amplifiers released, to much fanfare, a new range of "low profile" amps. A toroidal power transformer was used to fit the amplifiers into a 2RU chassis. Unfortunately the steel top cover of the case was just touching the top of the transformer mounting bolt. Initially this wasn't a problem because the crackle finish paint on the cover acted as an insulator.

Once some of the amplifiers had been on the road for several weeks, the inevitable bumps and vibration wore away the paint, and the amplifiers started having random power fuse failures. Once back in the workshop, these amplifiers were comprehensively tested, with the top covers off to allow ready access to the innards. Naturally they behaved flawlessly, no matter how brutal the testing became. As more and more amplifiers were returning with this apparently intractable problem, it was approaching the point where the entire production run would have to be withdrawn and purchase money refunded.

Fortunately a brainstorming session with the company's dealer network finally resolved the issue when somebody suggested that somehow the transformers were being short circuited. The solution was still costly, but it could have been worse. It was a very good lesson regarding the correct mounting of toroidal transformers.

[Aurora]

Over the years, I've used bucket loads of toroids for various purposes, and I always mount them as shown in the pictures above. Using the supplied metal disc and rubber gasket on top, does not create a shorted loop, - only if you for some peculiar reason should use a bracket ( of sorts) to tie the top of the bolt down to chassis again, - then you have a closed loop. I usually also use lock nuts or double nuts on the top...... I dont think I would use nylon screws to mount a transformer. If the wires on the "bottom" troubles you, just flip the thing.... who gives a shit if the text is upside down.... the transformer surely don't...
As for the DC thing, it is a general problem if it occurs, but it's not really DC, but a distortion of the sine wave that results in different energy content of the two sine half waves....

[Phil_S]

Probably. I had to go look. I translated the question as "Will a 3/8" nylon bolt provide enough pressure to clamp the transformer in place without breaking, yielding or creeping over time?"

R.G.: Yes, that is exactly my question. Thank you for articulating it so well, and, of course, for addressing it. Looking at what Aurora says, along with your answer, I don't think I'd use a nylon bolt. It does seem wise however (see what Darryl says) to insulate the top of the transformer bolt from the chassis cover. Interesting discussion...

[mirage_indigo]

I thought about capacitor isolating the AC mains. good to know It's not a crazy idea. Back to back electrolytics need only be 75 odd volts. A quick calculation reveals that a 1000uF cap would have a 2ish ohm impedance at 60hz if I did the math right. You'd be down to the ESR of the cap itself.

But I like the isolation transformer idea better. Tripplite makes a 8lb 250w unit for $125 available at that online store named after the river. I can leave the rest of the band to their own devices.

Thanks for your help everyone! I got the thing mounted and the heaters and everything twisted and installed. My main issue so far (as always) is being over optimistic about how many leads I could cram around a turret.

[mirage_indigo]

Also if you flip the toroid upside down I'd be worried about the Coriolis effect. But good to hear they mostly still work in Oz.