Antek Power Transformers

[R.G.]

Yeah chassis as a heat sink always kinda sketched me out. Just because there would be a lot of voltage only separated by a thermal pad and a screw to ground

That's a good reason to be wary. One slip or flake of metal from the tab of the MOSFET to the chassis and you have a B+ short if the chassis is properly grounded, and a B+ hot chassis if it isn't correctly grounded. There are many small aluminum heat sinks made to clip or bolt onto the common MOSFET packages, and these can be set up so they're not tied to chassis if you're careful. Several inches of air is a better insulator than a thermal pad.

[Tage]

Yeah just ordered up some beefy heat sinks that I think will do the trick just fine.

[jpfamps]

I have had a real dick ache ordering a PT from Antek. First there was a problem with the shipping fee which was my fault, then the damn thing got lost in the mail, then they were out of stock, then the stock didn't come in! Some 10 weeks or so later they're shipping me an alternative transformer which is probably a little better suited to what I want to do with it anyway. They do suck at communications, especially by email so phone them if you have any troubles. They do offer a lot of transformer for the cash so if this one does turn up I would use them again in the future as really, a lot of went wrong here was bad luck. Just thought I'd share.

If you are in the EU Tube Town do a good range of toroidal mains transformer for valve amps at a good price.

Many of these have bias windings.

The cores are very conservatively rated, and as I've used these with full wave bridge rectification (with the HT windings in parallel) at idle usually comes in at about 10% over the expected HT, so I take this into consideration when designing an amp around these units.

I've not used these transformers with half-wave rectification, and I would expect the HT to be more in line with the expected values.

If I want anything made to suit my specifications I use Tiger Toroids, in the UK, who are actually again very reasonably priced.

[Tage]

Anyone know of some north american custom toroid suppliers? Antek won't do custom stuff...

[Zozobra]

Finally got around to wiring up my AN-3T325 I ordered. They could have made the colour scheme a little less ambiguous for us folks wiring it for 230VAC use!
It seem to work fine although I've only tried it unloaded so far. Seems to pop the mains fuse on second power up for some reason. I'm only using a 680mA fuse though which might be a bit small for a 300VA PT although should that matter greatly if its entirely unloaded?

[jpfamps]

Finally got around to wiring up my AN-3T325 I ordered. They could have made the colour scheme a little less ambiguous for us folks wiring it for 230VAC use!
It seem to work fine although I've only tried it unloaded so far. Seems to pop the mains fuse on second power up for some reason. I'm only using a 680mA fuse though which might be a bit small for a 300VA PT although should that matter greatly if its entirely unloaded?

Toroidals are notorious for large in rush currents, regardless of the loading.

As far as I understand this is due to residual magnetization causing the transformer to saturate when first energized. This is "corrected" after a couple of mains cycles, but can cause fuses to pop.

You are usually advised to use some form of current limiting with larger units.

[imaradiostar]

Put a small voltage to one of the primaries- say 6.3 volts from some other filament transformer. By connecting the primaries in series you can get the phasing correct- but you probably already knew that.

My Antek TF seems to draw a lot of current upon initial connection of AC and it's only a 100VA model. Is your fuse a slow-blow?

Jamie

[Zozobra]

I'll try that to check. I should really scope it too.

Its a slow blow fuse but its pretty under rated for the task I guess. Its fuses are quite visibly blown as the inside of the glass envelope is black! Unfortunately my big box of bits with my fuses in is at my practice space from when I was voicing a big muff circuit a few weeks ago. I'll try cycling it a few times using my current limiter bulb too to see if that helps things.

Thanks for the tips guys!

[Zozobra]

and its absolutely fine with the lightbulb limiter in place. the bulb doesnt light at any point but i guess it tames the inrush just enough to save the fuse.

[Tage]

well i am a couple hours work away from powering mine up, and it's a 400VA. any suggestions on limiting teqniques?

[Steve Conner]

In my big solid-state amps, I use a hefty resistor in series with the transformer primary, and a time delay relay that shorts the resistor out after a few seconds. This is quite a common method, but there is a safety concern. If the relay fails to close, the resistor can get extremely hot and maybe set fire to things.

Nowadays you can get "NTC inrush limiters", little discs of a material that decreases drastically in resistance when it gets hot. They are widely used because they offer a big saving in cost over the step-start relay. They are also less likely to catch fire than the resistor, at least in theory.

These are refinements that go into the finished unit. For bench testing purposes I would bring the thing up on a variac.

[Zozobra]

Thanks for the tip. I was thinking of employing a primary side resistor on a switch as described in merlins power supply book but the NTC inrush limiter sounds interesting and easy to implement as I guess you can wire it straight between the IEC socket live pin and the mains fuse. The socket on my current play thing has the mains fuse built into it though so I cant use it here. How do you go about selecting a sensible value/spec for the part?

[Tage]

Hey Zozobra, i also use the iec sockets with a built in fuse, but i use them as the absolute, just in case fuse. I have a lower rated one on the back panel too, with that kind of setup the NTC can go between the fuse terminal and the IEC terminal.

I also would like some feedback on determining the proper thermistor to use.

[Wombaticus]

I'll try that to check. I should really scope it too.

Its a slow blow fuse but its pretty under rated for the task I guess. Its fuses are quite visibly blown as the inside of the glass envelope is black! Unfortunately my big box of bits with my fuses in is at my practice space from when I was voicing a big muff circuit a few weeks ago. I'll try cycling it a few times using my current limiter bulb too to see if that helps things.

Thanks for the tips guys!

Hey Zozobra -- One of my 4x6L6 amps that used the 300VA Antek AN-3T325 was wired for European use and is currently in service in Switzerland. Looking back in my notes, I specified a 1.5A slow blow for European operation, and I have had no reports of blown fuses.