Antek Power Transformers

[loudthud]

The link to the pdf file on the Antek site is broken so I can't look at the spec sheet. Off hand I would say no, don't connect the 70V taps and no, you can't use the taps when you are using a full wave bridge rectifier. After rectification, 70VAC is going to be about 100VDC. A little higher than you would like for elevated heaters. Use a voltage divider to get down to about 50VDC.

[Tage]

so I can't use just the one as a bias supply tap? That's junky, so I might go back to running this as a full wave center tap instead so I can tap off it to make a bias supply.

The spec sheet and hook up are a lot like this one

http://www.antekinc.com/pdf/AN-4TK400.pdf

Although it's 430V

[loudthud]

OK, you could hook the 70V taps together but it won't do any good. That point will swing +100V on one phase and +500V on the other phase. For a grounded center tap configuration you would hook one of the white wires to the yellow wire from the other winding and use that as your center tap. The gray wire from the winding that has it's white wire grounded is the one that will be 70VAC. The other gray wire will be 360VAC. Any mistake will result on a short or a -500V bias supply. Hope you have a variac.

This is confusing. They should have a better way of color coding the windings.

[Tage]

Yes I agree about the color coding, it is really not very intuitive. But I understand what you are saying and I shouldn't have any trouble doing that, I figured the one tap would be at a weird/too high voltage. I don't have my own variac, but I do have one I can borrow, I probably will in this case just to avoid any surprises.

[Wombaticus]

Yes I agree about the color coding, it is really not very intuitive. But I understand what you are saying and I shouldn't have any trouble doing that, I figured the one tap would be at a weird/too high voltage. I don't have my own variac, but I do have one I can borrow, I probably will in this case just to avoid any surprises.

I'm a bit suspicious of their diagram, probably some simple testing is in order to see what's what. I'd guess there's a decent chance that white to yellow is 400V and white to grey is 70. The way they have it shown looks bass-ackwards.
Easy enough to figure it out. Do testing with an AC wall wart to figure out the voltages..

[Tage]

So I hooked the transformer to a 8.3V ac transformer and got the voltages I expected. Yellow=430 Grey=70 and white=0, they all read a little higher, as they were unloaded. I had wired all these together in parallel, yellow to Yellow, etc... The Heater windings measured in at 6.4 a each, but I'll run them as a center tapped 12.6, center tap connected to a humdinger pot.

So if I wire the yellow and white to a bridge rectifier, I will not be able to use the 70V tap then? If I wire the yellows as a full wave rectifier and ground the whites, I should then be able to use one 70V tap to build a bias supply correct? or am I missing something...

[imaradiostar]

I don't think you'd connect the center tap to a humdinger- you'd connect the center leg of the humdinger to ground (or even somewhere about ground, floating positive) and connect the outer lugs of the humdinger to the filament leads- 12.6vac in this case.

I don't fully understand what they intended with the 70v taps on the Antek PT's. If you're using the PT with a bridge rectifier which is the most efficient use of the windings the 70 volt tap is referenced to ground one moment and B+ the next. If you use the PT with a full wave center tapped arrangement you won't get as much current out of it but you could use the other taps as intended.

What kind of output valves are you using?

For KT88's or 6550's I'd use the full winding (470 volts) and bridge rectify the whole thing for around 650 volts. I'd add a winding for bias (easily done with a toroid) and if needed use an additional tube or a mosfet for a screen voltage regulator with the gate fed by a voltage divider so that screen voltage tracks plate voltage. It should allow maximum power from big bottles.

With 6L6's I'd probably connect the PT using a FWCT arrangement with the 400v taps. It isn't the best use of the antek PT and the voltage is on the high side but it should be OK.

jamie

[Tage]

Yes you are correct on the humdinger wiring.

I will be using 6550's and so I will use a bridge rectifier, as far as bias I think I will make a capacitor coupled supply. I know it's not ideal but I have yet to see one fail in a commercial amp. Is there a reason you would regulate the screen supply? would it not still track the plate voltage if it was simply derived from a vlotage divider?

[defaced]

A voltage divider will be too high of impedance. It will sag alot under load. As I think about it for a second, I think if you feed the MOSFET via a voltage divider, it will bounce around with the B+, but just at a lower voltage. Definitely worth the 3 bucks in parts to try out.

[Tage]

So how do I go about biasing my Mosfet then? I'm pretty new with silicon stuff, I've only done a few pedals and mostly op amp stuff. My FET and BJT knowledge is mostly just switching purposes. You wouldn't happen to have a schematic excerpt that illustrates this?

[imaradiostar]

A voltage divider will be too high of impedance. It will sag alot under load. As I think about it for a second, I think if you feed the MOSFET via a voltage divider, it will bounce around with the B+, but just at a lower voltage. Definitely worth the 3 bucks in parts to try out.

What he said.

As for bias supply- I don't mind a capacitive supply on lower powered amps but i'd hesitate to use it on a 6550 powered amp because of the need for a lower impedance bias supply. It'll work but I'd say it's far from ideal. Merlin talks about there being a practical limit for the cap bias supply in his power supply book.

Jamie

[Tom Phillips]

Tage,
You mentioned a cathode biased 4 el34 type amp back in post #13. Now that you are talking 6550's how much power output are you shooting for? Are you planning 2, 4 or maybe 6 6550s?
Tom

[defaced]

So how do I go about biasing my Mosfet then? I'm pretty new with silicon stuff, I've only done a few pedals and mostly op amp stuff. My FET and BJT knowledge is mostly just switching purposes. You wouldn't happen to have a schematic excerpt that illustrates this?

This is where I would start with the design. Basically, the voltage that appears at the gate ends up also appearing at the source. So, if you feed the MOSFET a voltage, it spits it out the back end, but it can pass appreciable current, so it's similar to a cathode follower, it's a current buffer.

The two resistors are the voltage divider. The zener diodes are to protect the gate. They're probably not needed for this application, but for 5 cents, you can protect your MOSFET. Their voltage rating needs to be less than gate/source breakdown voltage as specificed on the data sheet. The MOSFET is just one I grabbed, I don't know if a IRF840 would work or not. You will want one that can handle the voltage, power, and current needed for what you're doing. It will also need to be properly heat sinked. I am not sure if you'll need the decoupling cap or not. Try it without, if you get hum in the supply, then add it back in. I'd start with a value that's used for elevated heaters, so like in the 22u to 47u range.

[imaradiostar]

I would add a gate stopper resistor between the gate and the junction of R9 and R10, just in case.

I would also tend toward a mosfet with a built in zener so you don't have to be as careful handling it.

These are cheap enough and easy to use: STW9NK90Z STMicroelectronics MOSFET

jamie

[Tage]

Yes I am now using an Antek for a 4 6550 amp, that is why I've gone with a higher voltage transformer, I'm looking to generate 150+ watts output from the quad. I have another thread going on it, UL taps and PI Drive.

I may do the whole running another transformer backwards to generate my bias but I've got limited real estate in the chassis.