Turning a 6.3 V secondary into a 5 V secondary on an Antek 2T350 - Page 2 - diyAudio
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Old 31st October 2010, 11:57 AM   #11
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Ok, using ohms law either in ones head or a calculator available on line it is simple to come up with a resistor sized to drop the voltage to the desired 5.0 volts. When you start winding wire around a toridal transformer I start to wonder what ga wire to use and if the added winding is going to be able to supply the needed 1.9 A the 5Ar4 is going to draw. Is 18-20 ga big enough? What is the calculation to come up with the ga of wire?
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Old 31st October 2010, 12:38 PM   #12
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I'm more concerned with the power-handling capability of the core. Lacking any hard data from the manufacturer regarding this, I would only add another winding if I knew I wasn't maxed out on the stock windings.
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Old 31st October 2010, 02:58 PM   #13
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Quote:
Originally Posted by tomchr View Post
These rectifiers have a 5 V filament. The trafo provides 6.3 V. What to do...
Thanks for sharing. I did something like that here. Tubelab SE - alternate power transformer
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Old 31st October 2010, 03:01 PM   #14
jrenkin is offline jrenkin  United States
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According to the seller the core is pretty robust, a bit over spec., so minor changes aren't too likely to be a problem unless you are pushing their limits anyway.
I tried the added winding technique, but the problem I ran into is that they no longer fit into the nice cover I have for them (from Antek) so no longer can be mounted on the outside of the chassis, if I want permission to place my amp in a visible location...
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Old 31st October 2010, 04:12 PM   #15
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Originally Posted by tomchr View Post
I'm not sure what your point is. I arrived at the 0.68 ohm by back of envelope calculation. The 5AR4 draws 1.9 A at 5.0 V.

(6.3-5)/1.9 = 0.684 ohm

0.68 is the closest standard value. Done. No trial and error. I hooked it up and got 5.0 V across the rectifier filament.

~Tom
Hello,
The short version is: I am skeptical. I donít think that it is that simple.
http://www.antekinc.com/pdf/AN-2T350.pdf
Assume that the 3 other secondary windings are connected to loads and we are considering the remaining 6.3 volt secondary. The output is rated at 6.3 volts, 4 amps with 115 volts input. Now increase the input voltage to a more typical 120 + value the output voltage will go up by 5% or so. Next change the load to 2 amps for a 5Y3 or 1.9 amps for a 5AR4. Considering the internal impedance (regulation) of the transformer my best estimate is that there is closer to 2.2 volts to burn across a resistor rather than 1.3 volts assumed.
I can see the adjustments to the value of R in option number 1 or the number of turns removed for option number 2 being done empirically (under test, one turn at a time).
DT
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Old 31st October 2010, 05:43 PM   #16
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I'm not sure what your point is. I arrived at the 0.68 ohm by back of envelope calculation. The 5AR4 draws 1.9 A at 5.0 V.......The short version is: I am skeptical. I donít think that it is that simple.
Mr. OHM hasn't been proved wrong yet, and it IS that simple IF your 6.3 volt winding actually produces 6.3 Volts with the entire transformer operating under the same load it will see in the amp, AND your 5AR4 actually draws 1.9 amps. Now the two conditions listed above are rarely true, but the calculation is a good place to start, and probably closer than what you would get off of the 5 volt winding of a Hammond transformer.

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According to the seller the core is pretty robust, a bit over spec., so minor changes aren't too likely to be a problem unless you are pushing their limits anyway.
It is usually a good idea to have some reserve capacity and an added rectifier winding will eat about 10 VA of capacity. My experiences show that even a 1T200 doesn't get too hot when running at 125 VA. The 4T360 that I am using could easilly power two of these amps, so it doesn't break a sweat.

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I start to wonder what ga wire to use and if the added winding is going to be able to supply the needed 1.9 A the 5AR4 is going to draw. Is 18-20 ga big enough? What is the calculation to come up with the ga of wire?
I have seen dozens of charts and calculations to arrive at the needed wire size. They vary over a 10 to 1 range! Look at this one:
American Wire Gauge table and AWG Electrical Current Load Limits with skin depth frequencies

Note that a #20 can handle 1.5 amps for "power transmission" and 11 amps for "chassis wiring". A 10 foot piece of #20 wire has about 0.1 ohm. It will dissipate about 200 milliwatts at 2 amps and barely gets warm. I use # 18 and it doesn't get warm at all.

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I've been winding my own "custom" EI transformers years, using cores salvaged from discarded solid-state gear.
Back in high school electronics class we learned all about vacuum tubes. There were a few chapters in the books about solid state stuff, but no reall applications since it was a public school running on donated junk for labs. I wanted to play with some of the new fangled solid state stuff, but power transformers were not available. You could not buy a 50 VCT 4 amp transformer in any of the usual catalogs in 1968. Digikey and Mouser did not exist yet. We had Allied (still around) Lafayette, and Radio Shack. So, you take apart the power transformer from an old TV set. Remove all windings except the primary and wind your own secondary. I used ordinary masking tape for insulation between the layers and the transformers lived. In fact I recently found an amp that I made in 1970 and the transformer was still working. I use Kapton tape today.
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Old 31st October 2010, 07:53 PM   #17
tomchr is offline tomchr  United States
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Originally Posted by jrenkin View Post
According to the seller the core is pretty robust, a bit over spec., so minor changes aren't too likely to be a problem unless you are pushing their limits anyway.
My current draws will be:

350 V: 180 mA --> 63 VA
5.0 V: 3.8 A --> 19 VA
12 V: 2.2 A --> 26.4 VA
TOTAL: 108 VA.

That's assuming I succeed in adding a 12 V winding for my switching regulators for the filaments. I also need the transformer to fit in the Antek steel cover...

If the primary can generate enough flux and the core has enough cross sectional area to transfer 200 VA, it should easily be capable of handling the 108 VA I'm throwing at it.

I'll be using two 12 V windings. Each will have a current draw of about 1.1 A. Designing for 1.5 A with 3 A/mm^2 current density yields 0.5 mm^2 cross sectional area of the wire. Recall that A = pi*r^2, hence the radius of the wire should be 0.4 mm. I.e. the diameter of the wire be 0.8 mm. According to Wikipedia, the corresponding wire size is AWG 20 (0.812 mm diameter).

The 3 A/mm^2 current density was mentioned as a rule of thumb by my instructor in a switchmode class I took in college. The instructor had spent a fair amount of his life designing power supplies - including winding transformers for use in switchmode power supplies. 3 A/mm^2 was a reasonable compromise between wire gauge and ohmic losses (temperature increase).

~Tom
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Old 31st October 2010, 08:14 PM   #18
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Quote:
The 3 A/mm^2 current density was mentioned as a rule of thumb by my instructor in a switchmode class
The wire losses are slightly higher in a switchmode transformer than in a conventional transformer since the conventional transformer runs at 50 or 60 Hz and a switchmode transformer runs somewhere between 20 KHz and 2 MHz. The skin effect will reduce the effective cross section area of the wire as the frequency increases.

I have used a #20 wire at 1.9 amps before, so it should be cool at 1.1 A.
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Old 31st October 2010, 08:25 PM   #19
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Originally Posted by tubelab.com View Post
Mr. OHM hasn't been proved wrong yet, and it IS that simple IF your 6.3 volt winding actually produces 6.3 Volts with the entire transformer operating under the same load it will see in the amp, AND your 5AR4 actually draws 1.9 amps. Now the two conditions listed above are rarely true, but the calculation is a good place to start, and probably closer than what you would get off of the 5 volt winding of a Hammond transformer.
Hello,
Kirchhoff's second law remains in force as well. Every time I have done that back of envelope calculation for a heater voltage dropping resistor I have ended up chasing the value.
Mr. ohm needs to plug in the total impedance; the heater, the added voltage dropping resistor and the transformer internal impedance. From your approach whack it with a bigger hammer then back off a little.
Or to look at it from the other side it is easier to remove transformer windings than put them back in.
My original point is the same as yours, that 0.68R value will require some adjustment on the bench.
DT
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Old 31st October 2010, 09:06 PM   #20
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Or to look at it from the other side it is easier to remove transformer windings than put them back in.
My wood shop teacher keeps telling me that it is far easier to remove wood than to put it back, yet I keep screwing up!
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