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Question: Wiring N68X iso transformer backwards to get 230VAC output

OK, here's another silly idea. I found a Triad N68X isolation transformer lounging around in my junk heap. (datasheet)

It has a dual 115VAC/230VAC primary, and a single 115VAC 0.435A secondary. Rated 50VA.

My silly idea is to use the transformer backwards to get about 230VAC 0.2A from 120VAC 60Hz mains (USA).

I've attached a diagram of how I think that would be wired.

I'd like to use this transformer to provide the DC plate supply for four 6P43P tubes in triode. I figure each will draw about 50mA (250V * 0.05A = 12.5W dissipation).

Do you think I can use this N68X transformer to get about 275VDC @ 200mA? Or is that 50VA transformer simply not up to that much of a load? (I could break down and buy a proper power transformer for this application, but that would mean spending money :D ).



  • _N68X_wiring_300VDC.png
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I'm 100% fine with using UF4007 rectifier diodes.
I figure I'll need a common CLC for the first stage of the power supply, so I'll get some voltage drop there. I have a few power supply chokes hanging around which I can throw at this project. One is a Hammond 159T, 2.5H 300mA 43R. That would drop about 9VDC right there. I'd be using cathode bias on the output tubes, so that's another 20VDC or so dropped off the plate-cathode voltage. That would bring that +310V down to +280V plate-cathode, which is within the limitations of the 6P43P when run triode.

I'm surprised this little 50VA transformer would be OK for this. I guess it's the lower voltage than I'm used to using (like +450V for four 6L6 or EL34 tubes).
On 2nd thoughts 0.2A @ 275V is 55VA so it's a little borderline but, one way to find out

Good point!
Yes, that is what I didn't consider. I thought there might be something.

Yeah, if there is 300V after rectification (230V*1.3 approx) and I put a 200mA load on that, then that equals 60VA. Now we're 20% over this transfo's 50VA rating.

Answer = Yes it can be done, but it would be borderline, or more likely the transformer will run hot and/or its core might saturate with big bass booms in the program material.

Oh well. Thank you for helping me think that through. I appreciate the help.
N68X will give you about 200 volts ac under a 30VA load.. I have just ried it last week on a project. (2x15 watt lightbulbs in series)

Real world experience trumps all. Thanks!
Hmmm.... It's rated kind of optimistically then.

Were your results from using it wired backwards? If so...
I guess this transfo is useful for preamps or headphone amps, but not for this kind of a stereo power amp. Time to buy something.
Most isolation transformers are not exactly 1:1. They are more like 1:1.05. Extra turns are added to the secondary to make up for the losses in the transformer. The exact ratio depends on the size and cost of the transformer.

Wiring it correctly and voltage doubling, you get 2X the (AC peak)

My testing with a N68-X got me about 320 to 330 volts of DC from a backwards N68-X into a full wave bridge made with UF4007's, and 330 to 340 volts when wired in the normal manner with a voltage doubler made with UF4007's. My line voltage runs 123 to 126 volts.

The N68-X will get a good deal hotter when running into a 50 VA DC load than it will when running into a 50 VA pure resistive load. This is due to almost all of the current being drawn in short pulses at the peak of the sine wave on the rectified load VS continuously on the pure resistive load.

I made a little guitar amp that ran a series string 100 mA AC heater (12.5 VA) and 165 volts of B+ at 75 mA (12.4 VA). The N68-X powers this perfectly and gets typically warm after hours of continuous use.

I made a bigger brother for it which ran a similar 100 mA series heater string, but added some tubes so I ran it on the 165 volt DC supply which also powered the output tube screen grids. (16.5 to 20 VA). The amp itself ran on a voltage doubler fed by the same transformer. (330 volts, 75 mA at full crank for about 25 VA, all silicon rectified). The N68-X DID NOT like this, and got quite hot at idle, and too hot to touch after some serious thrashing. That amp now has a 100 VA isolation transformer in it which works fine.

I have been in Florida for a while chasing the grandkids around Disney world, so I don't have my notes, or the ability to plug the amp in and test it. If you plan on doing a lot of amp building and testing you should get a Killa-Watt meter. They pop up on Amazon often for less than $20. It reads the VA and watts drawn by any line powered device and is a good tool for measuring and optimizing amps and their power supplies, and computers.

Many people spend big bucks for monster PC power supplies that they don't need. My recent Ryzen 7 PC build for 4K video editing sucks about 250 watts when running a full stress test to I used a 450 watt power supply....not the 650 to 1000 watt unit that's recommended on many web sites.
George makes an important point. Because of I2R heating considerations, you can safely access only a tad more than 50% of a winding's VA rating working into a cap. I/P filter. Adding fuel to the fire is the fact that the heating gets worse, as the capacitance increases. The charging pulses do get shorter, but the current in those pulses increases. Square variation divided by linear variation results in an increasing net value. :(

"Full wave" voltage doubling the O/P of Triad's N-68X and N-77U is an excellent source of low cost/high quality B+. The N-68X is good for "100" mA. and the N-77U is good for "200" mA.
Thanks everybody. Lots of great info, which I appreciate.

In the end, it all comes down to common sense and being a bit conservative with your design goals. I like the power transformer to stay only warm to the touch, not scorching.
As Eli wrote:
"Full wave" voltage doubling the O/P of Triad's N-68X and N-77U is an excellent source of low cost/high quality B+. The N-68X is good for "100" mA. and the N-77U is good for "200" mA.

That seems like a good rule of thumb. I have an N-51X and an N-68X, which I bought for trying with preamp builds. I suppose for this amp I'll get one of those nice Antek toroids. The 250V 100VA toroid costs about the same as an N-77U.

AS-1T250 - 100VA 250V Transformer - AnTek Products Corp
$48 with shipping.

Triad Magnetics - N-77U - Transformer; Isolation; Config Bobbin; 1; Freq 50/60Hz; Pri 115/230VAC; Sec 115VAC - Allied Electronics & Automation
$42.11 with shipping.