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Question about using Antek Toroidal as an Output

I'm thinking about using an Antek toroidal power transformer as an output transformer. For those who have tried this:

1) What VA ratings did you have luck with?

2) What kind of low-frequency performance could you get away with before core saturation became a problem?

3) Feedback stability. What sort of leakage inductance and shut capacitance did you measure? Alternatively, how much feedback could you apply before it was impossible to stabilize?

I'm thinking of using these for an 829B or 5894 amplifier. Possibly using class AB2, theoretically I might be able to get 40W out with a 600V plate supply, a 6k primary impedance and a 200V screen supply. Spice simulations look promising, but I had planned on using somewhere between 15 and 20dB of global negative feedback. I know I can get 16dB with the Hammond transformers, but I don't really know what I can expect with some cheap Antek toroids.

People have reported good results, so I'm hopeful.

Thanks,

H713
 
It seems that all the successful projects using mains toroids as output transformers are running relatively low primary impedances, using tubes with low plate resistance e.g 6as7 or triode connected sweep tubes. My experiments are with a hybrid drive at primary impedance of only 400-600 ohms. With a 120VA 115+115:30V Multicomp toroid it would saturate at about 15Vrms @ 20Hz or 19Vrms @ 25Hz. So about 25% higher relative voltage than the rated 30V @ 50Hz (which implies 15V @ 25 Hz). Saturation came on very suddenly and strongly so I would keep to a max of 10% over nominal. For a 230V primary, max voltage would then be 253Vrms @ 50 Hz. This equates to 10.7 watts @ 50Hz running into a 6k primary impedance, or only 1.7 watts at 20Hz. With 2 toroids in series (see Kodabmx's posts) you could get 4x this power.

I haven't measured leakage inductance and shunt capacitance, but high frequency performance is very good, and 20dB of feedback has been no problem. Although this is with low plate resistance triode drive, will be worse with pentodes.
 
If you want to try with a higher impedance valve then you could use the 2x toroid series connection and connect all the secondaries in parallel.

If you wire two discrete toroids in series make sure you use 1 section of each transformer on either side of the center tap to avoid any DC offset.

A few other points to consider.... the VA rating of the transformer has little to do with actual output power since you are essentially looking at line voltage (230V @ 50/60hz) as your determining factor. Assuming that a higher VA rating will allow you more power is a move in the wrong direction since lower VA transformers will have more inductance and higher copper resistances it actually becomes a tradeoff. I find that choosing a VA rating in line with the expected power output is a good place to start.

dave
 
I have used the 10 VA Anteks for a few single ended parafeed headamps with success. The highest voltage swing through the primary was 20 volts, so saturation is a nonissue. I avoid feedback all together, so I can't comment on the stability. I have also used 10VA Anteks for a 6N6P parafeed push pull output, but I don't recall ever taking measurements. I do remember it having very clean and deep bass, and was easily the most detailed push pull circuit I have heard to date... but I have only built a few.
 
I have used the 10 VA Anteks for a few single ended parafeed headamps with success. The highest voltage swing through the primary was 20 volts, so saturation is a nonissue. I avoid feedback all together, so I can't comment on the stability. I have also used 10VA Anteks for a 6N6P parafeed push pull output, but I don't recall ever taking measurements. I do remember it having very clean and deep bass, and was easily the most detailed push pull circuit I have heard to date... but I have only built a few.

Mmmm I'm breadboarding parafeed headphone amps right now. 120v primary? what secondary voltages did you use? Most of the Anteks have dual primaries and dual secondaries, did you hook these in series/parallel in any particular way? Or did you leave a coil unused somewhere? If this works out well, you could pretty much put any amp in front of an AC coupling right? To try a bunch of different headphone amps. Also if this works out, at Antek prices, you could load up several OPTs for low, medium and high impedance phones. Sowter has a nice mu metal multi impedance parafeed Headphone X former but it's pricey, I have a cabinet full of Anteks already. Or maybe with the right secondary voltage having two secondary's, one could figure a winding to get 3 outputs imps, series, parallel, or just one coil for the medium impedance? For low power headphones and parafeed, it seems like you can do a lot with cheap torroids over these expensive and hard to find headphone X formers.
 
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I've never used the Anteks, but Triads work well... The larger the VA rating, the less loses within reason. I wouldn't use these for much higher than 3K Ra-a though, as might higher implies large voltage swings. These coils work better for lower voltage with higher current (like my triode connected 6P45S amps that run 1k3 Ra-a, and put out over 110W@30Hz). NFB stability is better than with Hammond OPTs, and the frequency response is much better, too.
 
I have a set of old 55 ohm AKG's. Right now I am using a 6p36s with the screen as the anode in a spud SE parafeed configuration. The transformer I am using is a 115+115:12+12 with the primarys in series and the secondaries in series. This gives me a reflected load around 5k with the 55 ohm headphones. Toriods have a knack for having high parasitic capacitance, but biasing the output tube with enough current will alleviate any high frequency rolloff. Right now I am biased at 50mA, -10 volts grid-cathode, and roughly 120 volts on the "plate". I also used a 6p15p on another amp with a 115+115:5+5 giving a reflected load over 100k. This attenuated the volume, but came with the advantage of using a much smaller coupling capacitor... .47uF
 
... the VA rating of the transformer has little to do with actual output power since you are essentially looking at line voltage (230V @ 50/60hz) as your determining factor. Assuming that a higher VA rating will allow you more power is a move in the wrong direction since lower VA transformers will have more inductance and higher copper resistances it actually becomes a tradeoff. I find that choosing a VA rating in line with the expected power output is a good place to start.

dave

If you do a search for using mains toroids as output transformers there are repeated recommendations to use VA ratings many times the actual power output, due to the belief that the larger core is less prone to saturation. I never understood the logic of this as its the voltage/frequency that is the main factor. It's good to see a respected transformer winder backing this up. Although maybe the larger cores are a bit more forgiving with DC imbalance?
 
If you do a search for using mains toroids as output transformers there are repeated recommendations to use VA ratings many times the actual power output, due to the belief that the larger core is less prone to saturation. I never understood the logic of this as its the voltage/frequency that is the main factor. It's good to see a respected transformer winder backing this up. Although maybe the larger cores are a bit more forgiving with DC imbalance?

This was something I had intended to measure over the weekend, but my 40 year old scope seems to have finally given up the ghost. It was my understanding that a 230 volt primary rated at 50 Hz will only be good for about 90 volts peak to peak at 20 Hz before saturation.
 
My 6CW5/EL86 push pull amplifier uses Antek 50VA transformers with dual 5 volt secondaries wired in series. With an 8 ohm load I'm at ~4100k reflected load, it's flat past 50khz, and stable with global negative feedback to the cathode of the input tube. I'd build it again in a heartbeat.

My flea amplifier has noticeably rolled off high frequency, but it was meant as a cheap and easy build with the cheap 10VA toroids, and has respectable bass response for a one watt amplifier. A more hifi version could be done with a few parallel tubes into a lower DCR toroid at a lower reflected load.

In the long forgotten past (seven years or so ago?) I built a 5 channel home theater amplifier with Antek toroids as output, with 6AS7G tubes, for something around 1500k reflected load, I forget the exact model number. They were flat from 40hz to past 60khz or so, but I was maybe putting 9-10 watts through 25VA units. They were stable with the same sort of feedback as the EL86 build, and it was practically the same circuit, just with 6SL7 rather than 6N2P up front.
 
My 6CW5/EL86 push pull amplifier uses Antek 50VA transformers with dual 5 volt secondaries wired in series.

I built a guitar amp that used UL84's in push pull into a 50 VA Antek. I am 1000 miles from home now so I can't say for sure but I think it was a 9 volt unit with the secondaries in parallel. The UL84 is identical to the 6CW5/EL86 except for the 45 volt 100 mA heater. The amp ran 340 volts of B+ and 170 volts on the screens in cathode bias and made 25 Watts. I remember that the frequency response was far better than expected and good enough for HiFi, but that probably meant 30 to 40 Hz on the low end, not 20.
 
I once thought about going the other way - using a big one rather than a little one. The problem is getting a primary that can handle some voltage. If you used a 15T950 with a 4 ohm load on the 120V “primary” you’d get about 1k a-a on the 950-0-950 winding. And that would support a lot more than 170 volts peak. Probably could use 500 or 600 volt plate and get some actual wattage. Where else would you get a 500 watt OPT for $200? One of the bucket list of “retirement” projects is to try something like this out.

They also have a “AN4T400” which would imply that it doesn’t have the belly band. One could “ignore” the 120V winding (or use it for feedback), use the 400-0-400 plate to plate, and add a lower voltage 4/8 ohm secondary of the appropriate power level. That would get you somewhere in between the big one and the typical 50 VA 6-0-6.
 
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Interleaving the winding gives you "full fidelity" (30Hz - 50kHz+)

If anyone has a better term than "interleaving" I'm all ears BTW.

I get 110~ watts from PPP 6P45S in triode connection with this set up using a pair of VPT18-13800...
 

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