This has probably been discussed many times before but I can't seem to find any threads on the subject.
I need 240VAC at approx 160VA plus 6.3VAC at 50VA for a stereo EL34 40W/Ch amplifier.
Such a transformer seems impossible to source.
Are there any drawbacks to using two commonly available 6V transformers back to back ?
I need 240VAC at approx 160VA plus 6.3VAC at 50VA for a stereo EL34 40W/Ch amplifier.
Such a transformer seems impossible to source.
Are there any drawbacks to using two commonly available 6V transformers back to back ?
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Yes you can. The main issue is power loss in the transformers means 250VA in the first might be 220VA on the secondary, then down again in the step up. The secondary to primary step up is limited by the secondary's VA not the primary's so say 220 goes down to 190 (I am making these example figures up). So it's not quite as easy as it looks.
Also there'll be some EMI radiated from those lumps to deal with.
I'd suggest getting a transformer custom made to your spec. It'll do what you want, be safe, and be a lot smaller. JMS make them for example. They aren't expensive and they are pretty fast IMHO.
Also there'll be some EMI radiated from those lumps to deal with.
I'd suggest getting a transformer custom made to your spec. It'll do what you want, be safe, and be a lot smaller. JMS make them for example. They aren't expensive and they are pretty fast IMHO.
There is a possibility to choose a better transformer like R-core. There are a few sellers on Ebay that offer custom wound R-cores. I would let the primary winding be wound with a 250V tap just in case the secondaries are slightly too high under load.
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In the end I opted to go for two transformers. One providing the 350VDC HT and the other the 6.3VAC heaters. I've hidden them under identical transformer covers so the size difference is immaterial to the overall aesthetics.
This is a different solution to the one that I found in Post #6
This is a different solution to the one that I found in Post #6
Interesting! What's the advantage of R cores over traditional EI cores with tube output transformers?
Looking at the Softone website, they have a few nice no-nonsense thingies.
I've gone down the route of the original article.
T2 is a 250VA 240V - 240V isolation transformer. (Toroid)
T3 is a 50VA 240V - 6-0-6V (Toroid)
T4 is a 20VA 240V - 20-0-20V (EI)
T2 is a 250VA 240V - 240V isolation transformer. (Toroid)
T3 is a 50VA 240V - 6-0-6V (Toroid)
T4 is a 20VA 240V - 20-0-20V (EI)
I've gone for Hammond 1650H for the output transformers. This is EL34 in push pull at approx 45W.