back to back transformers...firefly

[swt]

Hi. I'd like to use a pair of back to back transformers to build a firefly. is this correct??: the first one must have enough current for the whole amp, including filaments. the second one can be smaller as it will only get current for the plates, etc. If i use a voltage doubler, i should double the current ratings.
Can i use for example: 220v-6v and 220v-9v to lower the voltage a bit??. Thanks a lot for your replies!

[puretube]

...Can i use for example: 220v-6v and 220v-9v to lower the voltage a bit??. ..

yes.

[bancika]

I used similar configuration for my hotbox build but I guess that transformers didn't like each other very much. They were both running very hot and finally first one died (even though it was rated 1.5A and hotbox has only two tubes) after about half an hour of playing with hotbox. Someone on forum told me it was impedance problem
Maybe you'll have better luck with back-to-back

[brett]

Hi
Back-to-back works with some limitations.  (your household supply comes from btb transformers)

I'm no expert, but I think that obtaining reasonable amounts of power from the second transformer requires it to be well magnetised.  The magnetisation current is sometimes quoted for transformers.  In Australia, most small transformers that I'd use to set up need about 25mA at 240V (6VA).  For a 6V transformer means a load of 1A.     Circuits that don't draw much current from the second transformer might only need a fraction of this, but it's better to err on the safe side.  It might explain your meltdown.

cheers

[bancika]

IMHO it's not worth it. Here one can get 20W toroid transformer for 10euros. It's custom made with as much secondaries as you need, CT or not, etc...and it's much smaller than B2B.
edit:
by the way, I now realised that it was my fault that previous transformers died (not so clever cap discharge switch) 
If you already have those transformers give it a go...Firefly is great little amp, I've built one myself...very smooth lead tone

[R.G.]

IMHO it's not worth it. Here one can get 20W toroid transformer for 10euros. It's custom made with as much secondaries as you need, CT or not, etc...and it's much smaller than B2B.

If I could get a custom toroid for 10 Euros, I'd say that it wasn't worth it too. That is not the case in the USA. There are many places where custom toroids are not available at  reasonable prices.

So, tell us - does your toroid provider ship worldwide? 

[bancika]

Actually I think they do ship anywhere
url is http://www.trafomatic.com/
20VA transformer costs 7.5eu IIRC without shipping no matter how many secondaries you have, also you can choose to make it shielded, CTs, etc...I bought two from them so far, one for firefly and one for hotbox. Very nice quality and you get mounting hardware...I'm really satisfied

[brett]

Hi
that's incredibly cheap.

For a Firefly or a real mctube, the transformers can be got from old wall-warts.  A charity here has a second-hand computer store where they sell donated stuff.  They sell wall-warts for $1 (US$0.75).  I feel so guilty for buying them so cheap.    
(But I gave them my old computer, so they got something at least).
cheers

[R.G.]

That is incredibly cheap. If they can deliver this stuff at those prices, there's no reason to do back to backs.

Back to back transformers are what you do when you can't get the right tube transformer and don't have the ability or skills to wind your own. Custom toroids were not a possibility up until recently. Toroids sold for a major premium because they were hard to wind. The introduction of automated toroid machines like in the pictures at that web site have had a major effect. As late as five to ten years ago, toroids were impossibly expensive.

This is really good news. If that company can do it, others can too.

[bancika]

I'm little supprised that you guys don't have this, I mean this is Serbia and we are usually 20 (or more) years late compared to the rest of the world. But good thing here is that you can do custom jobs for little quantities, and I'm not saying only for transformers. When I mentioned laser cutting, engraving and similar here some guys told it's impossible in "western" world cause nobody wants to do small batch (of 1 piece). Here this is not a problem, 1 transformer, 1 engraved panel - doesn't matter. You get quantity discount, but it's not a problem to order small..
Cheers

[R.G.]

It is an odd fact, but true.

In the USA, it is very difficult to find a supplier that will make one of anything at a reasonable price. Any supplier that will do one of something treats it somewhat like you'd expect for commissioning art - very expensive. The idea that one could get one custom transformer at around USD$1.00 per watt is unreasonable in the USA.

I think that it may be because the USA and other countries have become very good at harvesting talented individuals into more highly paid jobs. Within the memory of living people in the USA, it was possible to find a really smart, inventive plumber or carpenter. That is now almost impossible, because anyone smart enough to be a genius plumber or carpenter also has the ability to make much more money doing something else, and the opportunity to do that almost always gets presented to them. There are a few such people, ones who do a semi-technical trade and are very intelligent but willing to work for less money than they could get at some other profession, but they are rare.

So smart people who could do a good job of making one-off custom transformers don't because they can do better for themselves at something else. Or they charge a lot of money for it.

Just one more facet of the MBA mentality, I guess.

[bancika]

in case you didn't find pricelist, here's link http://www.interhit.co.yu/download/TorusniTrafoi.PDF
It's in serbian, but you'll get the point 
Cheers