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Old 19th September 2009, 04:02 PM   #1
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Default "Back To Back" transformers

Hi,

Since HT capable transformers are not available in my part of the woods, many times I use back to back transformers to achieve isolated high VAC for temp/breadboard builds, taking advantage of the local 230VAC mains here.
It's a quick fix but also a pain because each build uses three transformers, a "back to back" pair for HT and a third transformer for heaters. a "non 6.3V" tube rectifier of course bump the transformers count to four :-P
Question is, in order to cut on size and weight, can I use a single 6.3VAC transformer as heater supply - with the two 100 ohm resistors as virtual CT to ground or elevated DC ref - and also to drive a second transformer (6.3>230) for HT?

Thanks :-)
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Old 19th September 2009, 06:35 PM   #2
kevinkr is offline kevinkr  United States
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Yes what you propose is perfectly possible, you will just need to appropriately upsize the transformer providing the 6.3V power to the other transformer and now the filaments.

There isn't anyone in Israel who winds custom transformers that you could talk to about making some real ones for you?
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Old 19th September 2009, 07:11 PM   #3
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Quote:
Originally Posted by Hendrixon View Post

...can I use a single 6.3VAC transformer as heater supply - with the two 100 ohm resistors as virtual CT to ground or elevated DC ref - and also to drive a second transformer (6.3>230) for HT?
I say yes assuming the transformer has the current capacity. Any DC applied to a virtual CT will cancel out in the transformer's windings and cause no trouble.

Oops, Kevin beat me to it.
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Old 19th September 2009, 08:09 PM   #4
pointy is offline pointy  United Kingdom
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i would have thought you may need to use resistors to control how the current is split.

also it's good to have a 1M resistor to ground from the 6.3v loop to help keep things cool.

and if you are using 6.3v you will need a good few amps.
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Old 19th September 2009, 08:23 PM   #5
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Good to hear that
Of course, the 1st transformer will be of adequate size for both tasks.

Few years ago I went to a known local company here that manufacture transformers (wall warts, autotrafo, industrial isolation, toroidals, 115<>230 etc.). wanted them to make me a multi-tap HT transformer. it took me about 20 min' to explain to them that I don't do drugs and yea "I built audio amps with 400VAC transformers more then once"

You see, anything above 24V is custom order and they don't even remember ever building a transformer with more then 60V secondaries. when the sales rep gave up and said he has no idea if they can even make such a thing, he called the manufacturing manager there that, after hearing what I need, said:

"can't be, you're wrong, but if that's what you want we'll make it, no returns, no guarantee"

Well, I made my order (secondary with several taps from 80V to 250V, current 200mA), didn't pay attention when the guy said "0.5A is the smallest one I can make and 1A will cost you the same".

A week later I went back there to pick it up. it had several wires per the taps ordered, but it looked weird, too small. a quick voltage test showed very LOW voltages. what happened was the morons at manufacturing "figured" that 80V and 250V is a mistake, so they built it as 80 "wire turns" up to 250 "wire turns"

!!@$#$!%!&^!&$%&^$!!!!! <-- (enter your favorite curse here)

Back to back transformers is not perfect, but it doesn't saturate my nurves either
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Old 19th September 2009, 08:38 PM   #6
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Quote:
Originally Posted by HollowState View Post
I say yes assuming the transformer has the current capacity. Any DC applied to a virtual CT will cancel out in the transformer's windings and cause no trouble.

Oops, Kevin beat me to it.
You mean that about the use of elevated ref DC for the virtual CT, right?

Btw, up to what level does such a connection simulate a CT transformer? can you actually use such a setup to drive a full wave rectifier?
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Old 19th September 2009, 08:53 PM   #7
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I used a transformer to drop 240VAC to 18VAC then another transformer to up the 18VAC to 180VAC. I then rectified the 18VAC to get a DC voltage for the heaters through a voltage regulator.
So job done with two cheap transformers.
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Old 19th September 2009, 09:16 PM   #8
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Quote:
Originally Posted by pointy View Post
i would have thought you may need to use resistors to control how the current is split.

also it's good to have a 1M resistor to ground from the 6.3v loop to help keep things cool.

and if you are using 6.3v you will need a good few amps.
I'm not sure I follow... which current split control?
And how exactly you go about using a single 1M resistor? I mean what it does that a CT (virtual or real) don't?
Define "cool"

The first transformer that I will try this with is a 6.3/5A I have here. as the "main transformer" it should be enough to drive a multi channel preamp with several triode and pentode tubes or even a small SE guitar amp... of course as long as the power tube is not a hot biased KT88 and such
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Old 19th September 2009, 11:56 PM   #9
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Location: Mar del Plata, a BIG seasonal getaway city, can see the Ocean from our residence.
I'm having the same difficulty finding HV trannies down here too....I called a manufacturer in Buenos Aires, some eight hours away up North. They hadn't a clue what I was talking about. I think they are a distributor & not a true manufacturer.
So I will run two of these 220-110, 750 Watt versions. These step-down TX are all over the place at reasonable prices. I'm shooting for 330 VDC...so its' just right.
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Last edited by Richard Ellis; 19th September 2009 at 11:58 PM. Reason: Typo
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Old 20th September 2009, 11:28 AM   #10
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Quote:
Originally Posted by Richard Ellis View Post
I'm having the same difficulty finding HV trannies down here too....I called a manufacturer in Buenos Aires, ...
http://www.bazziamps.com.ar/transformadores.html
cheers
JJT
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