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
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
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
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
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?
There isn't anyone in Israel who winds custom transformers that you could talk to about making some real ones for you?
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.
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
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
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?
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
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.
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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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
Your original proposal was fine exactly as you described it.
something like............... the 1m ohm resistor goes from the loop between the two transformers it is not a ct. with back to back trannys you can get hot spots in the windings, which you can cool down by fitting a 1m ohm resistor from the low voltage LOOP TO GROUND.
it's not a ct at HT output
it's not a ct at HT output
I just built something like that. It's bread boarded but not yet put into a nice enclosure. I used 2 back to back 12V transformers. Then use the 12V to both drive the second transformer and to drive a DC heater. (It is a preamp using 12AX7 tubes so I run the heaters at 12V)
I may do this again but one a larger scale. I just got hold of an old golf cart charger with a 10 pound transformer inside.
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Rats!!...Just measured them & sure enough they are Auto-transformers. I really must now wind my own. I've read up on em..now to find a torched core that is big enough. The tricky one of course is going to be the OPTs'.
This country MUST be the scrounge capital of the world...nothing is thrown out, nothing is wasted. I found an old TV on the side of the road here once ...totally gutted, stripped.
_______________________________________________________Rick....
Several years ago when I started messing around with tubes I also got a pair of "step down/up" transformers for back to back connection. since I had no real electronics education, I used to measure everything possible (like a blind man who touches everything around him to get a grasp of the environment
). well, luckily my measurements showed that they BOTH had "shorts" in themI took them back to the store as "faulty"
, and came back home with a lessonAs for the "throw away" issue, I can relate to your frustration but from a different angle. since my country was pretty much in "dippers" during the golden era of tube gear, most people had no money for "luxury" items, so these days you can't find much tube gear in flea markets or second hand billboards
Here is an interesting issue to know about when doing back to back:
http://www.diyaudio.com/forums/showthread.php?p=1185433
Since some of my bigger 6.3V transformers are actually 2x6.3V (not 6.3-0-6.3), so if I'm paralleling the two secondaries and connected them to a 6.3>230 for HT, does this mean that if the second transformer's primary inductance is the same as the primary on the mains, I'm actually double paralleling it? which means triple the magnetizing current?
Btw, my reason to parallel the two 6.3 secondaries on those transformers and not use one for the heaters and the other for "back to back" is simply current distribution. if I have 2x6.3V@3A and I need only 1A to drive the HT and +3A for heaters (or the other way around), the load will balance over the two secondaries.
http://www.diyaudio.com/forums/showthread.php?p=1185433
Since some of my bigger 6.3V transformers are actually 2x6.3V (not 6.3-0-6.3), so if I'm paralleling the two secondaries and connected them to a 6.3>230 for HT, does this mean that if the second transformer's primary inductance is the same as the primary on the mains, I'm actually double paralleling it? which means triple the magnetizing current?
Btw, my reason to parallel the two 6.3 secondaries on those transformers and not use one for the heaters and the other for "back to back" is simply current distribution. if I have 2x6.3V@3A and I need only 1A to drive the HT and +3A for heaters (or the other way around), the load will balance over the two secondaries.
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