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Transformer problems

sabahan

Jul 14, 2013
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I was trying to find a 800V CT transformer for my tube amp project @ primary voltage of 240V,but I can only found a transformer with a 115V primary with 375VCT rating,so can I use the transformer and apply a 240 V over it,the output should close to 800V
 

KrisBlueNZ

Sadly passed away in 2015
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No!

But show us the schematic of the amplifier.

You can probably replace the rectifier toob with a bridge rectifier made from diodes, and you'll only need a transformer with a secondary voltage of around 370V.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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I would not recommend applying twice the rated primary voltage. You will likely saturate the core on the line peaks and overheat the transformer. Try purchasing two and connecting the primaries as well as the secondaries in series. You can also have a 240 V AC to 800 V AC center-tapped transformer custom-wound, price depending on power rating.
 

KrisBlueNZ

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Nice idea Hop but you can't connect centre-tapped secondaries in series!
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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D'oh! Brain failure. Of course you could do what you suggest.
The power supply I built for my Novice amateur radio transmitter was constructed with two transformers salvaged from TV sets. I wired the primaries in parallel of course for 115 V AC line operation in the States, but IIRC the high voltage secondaries were wired in series to provide AC to a 5R4GY dual-diode, common cathode, vacuum tube rectifier. Silicon diodes were available in 1966, but pricey compared to the "toob" version. I think later I did install silicon rectifiers in the base of a defective tube (after removing the glass and elements of course) but I liked the warm glow emitted from the tubes, especially at night with most of the room darkened while I pounded away on my telegraph key while trying to hear faint signals through my headset earphones.

One thing I found out that didn't work so well was wiring the 6.3 V AC filament secondaries in parallel in an attempt to provide more current. Unless the windings are identical and produce the same voltage, which is problematic from two salvaged transformers, there will be circulating currents among the secondary windings that will overheat the transformers. Not seriously overheat, but hot enough for me to realize something was wrong. I didn't need the extra current for my little transmitter, so I disconnected the extra filament windings. Back in those days there had to be a separate 5 V AC filament winding for the high voltage rectifier because the common cathode was at the B+ output potential. Hard to find that kind of transformer now because no one uses vacuum tube rectifiers any more... at least not for the relatively low voltages (0.5 kV to 4 kV) needed for amateur radio transmitters. I don't know about commercial practice, where the power required is in the tens of kilowatts or more, but I suspect silicon is king there too.

I have a couple of 15 kV neon sign transformers with center-tapped secondaries, but the center-taps are bonded to the case and (hopefully) power line ground. Those can definitely NOT have their secondaries connected in series, even if you "insulate" the cases by supporting the transformers on drinking glasses. Don't ask me how I know this!
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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One thing I found out that didn't work so well was wiring the 6.3 V AC filament secondaries in parallel in an attempt to provide more current. Unless the windings are identical and produce the same voltage, which is problematic from two salvaged transformers, there will be circulating currents among the secondary windings that will overheat the transformers. Not seriously overheat, but hot enough for me to realize something was wrong.
Hmm, that's interesting. I wouldn't have expected a problem.
Back in those days there had to be a separate 5 V AC filament winding for the high voltage rectifier because the common cathode was at the B+ output potential. Hard to find that kind of transformer now because no one uses vacuum tube rectifiers any more...
You could use an isolated DC-to-DC converter module e.g. http://www.digikey.com/product-detail/en/JCH1024S05/1470-1971-5-ND/4488351
I have a couple of 15 kV neon sign transformers with center-tapped secondaries, but the center-taps are bonded to the case and (hopefully) power line ground. Those can definitely NOT have their secondaries connected in series, even if you "insulate" the cases by supporting the transformers on drinking glasses. Don't ask me how I know this!
LOL :)
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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... You could use an isolated DC-to-DC converter module...
Yes you could, if you were serious about maintaining authenticity of appearance.

But what most hams did after about 1950 was to replace vacuum tube rectifiers with silicon diodes. They kept the vacuum tubes for RF power, but eventually changed to solid-state designs as high frequency, high-power, MOSFETs became affordable. Still, a resent issue, December 2014, of QST magazine (the journal published by the American Radio Relay League, or ARRL) featured a 1500 watt vacuum tube RF linear amplifier for all amateur radio bands from 80 m to 6 m. It was based on the 8877 power triode used in a grounded-grid design, which is typical of ham radio vacuum tube linear amplifiers.

In the January 2015 issue of QST a DIY 1250 watt MOSFET RF linear amplifier for amateur radio bands from 160 m to 6m, plus 2 m on a separate RF "deck," was featured. This amp uses a pair of Freescale (Motorola) MRFE6VP61K25H MOSFETs in a class AB push-pull linear design. No need for high voltage transformers, but 50 V DC at 30 A is necessary to drive the drains of the MOSFETs.

Low voltage DC operation has become the norm for commercial as well as DIY amateur radio equipment. Some don't even shy away from the notoriously noisy switch-mode power supplies, which can be tamed with proper engineering design and shielding.

I, OTH, still mostly stick with the tried-and-true line-transformer plus rectifier plus linear regulator approach. For example, the power supply for my Elecraft KX3 transceiver is a Variac-driven low-voltage transformer plus full-wave rectifier plus "brute force" electrolytic capacitor filter. It weighs in at about fifteen or twenty pounds and occupies a largish box about 8" wide by 12" high by 18" deep. I crank the Variac up to full output and it delivers about 14 V DC at 2 A or thereabouts. Why do this? Because I just happened to have it around doing nothing more serious than topping off the charge on my motorcycle battery. Since my wife has forbidden motorcycle riding until I get a trike conversion, I moved the power supply from the garage into my "shack".

The point is this: not many hams need high-voltage transformers, with or without filament windings, anymore. At the 2014 Dayton Hamvention® there was a booth in one of the exhibit halls where they were taking orders for custom-wound power transformers. They had brought along some examples of their work and they looked very nice indeed. Line-powered transformer engineering is not rocket science. You specify the primary voltage, the secondary voltages and currents, and they do the rest, selecting a proper-sized core and magnet windings. Not as cheap as an off-the-shelf transformer of similar specifictions, but not ridiculously over-priced either. Plus you get the design you want, not the design that happens to be available. This company (I forget the name) has the same booth year after year, so they must be doing something right.

73 de AC8NS
Hop[/QUOTE]
 
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