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[2E0ILY]

Last week I acquired a fairly big single phase 180 to 240 volt 50Hz (I am in the UK where the mains is 230 volts at 50 Hz) high voltage transformer. Secondaries are 6660 volts. Photos aew linked to at the end of the post. I was looking for a transformer to run a 3CX300A7 in a Henry RF generator being converted to single band amateur usage, but this turned up dirt cheap. Now the secondary voltage is too high, but the windings are on two bobbins with a very accessible centre tap joining them in series. Could I parallel them up instead? Any other ways to drop the secondary voltage and keep good current? Was thinking industrial size 230 to 110 tool transformer, buck transformer or a big Variac. This thing is apparently built to run full rated output continuous duty, so it would have some overhead on a lesser duty cycle. It looks pretty horrible as it's out of a wood glue drying machine, so it's covered in wood dust at the moment, but will blow off with the airline and fettle up OK. Thanks for any ideas.

http://www.gatesgarth.com/transformers/rfman4.jpg

http://www.gatesgarth.com/transformers/transformer1.jpg

http://www.gatesgarth.com/transformers/transformer2.jpg

[W8JI]

From all I can see, the transformer you have links the flux from winding cores into a common steel core running horizontally along the top and bottom.

This can cause some very weird things to go on when the vertical cores are run in common phase. In three phase systems flux does not add to double or subtract to zero in the common core area like it will in a single phase system. 

[KC8OYE]

I'm curious on an unrelated note.. is that 3CX300A7 or a 3CX3000A7?  I can't find any spec sheets on a 3cx300... and we have two broadcast transmitters on 98.9MHz and 105.5MHz running 2.1KW and 5.5KW respectively that both use 3CX3000A7's.. I'm curious if that IS a 3000A7 how that tube works running as low as 1.5KW   I was trying to talk the owner of 98.9 into letting me have his transmitter when he retires it

I figured a 1969 CCA FM3000D 3kw transmitter would make a FINE 2m FM amplifer 

[2E0ILY]

I'm curious on an unrelated note.. is that 3CX300A7 or a 3CX3000A7?  I can't find any spec sheets on a 3cx300... and we have two broadcast transmitters on 98.9MHz and 105.5MHz running 2.1KW and 5.5KW respectively that both use 3CX3000A7's.. I'm curious if that IS a 3000A7 how that tube works running as low as 1.5KW   I was trying to talk the owner of 98.9 into letting me have his transmitter when he retires it

I figured a 1969 CCA FM3000D 3kw transmitter would make a FINE 2m FM amplifer 

Sorry, it's a typo, I have left my reading glasses at work so I am a bit hit and miss with things today. It's meant to read 3CX3000A7 It's in a UK sourced Henry 2000D RF Generator, that was built for UK 3 phase. A US sourced mod to make it run on single phase by running the outer two transformer primary bobbins in parallel, and removing the resonant choke and fitting a capacative filter array. In this new format it  produced only 2.5kV. Using the other primary taps that are available sent the HV to 7kV and I thought I'd blown a rectifier module. The only thing that I am doubtful about with the conversion to single phase using the original Henry transformer (and it is a different part number to the US schematics, so presumably built for our 50Hz, on which it ran happily for years at a UK university plasma heating soil samples), is whether testing it with the zero load on the rectifier / capacitor array was liable to let the secondary voltage level rise too high. In use in the generator the RF deck would be connected and powered up before the HV was switched on.

Having acquired this other transformer above, I was wondering if the 6.6kV HV output from the secondariness could be toned down and it pressed into service until I can afford to have a bespoke transformer with several secondary taps wound, that I can use for other projects as well, utilising its various secondary voltage capabilities.

I am told a 3CX3000A7 based amp will work to quite low power levels if you just don't put much drive into it. Until I get an HV PSU sorted it is very QRP indeed

[KC8OYE]

in that 1969 CCA FM3000D that i was talking about.. we had constant problems with the IPA unit that used an 8122 tube (discontinued)  I rebuilt the entire IPA.. the transmitter ran at full output for about 10 days, then just quit..  turns out the 8122's that we got as 'nos' were actually used.. and couldn't handle it..

anyway.. we wired the 20watt exciter directly into the 3CX3000A which operates in a grounded-grid configuration.. as best we could tell the tube was making about 200-300 watts out with 20watts in, but with some noticeable reflected power to the exciter.. (it was a makeshift move)

since then we got a network analyzer on it, tuned the input circuit and added a solid state 600w IPA
now we're driving the 3cx with about 225-250w and getting about 2.1KW out of it
(the station is licensed for 2,140watts)  that little 'what if' bug got ahold of us.. we drove the tube with 500w and managed to get all 3,100 watts out of it that the CCA is supposed to be capable of

[KH6AQ]

Idea to reduce secondary voltage:

Wind additional primary turns over the outside of the existing windings. Connect them in series with the existing primary windings.

[2E0ILY]

That's interesting regarding adding more primary winding. Does it matter that the original primary winding is inside (concentrically speaking) the secondary, with about a half inch air space through a slotted former, for forced air cooling? Do I need to ensure the wire is the same gauge as the existing primary winding? One tends to take transformers as a given, all this has made me think a lot more deeply about these devices. Thanks.

[W8JI]

That's interesting regarding adding more primary winding. Does it matter that the original primary winding is inside (concentrically speaking) the secondary, with about a half inch air space through a slotted former, for forced air cooling? Do I need to ensure the wire is the same gauge as the existing primary winding? One tends to take transformers as a given, all this has made me think a lot more deeply about these devices. Thanks.

What Dave said works. It adds ESR but that might not hurt much. Use the largest gauge practical, and no smaller than the primary now. I use it on rare occasions to add a LV winding, but there is no reason it will not work as a buck-boost. That was a great suggestion.

[2E0ILY]

Thanks for that. What about running it not as two paralleled cores, but as a centre tapped transformer leaving the  secondary windings as they are now, in series, but with a spur off the accessible centre tap wire? As in http://upload.wikimedia.org/wikipedia/commons/5/5c/Fullwave.rectifier.en.svg

I am wanting to maximise current and halve the output voltage or thereabouts.

I have more detailed photos at http://www.gatesgarth.com/transformers/transformers.html

[KH6AQ]

Yes that would be the best way to use the transformer.

[AA4HA]

In the picture "transformer1" am I correct in seeing that the primary is wrapped under part of the secondary winding on the bobbin on the right side?

If so, it might be problematic to add turns to the primary winding as your additional turns would need to be carefully worked into that slot. I feel "edgy" about overwinding additional turns on top of the secondary high voltage winding. (just my fears of high voltage breakdown and a flashover from secondary to primary, making the thing into a giant tesla coil).

Overall it is a beautiful transformer. It looks like there is some damage to the phenolic and the ceramic insulators on the high side of the transformer. I would be tempted to make a new board out of thick acrylic or plexiglass and get a new set of HV insulators to replace that board. I like the little temperature sensor on top of the transformer, not a bad idea for cooling fan control or an alarm.

Too bad the voltage conditions are not perfect. I would be "antsy" about trying to make this into a split winding secondary unless I really understood how many turns are on each bobbin and if this would act like a 50/50 secondary with the phasing all making sense.

How about the idea of using a choke in series on the secondary to lower the secondary voltage? it would be a big one to handle 6 KVA but you could size it to drop the right amount of volts and improve the output of the rectifier/capacitor setup with several stages of capacitance- inductance - capacitance.

[2E0ILY]

I know people realise I am not 100% sure of what I am doing, but be assured I am playing safe. I can only learn by asking what I hope are sensible questions and slowly assimilating the answers. I am well aware that a shock off this thing could be fatal! With the windings just as they are I took a probe off the original joining wire on the inside of the secondary windings, and shorted the two original terminals on the ceramic HV take off insulators. I applied power via the same 5 amp fuse I used to check it worked ok "as is". I used a 20 amp Variac to apply the power slowly, as I have been doing before. It was immediately obvious from the hum at even 8%  on the Variac dial that it was not happy. A tad more and the fuse blew. I would have to assume the secondaries in this configuration are not in phase. I will probably split the centre "tap" wire and reverse the secondary phasing, as much as a learning exercise as anything. I am erring on running a ful wave rectifier on it, with a spur off the centre "tap". It's rated for 24 hours a day 5.75 kVA (it's from a wood glue drying RF generator that originally had two Mullard T-Y6800 tubes in it. In that configuration it had temperature sensor triggered fan cooling. I am wondering what sort of DC current it should give in low duty cycle operation, probably with permanent fan cooling and the temp switch left in place to effect a shut down should it get too warm?

Whilst paralleling the bobbins in the correct phase will give double the current at half the voltage, I seem to be entering a region where there may be other variables at play.

I don't feel confident adding primary windings over the secondaries. The OE primary windings seem to be copper tape, not wire. As it stands the primaries are around the core, on a thin former, then there are over a half inch wide phenolic type formers with air gaps separating the outer secondary windings by that distance. The primary and secondary windings are totally separated by the plastic spacers forming the "tubes" for the cooling air passages. I have new ceramic terminal insulators on their way, and have a new piece of material to replace the slightly cracked panel they are on, thanks for pointing them out. They will get changed once its final format is decided. I think I may be worrying about it not giving enough current with a centre tapped full wave rectifier circuit, as opposed to paralleling the secondaries. The 3CX3000A7 can run about 1.6 / 1.8 Amp plate current? Thanks everyone! Here are better photos of how the primary and secondary windings are separated:

http://www.gatesgarth.com/transformer3/transformer3.html

[G3RZP]

It looks a bit as if the HV secondary is split between the two legs of the core. Assuming that is the case and the two sections are equal, opening the centre tap should give two 3300 volt secondaries, with the outer ends in antiphase. You can check this by feeding the primary from a low voltage transformer -say 4 volts - when you should get about 55 volts from each side.

With 3300 - 0 - 3300, you would get around 4.5kV with capacitor input and around 2600 or so with choke input.

You will need a dedicated 30 Amp cooker type spur for the mains, too. What is the mains like where you live - many country areas will see appreciable sag when that sort of load comes on, and most UK properties are good only for feeds of 60 Amps - maybe 100 in newer ones. I presume you are well away from neighbours, because if not, EMC will almost certainly be a problem - and in your own property, too. You need a good separation from your antennas, for it is not unknown for the RCD (GFCI in US) to get upset with RF. Consider, too, the ratings of antennas and tuners in terms of voltage.

Additionally, although Ofcom turn a bit of a blind eye to exceeding the power limit, if they find you running 5 times the licenced power, they may well get a bit uppity!

[KH6AQ]

The full wave center tap bridge will give the same current rating as paralleling the secondaries and running full wave bridge. The RMS current per secondary winding is the same with either configuration.

[G3RZP]

The original probably used the self rectifying oscillator arrangement - quite common in industrial heating.

But once you get above a power level that needs more than a 13 Amp socket can supply, it's getting to be a whole new ball game.