The AM Forum

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The discussion the other night was "where will I find a 10.0 Volt, 20 Amp transformer for the filament of the 4X1 Rig Grant is building figuring on modulating it with 833's. I looked over on the shelf and spotted some iron I had removed from a 1500 watt UPS and a larger 2200 watt unit. The small one has a 14.0 Volt winding with # 10 or 8 square stock. I took my prick punch and found the 1/2 voltage point on the same side as the high current comes out. Inside winding on the bottom of the coil form. Scrape the varnish off the wire and solder the CT in place. Uses a small variac on the primary and instantly came up with the 20 amps at 10 volts. I tested it for several hours and it worked pissa. Don't throw out those units! Strip those suckers. The primary 120 volt winding is wound first. The secondary is easily available. If I had removed a few turns that would have worked also. I'll have this in the mail to Grant tomorrow. Everyone is throwing away those old UPS's. Grab Them!
Keith
WA1HZK

Hi Keith,

There is a filament transformer up on e-Bay right now that may just fit the bill. It is e-Bay item no. 150220936850, and was removed from a Gates BC-1G transmitter.

Check it out. 10 V at 20 A and two separate 10VCT at 10 A windings for the 833As.

73,

Bruce

Gates used individual center tapped windings for each mod tube and had front panel controls to set the bias on each mod tube. good setup

Yea but,....................4-1000A filament is 7.5v @21A ?? ??? ???

Whoa!!

As usual, Frank is right. And if anyone knows the specs on the 4-1000, it is Frank!

Well, you can still use this transformer for the 833A modulators!

Thanks, Frank.

73,

Bruce

The Iron was for the 833's. Find the CT, Split it, Parallel the windings & have 7.5 at 60 amps.
Still a good piece of Iron. I'll collect those old UPS's whenever they can be found. Did I mention that there is another 14 volt winding that will work as a buck/boost on those things!

PS
I gave the empty cases away for building E-Rigs!
Keith

microwace oven transformers can be striped and rewound to make nice fillament transformers, and if you leavve the plugs in they will act like current inrush limiters.

I have a couple of 7.5VCT? transformers that were used with 4CX5000's or 4CX10000's, but the primaries are 440VAC !! It is a pity I have no socket. There is one '5000 tube around here somewhere. -But I don't want to build anything that big, and plenty of projects already going on here.

If a jug requires 7.5 VAC I look for a 8-10 VAC filament transformer on purpose. Instead of step starting the filament transformer, I prefer to use a permanent dropping resistor for a gentle turn on. Or for 5 VAC I use a 6.3 VAC Transformer and resistor in series with the primary. 73  John

Thats a really good idea, I'l try it on my next project.

You don't want to use a filament transformer rated for a lot more current than the tubes pull, unless you have some kind of inrush current protection.

My preferred method is to use a transformer with excessive output voltage, and then put a wirewound rheostat or tapped wirewound resistor in series with the transformer primary to bring down the filament voltage to normal.  That gives you a filament voltage control to compensate for line voltage variations, and the  series resistance cushions the inrush current to the tube, thus prolonging filament life.

KL7OF mentioned having two filament transformers so that the bias can be adjusted for each modulator tube.

How is that adjustment accomplished?

73, Barrie, W7ALW

the gonset amp uses the tune-operate 4K 100W resistor for the same purpose. big warnings in the manual to have the switch in tune position before turning the unit on.