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ECA 1102 filament transformer

Discussion in 'Amplifiers' started by Crusher, Feb 24, 2021.

  1. Crusher

    Crusher Well-Known Member

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    anyone ever use one of these. These were used in the Henry 8k amplifiers. I picked up a couple a few years ago and just got around to using one. The ones I have seem to run high on voltage. My line voltage is 249vac so I put on 250V tap. I was getting 8.5vac at tube. Each leg on contactor pulls 1.9A. So I ended up using 5ohm @100w resistors feeding each leg to transformer. Dropped me down to 7.6vac. So I’m within spec now. It will drop some as amp is loaded up. Just wondering if anyone else has had this issue and if they kinda did same thing I did? ( originally I had it on 240v tap, was seeing 8.9vac at tube) man it was bright. Lol


     

  2. Shockwave

    Shockwave Sr. Member

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    I'm guessing your filament transformer is probably mounted on the RF deck and closer to the socket than it is in the Henry 8K. They anticipated more voltage drop in their secondary wiring and the pair of filament chokes used in that amp. If your added resistors are doing the job without getting too hot, call it a day and be done. Otherwise, consider using lighter gauge secondary wire to add the same voltage drop as the Henry had. If you do that, be careful to make good connections to the socket so you do not cause heating in this critical area.

    The added resistance, either in the primary as you did, or secondary, is also a simple form of inrush current limiting on the filament.
     
    Crusher likes this.
  3. Crusher

    Crusher Well-Known Member

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    I had to order proper resistors, using a pair of 10 ohm @50w in parallel on each leg. Will give me 5ohm at 100w. I measured current draw on each leg. 1.9A. So I rounded up to 2A. I figured my wattage dissipation to be about 20W. So with 100W rating I have lots of headroom. I’ll be using Dale aluminum resistors bolted to cabinet. I’m using 8awg wire so that’s why I don’t have voltage drop.

    Yes the inrush current limiting is a good thing. I had 2 contactors for soft start. But after finding this all out I removed the second one and that’s where I’m putting resistors. Filament is basically a 390W lightbulb. Definitely needs something inline.
     
  4. nomadradio

    nomadradio Analog Retentive

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    Old timers advised me to specify a secondary voltage 10% too high, and put an enormous wirewound rheostat in series with the primary of the cathode-supply transformer. Resistance would be chosen to drop a max of 30 or so Volts from the 240 Volt power line. Size of the pot dictated by a current rating at least equal to the max draw of the transformer's primary. These were guys who assumed you would have a AC voltmeter on the cathode secondary.

    The current limiting it would provide was a byproduct, especially for a broadcast transmitter. Just the same, that's why they insisted on a rheostat, and not a variac. Probably means a lot more to an amplifier that gets turned off more than once a week. Back then the only folks I knew with any big-transmitter experience were all broadcast engineers. Turn-on surge was not a big issue for them. They felt a few ohms in line with the primary would be sufficient.

    73
     
    wavrider, Crusher and Shockwave like this.

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