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Ameritron 811h

Discussion in 'Amplifiers' started by 552fl, Sep 29, 2019.

  1. nomadradio

    nomadradio Analog Retentive

    Apr 3, 2005
    Likes Received:
    A soft start will serve to remedy the cold surge to the cathode.

    One more pitfall to remember with an oversized filament transformer is that the output voltage marked on it will be ** AT ** the rated current draw.

    Less current draw will mean a higher output voltage. One conventional wisdom for powering large thoriated-tungsten cathodes like that is to specify a transformer with a slightly-higher output voltage than needed and place a large rheostat in line with the primary.

    Ordering a transformer that says "230 Volt" primary and powering it from 240 will typically allow this, even if the output voltage rating is what you want. Dropping that extra 10 Volts in a 100-Watt or 150-Watt wirewound rheostat allows you to set the correct filament voltage, with some leeway above and below the target.

    That rheostat will also serve as surge-limiting resistance at cold power-up. But the current rating on it must be sufficient for that surge current. This is what makes the typical 100 or 200-Watt rating necessary. That current rating counts, more than the power rating of the control.

    More than one way to skin a cat, just so long as your knife is sharp.


  2. Captain Kilowatt

    Captain Kilowatt Professional Amateur
    Staff Member

    Apr 6, 2005
    Likes Received:

    Agreed and I usually have a pretty sharp knife. ;) Most people never think to use a soft stat on a filament xmfr. Most smaller commercial amps use a single xmfr for both plate and filament.All the commercial broadcast tx I worked on all had a series rheostat on both the plate and filament xmfrs to allow precise adjustment of the filament voltage as well as a small variation in the plate voltage.
    Shadetree Mechanic likes this.
  3. Shockwave

    Shockwave Sr. Member

    Sep 19, 2009
    Likes Received:
    Filament transformers in large amplifiers (3CX3000A7 and up) present unique problems when used for intermittent communications duty. Line voltage sag under full output typically requires the filament voltage to be set too high in standby without the B+ load on the line. That's boiling more electrons off your filament than when you're transmitting! Voltage regulation with electronic current limiting would be ideal here.

    Wouldn't it be nice if there were inexpensive power supplies available for re-purposing in this application? If you think outside of the box, there are plenty of them around at prices far less than big transformers and they have performance characteristics that no transformer alone could ever match. I've purchased several 5 volt, 200 amp, surplus, main frame computer power supplies at about $50 each. Many models have three external adjustments for output voltage, current limit level and over voltage protection trigger point.

    With proper adjustment, they can produce up to 7.5 volts. Sometimes it requires adding a diode or two in the external sense line to hit that voltage. This tricks the regulator into putting out an extra .6 volts by showing the sensing circuit .6 less. The best part is the current limiting circuit. If you set it at the tubes rated current, it will take like 2 or 3 seconds to slowly bring the filament up to full voltage as it holds the current steady at the limited level. Nothing starts a filament easier than current limited, regulated, pure DC. It also eliminates the need for any hum bucking resistor in sensitive tetrodes.

    Their isolation from ground is ample enough to handle bias and cutoff voltages so long as a cutoff resistor is used in the cathode of a triode. Two ferrite cores with bypass caps on the power supply leads is all that's required to keep them happy in a high power RF environment. I've also added a timer and a relay to switch between two different filament voltages. Every time the amplifier is unkeyed, a 5 minuet timer begins counting down. If the amplifier has not been keyed during that time, filament voltage is automatically reduced by approximately 1 volt, until the amp is keyed again.

    That reduces filament emissions during extended standby periods and prolongs tube life. Line voltage would now have to sag more than 20% before the filament voltage could change even one tenth of a volt!
    #33 Shockwave, Nov 29, 2019
    Last edited: Nov 29, 2019
    Shadetree Mechanic and Woody-202 like this.
  4. Doc813

    Doc813 Member

    Mar 25, 2008
    Likes Received:
    Oh this thread just got better than sex ;)..I got my first 3cx3000A7 tube's in the late 80's early 90's i had never played with or had much info on them....But being full of piss an vinegar of a younger age i proceeded to throw an amp together....An it worked yee haa an look at that watt meter i needed a bigger one ......Anyway keep up the nice work an get lots of pics ....It is still one of my favorite tubes ..
    Shadetree Mechanic likes this.

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