"Glitch Resistor" in the AL 80 A

Discussion in 'Amateur Radio Amplifiers' started by KB4MNG, Oct 27, 2012.

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  1. KB4MNG

    KB4MNG Ham Member QRZ Page

    I was going to take my amp out of service to up grade the choke. I understand there is a glitch resistor mod that works as a fuse. I would like to do this while the cover is off. Is there some instructions on how to do this?

    Thank you Brian
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The real purpose of a glitch resistor is to limit the current available for an arc, if the tube gets gassy or develops some other problem that causes it to arc internally. It can save the tube (sometimes), and also save other components like the plate RF choke, HV rectifiers in the power supply, etc. No guarantees, but it's better than nothing.

    It needs to be a high voltage resistor, or a longish string of regular resistors (most "standard" axial lead 3-5W resistors are only rated in the 300 to 1000V range, depending on design and how long they are; so you could need 3 to 10 of them in series). It needs to be very well insulated from the chassis and all other components that aren't also sitting at 3kV, so routinely it would be mounted on ceramic standoff insulators and at least 1/2" away from anything else. Ideally it would be in series with the B+ from the power supply, between the rectifier/capacitor bank + output and the plate RF choke, so it can potentially protect the choke.
  3. AG6K

    AG6K Guest

     BRIAN == A successful glitch R does not act as a fuse. It acts as a peak current limiter. The problem with fuses is they do not act as peak current limiters because the metal vapour arc that forms as they are opening during a glitch has a V-drop of <20v. Even though a fuse arc lasts only a brief period of time, the lateral EMF generated by the peak-I can damage the filament in an 811A, 572B, or 3-500Z - either causing breakage or bending the filament until it shorts against the grid. If you need a surge rated resistor for glitch duty, Ohmite makes a 15-ohm surge-rated R that will safely discharge 120-Joules/a.k.a. watt-seconds of energy. One unit will work okay in many Amateur Radio amplifiers that have not been retrofitted with super-sized filter caps. For more filter-C use more than one 120J resistor in series.
    - note - to test a glitch R, charge the filter C to max V, switch the amp off and immediately short the HV+ to chassis ground. If the glitch R has not changed resistance and there is no physical damage to it, you have a winner. However, before doing the above test make sure that the grid-I meter shunt R and the meter movements are protected by at least 200a-pk Si diodes.
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