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Amplifier HV glitch resistor

Discussion in 'Amateur Radio Amplifiers' started by W5IAC, May 21, 2019.

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  1. W5IAC

    W5IAC Ham Member QRZ Page

    Instead of one very high voltage resistor in the HV line after the last filter capacitor, would it work to put less sophisticated resistors between the filter capacitors in the chain? Specifically looking at adding glitch protection to a Heath SB-230 I'm rebuilding.
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd say "no." Can't see how that would work.

    However if the SB-230 uses the original tube, a fault after the glitch resistor occurring while the amp is being driven should destroy the tube immediately, as its weak link is the grid, and certainly not the anode.

    If B+ is killed by a glitch resistor opening, applying drive to the tube should kill the tube permanently. If it's there to reduce the energy in an ion arc (inside the tube), my feeling is that would be a useless frill because these ceramic-metal tubes which operate at pretty low voltage don't fail that way. Way more likely with glass tubes having big pin seals and operating at higher voltage. I've never heard of an 8873/4/5 developing an internal arc due to gas or anything else.
     
    WA7PRC likes this.
  3. W1BR

    W1BR Ham Member QRZ Page

    Glitch resistors are supposed to limit the maximum current that can be supplied by charged filter capacitors. Placing the resistor before a filter cap is as effective as a fuse on the AC line.
     
  4. W5IAC

    W5IAC Ham Member QRZ Page

    Thanks for that. (W1BR, I was thinking between the caps in the string (six) effectively increasing the ESR of the bank.) It seemed like something you do for a 3-500z, but I've seen people adding them to SB230s, during Russian tube conversions and the like. I'm just rebuilding (unhacking, really) and old unit I came across to test the 8873 I pulled from another 230 I converted to run a 3CX800.
     
  5. K6BSU

    K6BSU Ham Member QRZ Page

    My amp has a 25 Watt wire-wound glitch resistor. I doubt it would save anything in case of a HV short in the tube. It would take too long for the resistor to heat up enough to blow. And the power supply is good for only 3/4 amp short-circuit. A good-size short anywhere should pop the front panel circuit breaker, or else the one on my AC power panel. I would rather replace the glitch resistor with a slo-blo fuse. It would do the same thing, but faster.
     
  6. WQ4G

    WQ4G Ham Member QRZ Page

    Isn't the purpose of a 'glitch resistor,' in the Anode circuit, is to protect the Transformer (not the tube)?

    Dan KI4AX
     
  7. AG5CK

    AG5CK Ham Member QRZ Page

    If the mains going to the hv transformer are fused properly it will be fine. I don't like the idea of a sacrificial glitch resistor.

    I had a gs35b amp with a 20 watt glitch resistor. I can't remember the resistance 15 or 20 ohms I think.

    Ive had a tube short and pop the 20a fuses immediately. The same tube actually did it twice. After cooking it for a couple of days the tube is still good.

    Ameritron does the same thing. If theres a short from b+ to ground it just pops a fuse. The stuff I'm talking about is modern transformers with cap input filters. Old iron with chokes may be a different game.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    IMO, no.

    If it's not sacrificial like the little 0.82 Ohm resistor in series with the B+ in Drake (and some other) amps, "glitch" resistors are generally intended to limit the current in an ion arc inside the tube. By limiting the available energy, a tube can arc internally in a non-destructive way (sometimes) and survive that. If the energy isn't limited and all the energy stored in the filter capacitors dumps into the tube in milliseconds, it's usually bye-bye tube.

    Best example I can think of occurred in my own homebrew 8877 6m amp, which is powered by a very LARGE power supply having two available outputs: 3kVdc at 1.5A or 6kVdc at 0.75A. The 6kV position is useful for 4-1000As and stuff, but an 8877 can't handle that (maybe the pulse rated 3CPX1500A7 can, I don't know).

    When the amp was first built and I was testing it, I applied 6kV to the tube (oops, cockpit error) and heard a "PING" from the tube itself. Metallic sound, I'll never forget it. I figured, "well, there goes that tube!" but the tube recovered and when switching to 3kV it still worked fine, thankfully. That was in 1986 and the same tube is still in service after 32 years. It had a 50 Ohm, 50W glitch resistor in line between the PS and the tube.

    I wrote a project construction article on that amp, with schematics, photos, etc. for CQ magazine and it was published. I also discussed the 6kV incident in the write-up. Only a week later I received a letter from Bill Orr W6SAI admonishing me for ever trying 6kV on an 8877, stating "dangerous X-radiation from the tube can occur" if you do that.

    I never thought about that, but it was interesting for him to point that out.

    In any case, evidently the glitch resistor saved the tube. That's pretty much the reason I use them.
     
    VK6APZ and K2XT like this.
  9. K6BSU

    K6BSU Ham Member QRZ Page

    WIK. That makes more sense. No one ever explained the real reason for a glitch resistor.
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Well, one could potentially save the rectifiers in the power supply, too...if the rectifiers used can't handle much peak current.

    I use 3Arms/300Apk rectifiers usually and a momentary short circuit blows the fuse(s) or circuit breaker(s) on the primary before rectifier damage can occur. But those using 1N4007s which have a much lower peak current rating might lose some in a similar event. A real short-circuit discharge can even damage filter capacitors sometimes, depending what they are and how they're made.

    So, I think there's a few reasons to use one; but in my case, mostly to help prevent a destructive ion arc in an expensive tube (or (tubes).

    A short on the B+ line should never damage the transformer if proper fuses or circuit breakers are used in the primary; they'd blow way before any destructive heating in the transformer.
     

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