What happens when a power valve lets go?

BeexterBeexter Frets: 597
When a power valve gives up the ghost, what else does it take out with it?
I guess they can die slowly which I assume leads to loss of volume and strange sounds so you have some warning but it's what happens when they fail suddenly that I'm curious about.

Any knowledge welcome. 
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Comments

  • jpfampsjpfamps Frets: 2723


    There are several modes of failure but these are the main ones I find.

    1) Internal short.

    This happens between the high voltage and low voltage pins. 

    In a well designed amp the likely outcome is that the HT fuses blows. Replacing the fuse and the valves will be all that's needed to bring the amp to life.

    However there is opportunity for collateral damage, which will need sorting out before the amp can be used again.

    Components in the firing line are screen grid resistors, power supply dropping resistors, cathode current sense resistors (not in every amp, but used to aid biasing), cathode resistors and caps (in cathode biad amps), filiament hum balance resistors / pots (can be destroyed of the HT end up on the filament supply), control grid resistors and bias supply (if the HT is imposed on the control grid).

    2) Valve stops conducting current.

    This can be a consequence of 1) but also mechanical failure in the internal connections.

    3) Valve emission too low.

    As valve age they experience reducing capacity to emit electrons from the cathode, and power loss will be noticeable.

    Most valve expire due to 1) before this happens.
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  • BeexterBeexter Frets: 597
    Eek! I guess there's not much you can do to avoid No.1 happening but I'm guessing  (hoping) it's relatively uncommon compared to 2 and 3?
    Is it just the design of the amp that stops No.1 taking out more than the HT fuse? (Mines a Mesa 5:25+ Express FWIW)
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  • digitalscreamdigitalscream Frets: 26465
    jpfamps said:


    There are several modes of failure but these are the main ones I find.

    1) Internal short.

    This happens between the high voltage and low voltage pins. 

    In a well designed amp the likely outcome is that the HT fuses blows. Replacing the fuse and the valves will be all that's needed to bring the amp to life.

    However there is opportunity for collateral damage, which will need sorting out before the amp can be used again.

    Components in the firing line are screen grid resistors, power supply dropping resistors, cathode current sense resistors (not in every amp, but used to aid biasing), cathode resistors and caps (in cathode biad amps), filiament hum balance resistors / pots (can be destroyed of the HT end up on the filament supply), control grid resistors and bias supply (if the HT is imposed on the control grid).

    The bits in bold happened to my Jet City JCA50H when I "upgraded" to KT66s. They lasted about 3 months, sounding phenomenal, then one of the valves developed a short and blew the HT fuse, but also took the screen grid resistor out leaving a scorch mark on the board.

    Quick visit to the tech with a pair of 6L6s, and a couple of days later (and about £45 lighter), all was well with the world again.
    <space for hire>
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  • ICBMICBM Frets: 71963
    All of what jpfamps said, plus in some amps a short can burn through a PCB trace before the HT fuse blows, even if the correct value is used. Engls, Fender Blues Juniors and Marshall DSL/TSLs (old UK-made series) are the most prone to this that I've come across.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • BeexterBeexter Frets: 597
    Should you pre-emptively change power valves to prevent this or do you just have to wait for them to go bang?
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  • ICBMICBM Frets: 71963
    Beexter said:
    Should you pre-emptively change power valves to prevent this or do you just have to wait for them to go bang?
    They can - and do, often - fail when they're very new. In fact the failure rate for nearly-new valves is higher than the average for old valves, until they're really truly knackered... a lot of people don't realise this and keep changing valves for 'reliability', when in fact leaving a good known working set in is usually a better plan.

    I would not generally change old power valves unless there is some fall-off in power or tone, or a noticeable drift in bias which can be a warning sign.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • springheadspringhead Frets: 1583

    So I powered up my 5E3 Deluxe at the recent Water Rats meet.  Big hum, motorboating, varying with the volumes and adjustable power.  After much poking around and head scratching at home it turned out one 6V6 had failed - heater was open circuit.  The heater must have flapped around inside the bottle and hit cathode/anode or screen and the two 100 ohm heater 'centre tap' resistors had blown.  Visually you couldn't tell but they measured open and the heater supply was floating up to silly amounts of AC.

    Not had that happen before!


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  • ICBMICBM Frets: 71963
    The virtual centre tap is one of Leo Fender's few bad ideas. It's very common to find old Fender amps with the resistors (or the hum balance pot, in amps with one) blown. Presumably it was cheaper than spec'ing a PT with an additional wire...

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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