Valve life expectancy - cathode vs fixed bias ?

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paulphoenixpaulphoenix Frets: 70
Will a cathode biased amp burn through power valves quicker than a properly biased fixed bias amp? 


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  • ICBMICBM Frets: 39491
    edited May 14
    If the cathode-biased amp isn't biased properly, yes.

    Most aren't.

    "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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  • ecc83ecc83 Frets: 944

    To add a little to IC's response? As with most things to do with valves and guitar amps, it all depends.

    You see Cathode Bias was used universally for valves in just about every application, mainly because it is much cheaper to deploy than generating a negative bias voltage anywhere from 10 to say 100 volts. Not only cheaper but in many ways better because it is also called "Auto-bias" because it self limits the anode current of the valve to a safe level IF THEY DO IT RIGHT!

    Mains radios and the sound stages of TVs of yore  had CB pentode and tetrode OP stages and these lasted for decades, in the case of the 6V6 they were virtually immortal!

    But as ICBM says, guitar amp makers biased too hot. To be a bit fair they probably looked at the "book", saw a value for the cathode resistor and used the nearest preferred value.Then, were not so fussy what the HT voltage was! The Book you see probably gave a value for 250V HT.

    Fixed biased amps CAN run into thermal runaway, not common but can happen. You are also dependant on a rather complex collection of components to provide the essential bias whereas CB just needs one adequately rated resistor.

    Dave.

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  • Modulus_AmpsModulus_Amps Frets: 1035
    tFB Trader
    A lot of amps run the power valves past the datasheet max recommended values, this has more to do with valve failure than the type of bias. Saying that cathode biased amp are generally biased warmer, actually giving off more heat and that may create more issues inside the amp, leading to failure.
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  • paulphoenixpaulphoenix Frets: 70
    Thanks to all for your replies. 

    At my last gig the amp (Friedman Dirty Shirley Mini) lost some output volume near the end of the night. 

    I have only owned it for 11 months and have gigged it every couple of weeks (plus rehearsals) with the factory installed EL84 valves. 

    I have put a new matched set in (I always keep a spare set handy) so all good but, it just got me wondering about valve lifespan relating to cathode biased designs. 

    As an aside my other cathode biased amp (Orange R15) was used for over twelve months (same sort of gig schedule) without skipping a beat. 

    Here's one of the pair I removed from the Friedman (both had similar tarnishing), so I guess they were ready to be retired. 


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  • ecc83ecc83 Frets: 944
    edited May 15

    Paul, that looks like some form of staining  I would investigate the amp's internals further for the source.

    There is no reason why a good (ha!) EL84 should not last several years in a properly designed circuit that kept anode dissipation (Pa) under the 12W limit.  The El was not common in domestic kit but its crappier brother, the Ul84 was used in 1000s of record players and even though its life was short compared to say the ECL82/ECL86 they did run for 4 or five years. I had a Rogers stereo hi fi amp and never changed the ELs in 3 years. AND that got used a lot!

    You should check the valves for "red plating". Put the amp in a darkened room and look at the anodes. If there is the slightest sign of a dull red glow on the spine of the anodes get it to a tech. Just 10 or 20 more Ohms in the bias resistor can bring them down from RP.  You will never hear the difference. N.B. Some brands will red plate, others not but that does NOT absolve the manufacturer in my book!

    As an aside. The cathode resistor should be many times the wattage rating of the amp when running normally. If just on rating a faulty valve (of two) can burn it out. That puts HT on the cathode decoupling cap...Messy! Of course, proper HT fusing is also required.

    'King junk design some gitamps!  (bit ambiguous that. What I mean is, say the power in the cathode R is <1W when running normally? A five, better ten watt resistor should be used. Same goes for 1R in a fixed biased amp although a 1W plus diode is ok)

    Dave.

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  • ICBMICBM Frets: 39491
    I don't know what that staining is, but it's not a normal sign of cooked valves. You would expect to see some slight silvering on the sides of the bottle level with the middle of the plate structure, or the plate itself having "ribs showing" (which is actually the shadow of the screen grid wires) if it's been redplated.

    Other than that it can be difficult to tell visually if a valve is worn out. On very old valves the silver getter at the top (or sides on some valves) can turn a pinkish-grey or semi-transparent, which indicates a tiny amount of air has got in the valve and means it's very close to the end of its life.

    In general cathode-bias guitar amps do run the valves far too hot as Dave said - often over 100% of the dissipation rating even at idle, and more when driven hard. This goes back as far as the Vox AC30 and is probably partly responsible for the incorrect belief that they're Class A, since in a Class A amp the valves *can* (but don't have to be) run at 100% at idle, since the dissipation does not increase with output power.

    "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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  • paulphoenixpaulphoenix Frets: 70
    I've disposed of the old valves now, but I'll fire the amp up and check for red plating later tonight under cover of darkness. 
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  • ecc83ecc83 Frets: 944

    !! Don't you have curtains? I would never bin old valves if they still work. New bottles can blow anytime and even a worn set will get you through the gig!

    One other benefit of CBias of course, you can just swap valves, no re bias necessary.


    Dave.

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  • Modulus_AmpsModulus_Amps Frets: 1035
    tFB Trader
    My experience with JJ EL84's is that the red screen print goes yellow/faded quite quickly if they are run too hot.

    Are those amps supplied to the UK for 230v or 240v, that will make a difference.


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  • paulphoenixpaulphoenix Frets: 70
    edited May 16
    I checked for red plating last night and found that maybe the amp is running ever so slightly hot?

    So it'll be off to my local tech for some jiggery pokery 


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  • ICBMICBM Frets: 39491
    That's *way* too hot. If you look carefully you can see the shadows of the grid wires, four or five bars of very slightly less red across each patch, which is the 'ribs showing' effect. When they cool down you'll probably see some discolouration.

    "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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  • paulphoenixpaulphoenix Frets: 70
    I managed to rescue the old valves so I'll show them to my tech and have the amp fully checked over .
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