Short tube life in class A

Discussion in 'Amps and Cabs' started by aussie_gear_maestro, May 1, 2019.

  1. aussie_gear_maestro

    aussie_gear_maestro Member

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    Hi guys I have an amp II really like (fuchs) running single ended class A however output tubes are only last like 6-8 months max and I play at very low volume for about 30-45 mins a day. I have never owned an amp like this before, is this usual for class A amps to kill tubes this fast.

    Obtaining tubes in a country town in Australian is a stuff (including via mail) around so anyone have suggestions for long lasting tubes.
    Thank you and rock on.
     
  2. fjrabon

    fjrabon Member

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    That seems a little fast, but yes, they eat tubes. It’s usually more like a year. But I also wouldn’t say 6-8 months is unusual.

    Also with class A the volume you play at doesn’t really matter. The tubes are taking the full brunt at all times regardless. The only way to extend tube life with class A is to turn the amp off.
     
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  3. RAG7890

    RAG7890 Member

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    What Tube are you using in this Amp?

    If it is an EL34, I’d suggest Shuguang EL34B’s. These can take a hammering so should last longer in Class A IMHO.

    Good luck.

    :beer
     
  4. aussie_gear_maestro

    aussie_gear_maestro Member

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    I'm using electro Harmonix output tube and some preamp Which have ALL disappointed me . Soon as I put a ring sol in V1 or it was like bam theres the sound.
     
  5. aussie_gear_maestro

    aussie_gear_maestro Member

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    Would the military equivalent be a good choice.
     
  6. aussie_gear_maestro

    aussie_gear_maestro Member

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    Sorry yes its electro Harmonix el34.
     
  7. jcarpenter

    jcarpenter Supporting Member

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    I wonder if you could try a KT88 or 6550. More pricey but may last longer.:huh
     
  8. aussie_gear_maestro

    aussie_gear_maestro Member

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    Isn't it a kt66 which is an el34 equivalent and required much more voltage. Have to email Fuchs to make sure that's safe.
     
  9. HotBluePlates

    HotBluePlates Member

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    I bet @Fuchsaudio could provide some insight on this. However, I'd like to understand how you determined the EL34s you used to be "dead" or end-of-life.

    I'm going to disagree with @fjrabon and say you should get longer tube life, and that the amp being Class A doesn't matter (much).

    Class A vs Class AB is kind of like highway driving vs stop-n-go city driving; cruising at a continuous 65mph (Class A) is less stressful to the engine than swinging from 0 to 35 to 0 to 55 to 20 to 45 to ... (Class AB). That is, Class A always runs the tube up around its max dissipation, but Class AB swings to much higher peaks and rapidly (if you play loud). Class AB compensates for the extra heat when you're playing by reducing the heat when you're not playing.

    So consider Class A operation a baseline of tube operation & life. Now if you run Class AB, you might get longer life than Class A if you play quieter (never swing those large peaks), and shorter life if you play louder (push the amp to its designed max power output).

    All that said... If you're not abusing the tube, the above shouldn't really matter either way. Tube life should be acceptably long. I've got both Class A and Class AB amps that are running on the same tubes I've had in them for the past 5 years.
     
  10. Fuchsaudio

    Fuchsaudio Member

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    The entire Casino line runs about 400 V B+. The current of the power transformer changes depending on the model. The Lucky-7 uses a single EL-34 running at 40 ma, so that's about 90% dissipation about 15 watts. Using quality tubes, there should be reasonable tube life using quality tubes.
     
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  11. handtrix

    handtrix Member

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    Yes, more headroom than the EL34 types. Doesn't necessarily change the tone, but alters it slightly and would be more noticeable in feel or the responsiveness of the amp. Even in preamp tubes, respectively in different positions; how they play their part in shaping the tube amp.
    Then there are the filter caps. Change the tired electrolytics, and now you have a different amp. Some will take it in bad ways and some will like it for the better. YMMV
     
  12. gunslinger

    gunslinger Member

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    Try some JJs if you can. Or maybe some New Sensor varieties.
     
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  13. ProfRhino

    ProfRhino Member

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    not sure about true Class A, but my tech says something similar about those popular Vox style cathode biased circuits.
    he claims they wear tubes out quicker at low volume than when being properly cranked.
    ymmv,
    Rhino
     
  14. Fuchsaudio

    Fuchsaudio Member

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    This is fixed bias (not cathode bias) and is adjustable (obviously). It could be run cooler, but not by too much and still remain Class-A.

    I checked the Weber bias calculator, and 56 ma is actually class-A @ 90% @ 400 V for an EL-34 with a 15 watt dissipation, so 40 is fairly tame. Unless the line is high and the B+ higher, so that could be checked too.
     
  15. ProfRhino

    ProfRhino Member

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    cool, thanks for the info !
    Rhino
     
  16. HotBluePlates

    HotBluePlates Member

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    I made some key deletions in the quote above. Bottom line, Class A results in more heat for the output device(s) to lose to the air when idle than at full tilt.

    The reason is the average power pulled from the power supply and into the output device(s) stays at ~100% both at idle and at full tilt; however, the power transferred to the speaker rises from zero to max power output. The power transferred to the speaker is deducted from that average power pulled from the power supply, leaving less for the output device(s) to waste as heat into the air.

    The above means the output devices run "cooler" the more you're transferring audio power to the speaker (playing loud).

    I kept using terms like "output device(s)" and deleting "True," "cathode biased," and "Vox style" because the above is true for every amp, using every bias method, regardless of tube/transistor, etc.

    Class A device(s) dissipate less power the more they're driven; Class AB & Class B devices dissipate more power the more they're driven.
     
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  17. ProfRhino

    ProfRhino Member

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    thanks, very good explanation, even understandable for an interested amateur like me. :aok
    I'm not on a quest to become an expert tech, I have buddies who are, but I love to learn about how my amps tick in general.
    you guys make TGP a neverending source of info and inspiration.
    much appreciated !
    Rhino
     
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  18. sickboy79

    sickboy79 Member

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    6-8 months in single ended class A with new production tubes seems pretty darn reasonable to me.
     
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  19. HotBluePlates

    HotBluePlates Member

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    Perhaps I'm unreasonable, but I'd like to see a lot more than that, based on the conditions Fuchsaudio described. It's possible I'm out-of-touch, but even new production tubes have lasted me a lot longer than 6 months.
     
  20. No457 Snowy

    No457 Snowy Member

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    I have a Trinity Tramp that runs Single Ended Class A and has had the same JJ 6L6GC tube installed for over 5 years and gets played for several hours most days and it's still going fine, the amp sounds great. I checked the dissipation and it runs at 29.4W (just under the published 30W max spec for the JJ tube).
    One notable feature is the amp does have an advantage with the VRM (Voltage Regulation Module) that allows lowering the voltages in the amp by setting to "idle" during a break but during play I run it at the Max setting. In any case the tube is still going fine after all that time, I occasionally swap in a new spare tube (the other tube from a matched pair) to check if there is an improvement but so far there was none to speak of so I just put the original one back in.
     
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