class A, cathode bias, and tube life

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by kingink, Oct 16, 2003.


  1. kingink

    kingink Gold Supporting Member

    Messages:
    732
    Joined:
    Sep 17, 2003
    Location:
    austin, TX
    Okay, so I've heard/read that cathode bias and class A shorten tube life? Can anybody weigh in on this issue?

    Does tube type (6L6 vs. EL84 etc.) or NOS vs. new production play any part in this?

    Thanks much,

    David
     
  2. Fuchsaudio

    Fuchsaudio Member

    Messages:
    7,200
    Joined:
    Jan 3, 2002
    Location:
    Bloomfield, New Jersey
    This question reminds me of "how long will my car or tires last". Okay, how much do you drive, and how hard are you on a car ?

    Firstly, you never know how good the tubes you have are. They will prove themselves over time. A premium grade NOS or good current production should last a good 5 years if used a reasonable amount and properly biased. The other question is really what a "normal" tube lifespan would really be. I get mod amps here that have GE and RCA black plates inside, and they are 30/40 years old. They still make 90% or rated power and show a good vacuum. They also sound (sometimes) better than the Svets or Sovteks I would put in their place, if I replaced them.

    Theoretically: Class-A would lower tube life expectations, as the tubes are "on" all the time. AB uses a tube less of it's duty cycle, but if you play 12 hours a day, on "12" with an attenuator, the AB tubes might last less than the class-A tubes that were used 2 hours a month.

    If it's not obvious already: It's a hard question to give a solid direct answer on.
     
  3. KLB

    KLB Member

    Messages:
    3,047
    Joined:
    Feb 9, 2002
    Location:
    West of the Sun, East of the Moon
    I would add to Andy's excellent description a few points:

    1.) An amp can be "cathode biased" just like a fixed-bias amp, with the tubes dissipating a conservative 50-60% of idle current. Because of the cathode bias, they will have a softer high end, a little less output, and a bit more compression.

    2.) An amp that is cathode biased at or near full idle current isn't necessarily in Class A mode. See the paper on Randall Aiken's site for a description of why this is so.

    3.) Tube life is greatly affected by how well ventilated and/or cooled the tubes are. Amps with the tubes and transformers pointing up (and epecially with overhead vents) stay much cooler than when they point down. A cooling fan can usually accomplish equal or better ventilation, regardless of tube orientation. Combos played loudly are rougher on tubes than are heads, due to the vibration.

    4.) Some tubes cool better than others. Consider the skinny EL-84 vs. the chunky KT-66. There is much more surface area compared to internal volume on the EL-84. (This is also why skinny people deal with heat easier...)

    I would avoid a cathode biased (full current and/or class A) combo with the tubes pointing down with no cooling fan. These amps eat tubes! (Can you say Matchless?) Still, it is amazing how long a good pair of old 6V6 can last in a Tweed Deluxe.

    - Ken
     
  4. kingink

    kingink Gold Supporting Member

    Messages:
    732
    Joined:
    Sep 17, 2003
    Location:
    austin, TX
    Thanks guys!

    There are some amp companies that argue that cathode bias and/or class A circuits shorten tube life. I was simply looking for some perspective on the matter and now I've got it. :)

    Intersting point about EL-84s, Ken. One thing that was in the back of my mind when I posted was: how expensive is it to maintain a MAZ 38 or similar amp with a quartet of EL-84s in class A etc.

    Thanks again

    David
     
  5. reaiken

    reaiken Member

    Messages:
    1,773
    Joined:
    Jan 4, 2002
    Location:
    Greenwood, SC
    FWIW, and to further confuse the issue and stir up debate, I have another take on the subject:

    Running tubes in true class A doesn't shorten their lifespan at all - they are rated by the manufacturer to run at a particular max plate dissipation, and as long as you don't exceed that rating, the tubes last their "normal" lifespan. Running them cooler will likely extend them beyond their normal lifespan, whatever that may be. Now, this doesn't change a damned thing, but it sure makes you feel better about running tubes at their max ratings when you look at it this way. :)

    Actually, it is a common misconception that tubes in class A are running at their maximum all the time - they do indeed conduct *current* throughout the entire cycle, but the plate dissipation is not at maximum for the entire cycle. In fact, the average plate dissipation drops as the tube amplifies a signal. Dissipation is at maximum at idle, and drops to a minimum of half the idle power dissipation when putting out full power - the part of the total input power that is passed on to the load is subtracted from the idle dissipation in the tube, so they actually run cooler at full power than they do at idle. For more on the subject, read this paper: http://www.aikenamps.com/Why70percent.html

    So, if you want to extend your tube life in a class A amp, play it at full volume all the time. When your bandmates tell you to turn down, inform them that you must play that loud to preserve the life of your valuable NOS output tubes.

    Randall Aiken
     
  6. Dr Rico

    Dr Rico Member

    Messages:
    1,772
    Joined:
    Jan 4, 2002
    Location:
    Dade County, FL
    My hero.

    Hey Randall, how far are you from Bluffton? I'll prolly be heading up to hunker down with me folks at T-Giving and would love to run by to check out yer wares.
     
  7. reaiken

    reaiken Member

    Messages:
    1,773
    Joined:
    Jan 4, 2002
    Location:
    Greenwood, SC
    Bluffton? I dunno, I've never heard of it. The shop is located in Buford, GA, which is just outside of Atlanta on I -985, second exit up from the I-85/I-985 split.

    Randall Aiken
     
  8. KLB

    KLB Member

    Messages:
    3,047
    Joined:
    Feb 9, 2002
    Location:
    West of the Sun, East of the Moon
    "Hey, turn that damn thing DOWN, and give it a break!"

    "Sorry, no can do, 'cuz Randall sez I have to crank it and play it full speed."

    The Moral?

    If 'ya love 'ya tubes, 'ya ears need to be Aiken.

    Ka-boom.

    :D
     
  9. Dr Rico

    Dr Rico Member

    Messages:
    1,772
    Joined:
    Jan 4, 2002
    Location:
    Dade County, FL
    Ah, a good piece north. Bluffton is in SC, not far from Savannah, GA. I head up to Augusta a bit, but rarely get as far north as Atlanta, alas. It take a whole day of driving to get from these parts to Bluffton...we're on the edge of civilization.
     
  10. kingink

    kingink Gold Supporting Member

    Messages:
    732
    Joined:
    Sep 17, 2003
    Location:
    austin, TX
    Aha! At long last, the excuse I've been looking for!!:D

    Thanks Randall, not just for this but for your whole post. Excellent info.
     
  11. alwebber

    alwebber Guest

    Hi, my first post on the gear page!

    To go back to the original question about tube life in class A cathode biased amps I have a Fender Champ 12 (single ended 6L6). It has been modded - the previous owner carried out a Dan Torres mod on it and I have upgraded the speaker.

    I bought a NOS GE 6L6 for quite a lot of money and it only lasted about 10 months playing through the amp an average of 3 or 4 hours a week. From what has been said above, I would have thought I should get more life out of it than this. Is there anything that could shorten valve life that I should check? Should I check resistor values etc to make sure that they are correct or did I just get unlucky with that one valve?

    Al
     
  12. KLB

    KLB Member

    Messages:
    3,047
    Joined:
    Feb 9, 2002
    Location:
    West of the Sun, East of the Moon
    Alwebber,
    In case you don't get an answer here, the AMPAGE forums are generally better for this sort of question.

    By the way, my favorite sound with a Champ was when I used a small bottle GE 6L6!

    - Ken
     
  13. reaiken

    reaiken Member

    Messages:
    1,773
    Joined:
    Jan 4, 2002
    Location:
    Greenwood, SC
    6L6's can usually take quite a beating - some old Fenders I've seen had the original tubes in them. :)

    First question - how did the tube die?

    Did the filament quit glowing? If so, the filament voltage might be higher than the recommended 6.3VAC. If it is, a series resistor, appropriately rated, can be used to reduce it.

    If it died from other means, you could have too high an idle plate dissipation. Check the bias current and plate voltage and calculate the dissipation. It should be within "safe" 6L6 range for optimum lifespan.

    You didn't mention what the Torres mod was - if anything was modded in the output stage, it could be suspect in reducing the lifespan of the tube, such as parasitic oscillations that make the tube run hot or too hot bias.

    When you upgraded the speaker, did you use the correct impedance? For that matter, is the amp running the correct reflected impedance on the primary for use with a 6L6 and the speaker you are using? If the tube arced internally and shorted out, you might be running too high an impedance, and likewise, if the tube just plain burned up, you might be running too low an impedance.

    Then again, you could have just gotten a marginal tube - it happens, even with NOS stuff. I would try another tube after checking the above stuff and see how long it lasts. Check the tube in the dark to make sure the plates don't glow red at idle, at full power, and at points in between.

    Randall Aiken
     
  14. alwebber

    alwebber Guest

    Thanks for the help.

    The tube didn't totally die (although I think it was well on the way) but it started making frying pan type noises which didn't go away when I turned the volume knob down. I figured that the problem had to be after the preamp and my first reaction was to change the power tube for a spare which fixed the problem and made the amp sound better as well. My worry is how long this one will last.

    The speaker is an 8 ohm Celestion G12H30. I've checked the schematic and this is the right impedance.

    As for the Torres mod, I don't exactly know what it involves. If you go onto the Torres website there is a kit available for a Champ 12 for $15.00 but they don't give any details of what it is.

    Again thanks for the input. I will try the stuff you mentioned.

    Al
     

Share This Page