12AX7/ECC83 Valve life

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by alwebber, Nov 4, 2003.


  1. alwebber

    alwebber Guest

    Of the modern production 12AX7/ECC83 type valves which one has the longest life. My Classic 30 eats the phase inverter tube and its getting expensive - especially if you pay to have the 2 triodes matched.

    I have been using 12AX7 EH for this but I was just wondering if there is anything better. I seem to remember reading that the Sovtek 12AX7 WA has long life because it is lower gain - is this correct and will it adversely affect tone as a phase inverter compared with the pricier EH?

    Al
     
  2. Braciola

    Braciola Silver Supporting Member

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    Unlike power tubes, preamp tubes should last many years with no problem.
    If you amp is going through them, you might want to have it checked out.
     
  3. alwebber

    alwebber Guest

    Regarding the NOS idea, I did use a Brimar CV4004 (military grade) valve in this position once and it did last about 10 months instead of 6 or 7 months which is what the current production valves tend to last but the Brimar has tripled in cost over the last 3 or 4 years and are now about 4 times as much as a 12AX7 EH.

    The other 2 preamp tubes do last much longer. It's just the phase inverter that doesn't last that long. I know a lot of people recommend that you change the phase inverter more often - generally when you change the power tubes but the phase inverter isn't even lasting long enough to do that.

    Al
     
  4. John Phillips

    John Phillips Member

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    Sounds like there's something wrong with the amp if it's going through them like that. These amps are hard on tubes, but that sounds excessive even at that.

    Like the others said, an old-stock tube should last years, maybe even decades - I've worked on many 60s and 70s amps with original preamp tubes throughout, working perfectly.

    BTW, IMO matched phase inverters are a total waste of money - it could only make any difference if the power tubes were perfectly matched (they won't be), and so was the OT, and so was the entire phase inverter circuitry - which it won't be, since the two halves of the normal 'long-tailed pair' PI have different operating characteristics (which is why the plate load resistors are different values, to crudely compensate for this). And that does not even account for component tolerances which are usually +/- 5% in this kind of product. That's a lot of variables which would all have to be eliminated before there was any guarantee that a matched tube would produce a better result than an unmatched one - in fact, there is a small chance that a particular unmatched tube could produce a perfectly balanced result, by compensating for the inherent mismatch in the circuit.
     
  5. alwebber

    alwebber Guest

    John

    Thanks for the advice. Do you have any idea what kind of things could cause the problem or should I take it to a good tech? The thing is that the amp sounds good and I've had no other problems with it.

    One thing that does come to mind is that I have a Fender Champ 12 where a power valve did not last nearly as long as I would have expected it to. I was wondering if I am doing something wrong like moving amps too soon after using them before they have had a chance to cool down.

    Al
     
  6. John Phillips

    John Phillips Member

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    The Champ 12 is also quite hard on the power tube - it's a single-ended amp (unusual with a 6L6) - these run the tube continuously at full dissipation. They also came stock with particularly nasty Chinese tubes, but I'm assuming yours had been changed already...

    Not sure what the cause with the Peavey would be, unless some component (resistor or cap) has gone out of spec and is running the tube outside its normal rating - but a big problem with the Classic 30 is the thin and flexible chassis, which really rattles the tubes around. They do tend to eat tubes in general, but most of the ones I've seen go have been the first preamp tube not the PI. But something is not right if you got less than a year out of a Brimar - unless you were just unlucky. What was the failure symptom? Did it stop working or go microphonic?

    It is definitely bad for the tubes to move or bump the amp while they're still hot though - I like to leave it at least a couple of minutes.
     
  7. Del

    Del Member

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    Iwould contact watford valves as they have some preamp valves which should last more than 10 months:cool:
     
  8. alwebber

    alwebber Guest

    John

    In the Peavey, they don't stop functioning but the way they go is that the drive channel starts to sound exceedingly nasty - like a cheap 80's practice amp. It once got to the point where the drummer (of all people) noticed and asked what was wrong with my sound! If you leave it long enough the clean channel starts to sound nasty as well although I generally change the valve before this gets too noticeable.

    With the Brimar I did swap it into the first position when it went like this and it gave me lots of hissing/fizzy noises.

    I try to leave it for at least 5 minutes before moving the amp. A sound engineer once picked the Fender up and moved it while it was on (and had been on for over an hour so it was nice and hot) which I wasn't terribly pleased with at the time. This may have contributed to that one - a pity as I'd managed to lay my hands on a NOS GE.

    I've got a schematic for the Peavey and a multimeter so it's probably not beyond me to check a few resistor values. Do I have to disconnect capacitors from the circuit to measure values? I live 2 miles from Chandler's so the other option is to let Dave Petersen have a look at it.

    Again thanks for the advice.

    Al
     
  9. DreamTone7

    DreamTone7 Guest

    If the amp checks out, try a NOS Telefunken. Their best 12AX7s are rated at 100,000 hours. This means you can play them 10 hours a day, every day, for 30 years! (According to the claim) Some of the modern production tubes, in comparison, will only last a good 1500 hours.
     
  10. John Phillips

    John Phillips Member

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    Al - I'm not sure what's wrong there. I'm guessing that perhaps the tube is being run too hot - a shorted or leaky coupling cap might do this, and if it's affecting one side of the PI only the tube will go badly out of balance and might make the distortion sound bad, more noticeably than the clean anyway... but just a guess.

    If you want to check this, measure the voltage drop across each of the plate-load resistors for the PI, from which you can work out the tube current (V/R) and and calculate the power dissipation (current x plate-cathode voltage) - if it's over 1.2W for either half the tube is cooking... certainly don't risk anything expensive in there until you've found the cause.

    You'll need to do this with the amp on, so unless you're confident you can work safely, I'd suggest paying someone else to fry themselves... ;)
     
  11. Rog

    Rog Member

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    My guess is that this is based on a signal that doesn't cause the valve to seriously clip. I've found myself replacing V3 on my MKIV Boogie several times due to tone degradation.
     
  12. alwebber

    alwebber Guest

    I was wondering how much effect the lack of a standby switch on the Classic 30 (and champ 12 for that matter) would have on valve life.

    Al
     
  13. alwebber

    alwebber Guest

    I've had a look inside the Peavey. The problem is that the design (which I guess is cheap to build) is a pig to work on. Taking measurements of plate voltage etc. seems impossible with the PCB in place. And if you take the PCB out of the chassis which is quite a major operation then you lose the connection to earth (without some kind of bodge) and you can't switch it on to take the measurements.

    The thought of paying someone £40 to £50 an hour (at London prices) to work on the amp when it would probably be an hours work just to take it apart and put it back together again does not appeal. I think I might just put it back together and put it on Ebay, along with the Fender and put the proceeds toward a nice Cornell or Brunetti.

    Al
     
  14. myles111

    myles111 Member

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    Al ....

    Download a free copy of my Tube Primer v3.20 .... it will answer a lot on tubes, and give specs and test details on all current preamp tubes that are acceptable.
     

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