Tubes to AVOID in cathode follower position of amps

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers T' started by markflo, Mar 26, 2010.

  1. markflo

    markflo Member

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    so i've been reading up on some threads and forums about tung sols and sovtek lps being not good for plexi style amps or amps with cathode followers on them. any other tubes we should avoid placing in this position?
     
  2. Blue Strat

    Blue Strat Member

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    Those are the only 2 I know of.
     
  3. Roe

    Roe Member

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    ditto
     
  4. donnyjaguar

    donnyjaguar Member

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    Is this an issue with the voltage differential between the filament and cathode? My thinking is a CF imparts next to nothing on the signal because it has no voltage gain.
     
  5. Blue Strat

    Blue Strat Member

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    That's the general consensus. This is the only type of circuit where the cathode "sees" 100 volts or so.
     
  6. Ronsonic

    Ronsonic Member

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    Back in the dark and dim past, there were spec sheets provided by each manufacturer for the tubes they produced. These had specs for things like cathode/heater voltage. Nowadays, no spec sheet, no spec. Makes manufacturing so much easier to not have a bogie value to shoot for.
     
  7. conundrum

    conundrum Member

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    Cathode followers have unique overdrive characteristics even though they have less than unity gain.
     
  8. Blue Strat

    Blue Strat Member

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    Funny about that, eh?

    My favorite is when new manufacturers release a "high grade" version of a standard guitar amp tube (say, a 7025) that doesn't meet the spec of the standard grade item (12AX7). ;)
     
    Last edited: Mar 26, 2010
  9. Roe

    Roe Member

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    in the early marshall 100watter, the cathode sees more than 200v
     
  10. markflo

    markflo Member

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    so to all you guys with cathode uhm...amps...what do you guys use? right now i have an ehx 12ax7 in there. is that ok?
     
  11. TweeDLX

    TweeDLX Member

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    I used those in my JTM 45 build. Worked OK... no issues.
     
  12. WaltC

    WaltC Silver Supporting Member

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    in contrast to a lot of folks, I've had good luck w/ the JJ ECC-83's, both in V1 situations and in cathode follower roles. Not as good sounding as the tung sol re-issues, but not as failure prone either.... I really need to start buying my tung sol's from Mike K <G>...
     
  13. 59Vampire

    59Vampire Silver Supporting Member

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    i have a marshall tmb 18 watt clone.. i like to switch between delta hollands and beleive it or not a tungsram or a telefunken. currently im running a brimmar cv4004 with good results
     
  14. Mickey_C

    Mickey_C Gold Supporting Member

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    Do you have any links (other than this thread) that discuss the issues with this tube for a CF? I've been using them for sometime without a problem, so I'd like to understand what the issue is.

    I am also wondering if this is a recent problem - as I've been shipping amps with these since they first came available. I do an 8 hour tube burn in with new amps, and any tubes that make it through that last longer than I know about.
     
  15. hasserl

    hasserl Member

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    It's been some time ago now, but I had 2 Tung Sols fail in a very short period of time in a CF application, one right after the other. I stopped using them there after I lost the 2nd one.
     
  16. RockStarNick

    RockStarNick Supporting Member

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    This is a dumb question, but how does one know if your amp has a cathode follower in V2 or not?
     
  17. bonertone

    bonertone Member

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    We had some terrible issues with the 12AX7EH but only in certain positions...cathode follower positions being one of the most common points of issue.

    The problem is either right away or....worse.... everything seems fine then 30 or 40 hours of service then they start to pop or crackle.... most common an annoying background hum that grows in intensity over time.

    I ended up sending several hundred 12AX7EH back.

    We swapped them for TS RI and Mullard RI ...had the same issues with the Tungsol RI but for some reason the Mullard RI is better...still have had a few problems with noise but nothing like the EH or the TS RI.

    The tube from New sensor that seems to survive that position is the Sovtek 12AX7WA. also the chinese 12AX7's seem to work as well...neither being my first choice.


    Out of frustration I emailed New Sensor for an explanation and this is what I got:

    Spiral filament tubes do not work well in cathode follower circuits and are prone to failure. This appears to be due to the closer spacing of the spiral filament to the cathode. Spiral filament tubes include the 12AX7LPS, 12AX7EH, Tung-Sol 12AX7, Mullard 12AX7 reissue, and JJ ECC83s/12AX7. For cathode followers and phase inverters, I have had good luck with Sovtek 12AX7WA/WB/WC series tubes.
     
  18. ripoffriffs

    ripoffriffs Supporting Member

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    Here's my experience from yesterday.....

    I was trying out a different pair of EL-34s in a JCM800/1987 (4-input non-master). I also decided to replace V1 with Tung-sol RI's. This is before I even read this thread. I was biasing using the Weber Bias Rite. I finished and plugged a guitar in just to see how it sounds while bias rite was still attached.

    After 30 seconds of playing, volume began dropping steadily. As if the standby was switched. I look at the bias rite and plate voltage and idle current is still there unchanged.

    Turned it off and let it cool. Replaced the Tung-Sol with the stock tube. Works perfectly as if nothing happened. Meanwhile Tung-Sol is dead.
     
  19. bluesy

    bluesy Member

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    .....how does one know?
     
  20. Jack Dotson

    Jack Dotson Member

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    Add me to the list; I'd like to know as well. I don't know if my Lucky 7 has the CF or not, but the issues described sound like what I've experienced as well.

    My amp came with a 5751 in V1 and a JJ in V2. Noisy, but worked and sounded pretty good. As always, I can't resist trying different tubes and rolled in a set of TS's and got nothing but hiss and hum. Tried the TS's again, but this time using only one at a time with the JJ, but the result was that same. The JJ is noisy too, but usable. I decided to try a couple of cheaper tubes that I keep for spares, a Sovtek and a Ruby and the amp likes both of these regardless of the position their used in.

    Funny thing is that once I rolled the 5751 back into V1 it died. I've got the Ruby in V1 now and JJ in V2, a little hiss, but not too bad (seems to be a characteristic of the amp) and practically no hum.

    Maybe just a bad batch of tubes or do I have a CF in my amp? Anyone know?

    Tks.
     

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