What's V3 In A Marshall Dsl40cr? It Eats Tubes!!??

Discussion in 'The Workbench' started by cspencer, Apr 28, 2019.

  1. cspencer

    cspencer Active Member

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    I just got a dead tube in V3. Two previous tubes became crackling in V3. My amp was playing fine and suddenly there was no sound. I pulled every tube and it was V3! What's strange was that the tube was hotter than the others but of same brightness. There was an earlier thread about V3 eating tubes in a JVM205H. I'm curious to know what's so demanding in V3 that it eats tubes. Maybe I shouldn't use NOS in V3? Any advice welcomed. Thanks.
     
  2. Robert Herndon Project

    Robert Herndon Project Well-Known Member

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    V3 is the cathode follower. I am quite certain another member can offer a more scientific explanation, but suffice to say It's a hard working tube. I've had very good results with ECC83MG or 5751 in this location on DSL40C...
     
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  3. cspencer

    cspencer Active Member

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    I found a reference post I read earlier http://www.marshallforum.com/thread...nformation-thread.53780/page-285#post-1484473

    V1a is the input buffer for both channels of the amp. V1b is the first gain stage of the Ultra Channel. V1B is bypassed for the Classic channel.
    V2 is strictly a cascaded gain stage tube, the output of V1 feeds V2a, and that output feeds V2b. This is essentially the difference between Classic and Ultra, Classic goes V1a->V2a->V2b and Ultra goes from V1a->V1b->V2a->V2b. Follow me so far?
    From V2 onward, both channels go thru basically the same path, with only slight variations.
    V3 is what some call the 'tone stack' as the EQ controls are located right after the circuitry of this tube. It essentially is yet another 2 gain stages, as the output from V2b feeds V3a, which in turn feeds V3b. The EQ controls come next, and then the reverb circuitry. After this is the Master Volume (MV) for each channel. Yhen, just before V4 is the FX Send and Return.
    V4 is actually NOT a preamp tube, it is part of the power section. Commonly referred to a the Phase Inverter (PI) it is what allows the signal from the preamp to push the power tubes in a Class A/B (Push-Pull) operation. It splits the signal into 2, and then flips one side out of phase to feed the finals.


    But is V3 doing a lot of work that causes it to kill tubes? I read in another thread that Chinese tubes lasts the longest in V3. Why is that so?
     
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  4. ampmadscientist

    ampmadscientist Well-Known Member

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    V3 being a cathode follower pushes the most current.
    I'm not too sure that NOS would help....
     
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  5. cspencer

    cspencer Active Member

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    So, cathode follower pushes more current and therefore a new tube is preferable. Seems logical. I didn't know that it will kill tubes until today. I've always thought that the tubes had gone bad. I've used NOS thinking that since V3 is a gain stage, it'll shape the tone just as V4/PI will. Wow, I'll have to see how long the factory tube will last in V3.
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    The thing is that most NOS tubes are not really NOS. They are just old used tubes.
    A tube tester does not really tell you how a tube will work in a guitar amp.
    That's why new tubes may work better.

    A 12AT7 is designed for more current but it doesn't have as much gain.
     
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  7. Micky

    Micky Well-Known Member VIP Member

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    I wrote that a few years ago, and the CR version had not been released.
    All the details apply except for the PI, which I believe is solid state in the newer amp. I also believe the voltage is lower in that section of the new amp, which would result in higher current in that section.

    A true NOS tube may help, but in my opinion you just need a QUALITY 12AX7 that can handle the current. For example, a JJ tube here just won't last...
     
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  8. cspencer

    cspencer Active Member

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    Thanks Micky,
    Can I pick your brain and ask why won't JJ last?
    The tube that died today was a NOS Mullard CV4004 M8137. I know it's used.
     
  9. Micky

    Micky Well-Known Member VIP Member

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    In my experience JJ tubes are not quality tubes.
    Many people rave about them, and sometimes in certain positions they sound great. But again, in my experience they just don't last.
    I have had JJ go microphonic, crack the glass envelope for no reason, as well as fail quickly after installation. No other tubes I have purchased have done this, even inexpensive Tung-sols.

    All tubes fail eventually. Older NOS tubes were built when sturdier, more toxic materials were allowed. That is why they are preferred.

    Current Production (CP) tubes can also be built well, and if you ask here you will get hundreds of recommendations. I prefer TAD, Ruby, Svetlana and EH CP tubes, and you may find there are other brands made at the same factories. But then again, you didn't ask for my recommendation...
     
  10. MoreAmpsPlease

    MoreAmpsPlease Active Member

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    I bought some JJ's and they're OK. They appear to have more gain but don't last as long and/or not as quiet. Recently bought a new Mullard 12AX7 to try and it's heavier glass, looks very well made, put in V1 and it's super quiet. Had an old 7025 in there, but it just became microphonic at some point recently, but it had been there for years.
     
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  11. MoreAmpsPlease

    MoreAmpsPlease Active Member

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    +1 for Ruby and TAD. Haven't tried Svetlana's yet. Also Sovtek 12AX7-LPS (long plate) as phase inverter just works. Never replaced one yet.

    I've got NOS tubes. Don't use them. I can't hear any difference, so they just go into the collection. Maybe trot them out again at some point and retest. Super happy with new production stuff I'm seeing now.
     
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  12. Matt_Krush

    Matt_Krush Well-Known Member

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    For V3 (Cathode Followers), spiral filament (90% current production Russian tubes) will fail here quicker than the rest of the preamp tubes.

    Try and use either some old US/European 12AX7's or currrent production Shuguangs (Chinese).
     
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  13. ampmadscientist

    ampmadscientist Well-Known Member

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    It's the current that kills it faster...

    I don't doubt that old tube was built better, it's just finding good ones that's the hard part.
     
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  14. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    There are many amplifiers that do not have cathode follower tubes getting blown out. I have never had a fail.

    Yet no one is asking pertinent questions. Yes the cathode follower can be hard on tubes but the conditions need to be know and no you cannot just throw any preamp tube in that slot.

    What are the tube brands and models which went bad?

    What are dc voltages in this amplifier at the cathode follower's pins/lugs for the cathode, control grid and plate?

    What is the dc voltage in this amplifier at the 100k cathode follower resistor?

    Are the filament/heaters raised? If so what is that dc voltage in this amplifier?

    I guess no one has a schematic yet for this amplifier. Yes, no?
     
  15. cspencer

    cspencer Active Member

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    I wish I had the tools to measure some of the numbers above. If memory serves me correctly, I've had 2x old Mullard shortplates in there that's gone microphonic and a Marshall factory tube (Chinese made). I've had CP Mullards there before and they were fine. Also had a CP Gold Lion Gold Pins that worked too.
     
  16. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Give good pictures of the Marshall factory tube showing plates, innards and all markings if still possible.

    If you are going to own tube amplifiers, speaker cabinets, cables and such then you need at least one decent multimeter. Learn how to use it. It is handy around the house and vehicles.
     
    Last edited: Apr 28, 2019
  17. cspencer

    cspencer Active Member

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    These came with the amp. There were a pair of each.
    [​IMG]
    [​IMG]
    I read on Marshall's website that said, you should replace the preamp tubes every 6 months! And replace power tubes every 4 years.
     
  18. anitoli

    anitoli Well-Known Member

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    Red label could be a JJ and the White label is Sino.
     
  19. cspencer

    cspencer Active Member

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    There is a reference thread here that says:
    VLVE-00055 12AX7B/ECC83 Shuguang No Selection White logo
    VLVE-00066 ECC83S JJ Low Microphony Red Logo

    Mine is VLVE-90066 so I guess it would be ECC83S JJ Low Microphony Red Logo.
     
  20. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Which one blew in V3?
     

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