Marshall Jvm410h Fx Loop Intermittent Volume Drop.

Discussion in 'The Workbench' started by PeterBacon, Aug 1, 2018.

  1. PeterBacon

    PeterBacon New Member

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    Hi guys,

    I’ve seen people get help with similar issues on here, so thought I’d reach out.

    Im running a Marshall JVM410h and using the FX Loop for my noise gate. Sometimes, but not all the time, I will lose about 40% volume when the loop is engaged. If I turn the loop off when this happens, my signal returns. The I have the FX loop on fully wet, at -4db with the second loop bypassed. This happens with both of my noise gates (TC Sentry/ISP Decimator). I’ve tried different leads etc etc.

    I’m aware of the series/parallel feature on the FX loop dial, and it’s not to do with that as it’s on full (100%) wet.

    Is this just an issue with the loops pre amp tube?

    Grateful for any help/pointers.

    Peter
     
    Last edited: Aug 1, 2018
  2. spacerocker

    spacerocker Well-Known Member

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    Clean the jack sockets with contact cleaner?
     
  3. PeterBacon

    PeterBacon New Member

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    Worth a shot. I have a rehearsal tonight, will report back!

    Thanks for your help.
     
  4. MonstersOfTheMidway

    MonstersOfTheMidway Well-Known Member

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    First thing that jumps out at me is your "FX LEVEL" switch setting. You wrote that you have it set at -4db as quoted above; but, printed on the rear panel of the JVM is +4dBu. Did you mean to write +4dBu? If so, then +4dBu is not designed for pedal effects like TC Sentry and ISP Decimator. As stated in the JVM manual, pedals should use the -10dBV setting. Try resetting your "FX LEVEL switch" to see if it makes a difference.

    If that doesn't help, try a bit of basic diagnostics: remove all effects from the "SERIAL/PARALLEL FX LOOP" and insert a patch cable connecting the "send" and "return" ports. Set the "FX LEVEL" control to -10dBV and the "MIX" control to 100% wet. Finally, enage the effects loop via the front panel "FX LOOP" switch. Do you still hear the same volume drop as before? If you do hear the same volume loss, then you can start looking at the internals of the amp. If you hear no volume loss, then the problem could be your effect(s) (i.e. the settings, some internal issue, etc.) and/or a cable(s) you are using.

    That's good to know and one less things to worry about. As a side note, noise gates/suppressors don't work as well if the "MIX" control is less than 100% wet.

    Yes, it could be a preamp tube that is faulty. Try the simple diagnostic check I listed above. If you still hear the volume loss with nothing in the effects loop, then you can turn your attention to the internals of the amp, such as a preamp tube.

    The preamp tube that mixes the effects loop (which also mixes the reverb) would be a good place to start. Here is a picture of the tube that mixes the effects loop/reverb (all credit goes to the person who drew it):

    Marshall JVM Preamp Tube Functions.jpg

    The first and easiest thing to do with the preamp tube that mixes the effects loop/reverb is to clean its socket as suggested by previous post. If that doesn't work, then try changing this preamp tube for another that you know is in good working condition.

    Good luck with your amp. Let us know what you learn.
     
    Last edited: Aug 1, 2018
  5. PeterBacon

    PeterBacon New Member

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    Hi,

    Thanks for your reply.

    I meant -10, sorry. I wrote my post at work and I was a bit harassed!

    I’ve tried the diagnostic approach you recommended. The problem is, the drop in volume tends to happen after 45-60mins of use, but, not every time. Which makes it harder to pin down exactly what’s going on.

    I cleaned the send/return for the loop with contact cleaner tonight at my bands rehearsal and there was no drop in volume, despite 2.5hours+ of constant use.

    I’ve ordered a couple replacement tubes, just to be sure. The tubes are stock and the amp is from 2009, so I would say swapping them out is long overdue!

    Thanks for that diagram, it will come in very handy :)
     
  6. MonstersOfTheMidway

    MonstersOfTheMidway Well-Known Member

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    Hello.

    If the problem was nothing more than a cleaning, I think you may have solved the problem with little time, money, and effort.

    You mentioned that the tubes are stock and the amp is from 2009, but I'm not sure if you're talking only about the preamp tubes being stock, the power amp tubes being stock, or both. Did you buy preamp tubes, power amp tubes, or both? What brand and type? I'd recommend changing out any tubes that are stock in your 2009 amp. If you think some tubes may still be good, you can keep them for spares, but without question a change of any original/stock tubes in an amp that old is a smart move. Also, if you're changing power tubes, don't forget to bias the amp.

    Let us know how the amp sounds after new tubes are installed. Good luck with your amp.
     
  7. PeterBacon

    PeterBacon New Member

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    Hi,

    I ordered 2 EHX 12AX7EH (one for a spare), on recommendation from a friend. I intend to just swap out V4 at this imminent moment, but I would like to change everything ASAP.

    Do you have any recommendations, in terms of tube choices? Seems there are a plethora of options!

    Thanks
     
    MonstersOfTheMidway likes this.
  8. MonstersOfTheMidway

    MonstersOfTheMidway Well-Known Member

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    Hello.
    Not sure of your situation, but if money is tight, then I'd do what you did and just get what is absolutely necessary; it's simply better than doing nothing.

    I, too, use Electro-Harmonix 12AX7 preamp tubes, though I do switch them out to try other brands. The EH 12AX7 I use is their basic model (i.e. no gold plated nonsense); it's served me well as a reliable tube that produces a good balance of low, mid, and high frequencies. But, as you said, there's a wide selection of brands as well as types.

    For preamp tubes, in addition to Electro-Harmonix, I've also used JJ ECC83-S, Tung-Sol 12AX7, Shuguang 12AX7-A, Shuguang 12AX7-B, Svetlana 12AX7, Tube Amp Doctor 7025-S, Genalex-Gold Lion ECC83 (standard), and JAN-GE 12AX7WA. All performed very well with low microphonics and very good reliability (some may have lasted a little longer than others, but they all gave at least six months of moderate to heavy use. Some, I felt, had their own unique sound characterstics, thoug. The JJ ECC83-2, for example, seemed a little "dark" while the Tung-Sol 12AX7 seemed a little bright. I like these differences because it allowed me to have choices in how I wanted my sound represented. Additionally, I didn't go for anything special like gold plating on the pins or whatever; just the basic stuff. I also usually don't get into New Old Stock (NOS), but if I can get a good deal on something NOS, I will certainly look into it.

    For power tubes, I prefer SED Winged =C= EL34. Back when I first bought my 410H, the SED Winged =C= El34 power tube were not as expensive as they are now. Still, the SED Winged =C= EL34 has been the best current production power tube I've ever used, so I always pay close attention for deals on these before I look at other brands. Other brands that served me well as Electro-Harmonix EL34, JJ E34L, Tung-Sol EL34-B, Svetlana EL34, and Genalex-Gold Lion KT77. Again, all generally performed well with low microphonics and good reliability.

    One last note: when it comes to preamp tubes, I generally like to use all the same brand and type for the sake of consistency. I used to mix brands and different types, which is possible to do without damaging the amp so long as you are using types that compatible, but I changed my philosophy and now stick with one brand and type. I generally favor the Electro-Harmonix 12AX7 or the Shuguang because they consistently give me a sound I like.

    That's about it. I'm sure you'll read more about other player's choices, preferences, and experiences regarding tubes for the JVM4 and JVM2 amps that will be worth reading. Good luck with your amp and keep up informed.
     

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