Gain reduction mod for the JVM

Discussion in 'The Workbench' started by MatFnEhUK, Jan 4, 2021.

  1. MatFnEhUK

    MatFnEhUK Member

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    Quick question about the gain reduction mod for the JVM for those of you who have tried it. Is there a big drop in gain to the OD orange and red modes with this mod?

    At the moment I use a 5751 tube in v1 to improve the clarity and remove fizziness by slightly reducing the gain in these two modes. It works but unfortunately it also seems to effect the other channel/modes, which isn’t ideal for me. I was hoping the gain reduction mod would have the same sort of effect as the 5751 tube but without effecting the other channel/modes. I’m just trying to tame the OD orange and red modes a little without neutering them too much if that makes sense.
     
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  2. stickyfinger

    stickyfinger Well-Known Member

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    On my 205 I've completely modded it as close to a 1987 and a 2204 JCM 800. It helped a lot but I simply dont use od orange red. This included the plate resistors swap to 100k , negative feedback to 70s 80s spec, cathode follower resistor 100k, od2 to od1, a few bright caps and added high pass filters and a few other things and I still dont like OD orange or red that much lol. Ive thought about pulling C83
    Sorry dont know if that answered your question or helped any.
     
  3. SkyMonkey

    SkyMonkey Well-Known Member

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    A 5751 has the same mu (i.e. amplification factor) of 70 on both triodes.
    Looking at the triodes used in the V1 & V2 gain stages of a 410, an unequal tube like an ECC832 or ECC823 in V1 would have a better chance of only lowering the gain of the orange and red channels.
    They are labelled as V6 & V7 in a JVM410.
    https://www.marshallforum.com/threads/jvm-410-preamp-tube-layout-and-mods.101253/
    The ECC832 has ECC83 characteristics (mu=100) in system 1 (pins 6,7&8) and ECC82 characteristics (mu=17) in system 2 (pins 1,2&3).
    An ECC823 has the systems reversed.
    https://www.jj-electronic.com/en/ecc832-12dw7
    I don't know which way around would work for a JVM though.
    The schematic would help.
    http://musiciansroadhouse.com/viewtopic.php?t=11597

    ECC832 tubes are easy to find in the UK, but the only EU stockist of ECC823's I know of is ToneFactory in the Netherlands.
    You would have to request it.
    https://www.tonefactory.nl/jj-ecc823
     
    Last edited: Jan 5, 2021
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  4. MatFnEhUK

    MatFnEhUK Member

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    I appreciate your input. I reckon your jvm sounds killer with those mods. I’ve got a 205 as well and I like it for the most part, but I think with a few tweaks I will love it.

    I’m planning to get the compression reduction, OD2 to OD1, choke, and variable negative feedback mods to start with. I’m hoping that’ll be enough to tame the OD orange and red a little. If not I’ll look at doing the gain reduction mod as well. I actually like orange mode with the gain backed off a little and a 5751 in v1, so the mod might work for me. Can’t see myself using the red mode very often though admittedly. It’s overkill.

    I’ve thought about C83 as well but that would effect all modes and not just OD orange and red wouldn’t it?
     
  5. MatFnEhUK

    MatFnEhUK Member

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    Yeah I’ve read about the ecc823 and tried to find some here in the U.K. No dice. I’ll try tonefactory. It might be worth getting some shipped from the U.S. if that doesn’t work, if it saves me having to do the gain reduction mod. Cheers!
     
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  6. stickyfinger

    stickyfinger Well-Known Member

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    v6 triode a is only used on the OD orange/red.
    I'm not quit sure where red is getting the gain increase. I suspect R76 RL2b but not sure why that would increase gain.
     
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  7. gsgtrman

    gsgtrman Member

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    I did this mod to my 205 head. I like the result. It does reduce Orange and Red gain amounts, and takes away some of the fizziness, but it also lowers the volume a tiny bit while leaving OD Green's volume alone. As I use this amp more for recording one sound at a time than anything else, the volume difference doesn't bother me. The discrepancy may prove to be more of an issue in a channel-switching performance situation. I haven't been switching back and forth much, so I forget exactly how much of a drop in volume it is. Hope this helps, good luck!
     
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  8. MatFnEhUK

    MatFnEhUK Member

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    Thanks for this. I don’t think the volume drop would be too much of a problem for me either, but it’s useful to know. Stock I feel the amp already has a volume difference from one OD mode to the other. It seems to add a bit of volume as you add gain with each mode. It’s never been a problem though because I tend to stick to one OD mode within any given tune. I can also use the second MV if I do need to switch OD modes. If the mod removes fizz without sacrificing too much gain from OD orange and red then I think I can live with the volume drop.
     
  9. RickyLee

    RickyLee Well-Known Member VIP Member

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    So, are you doing the mod that lowers the three plate resistors of the three stages down to 100K? Or does that just lower the 220K plate of V6A?

    I lowered the value of the grid load R128. That only affects the V6A stage, so only lowers the saturation of OD1/2 on my JVM 410H.

    I know it was a great improvement for me . . . .
     
  10. MatFnEhUK

    MatFnEhUK Member

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    Cool. I’m not too savvy about the amp’s circuitry I’m afraid. I’ll be taking it to a professional if I go ahead with the mods. I’m happy to mod pedals, guitars, and speaker cabinets, but an amp is a little too much for me I think. :ohno:

    The gain reduction mod I was thinking of is the one that affects only OD orange and red, and involves R128 I believe. I’m not familiar with the other mod you mentioned. Does it have a similar effect? Which other mods have you done to your jvm? There’s a few I’d like to do.
     
  11. RickyLee

    RickyLee Well-Known Member VIP Member

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    I lowered R128. I did not touch the plate resistors. I was thinking changing those plate resistors was the Compression mod from the JVM forum.

    I tinkered with increasing the value of the negative feedback loop resistor. I put a small trim pot in the loop which is similar to the NFB mod. It was not all that drastic to my ears like I read on the innerwebz. It adds a touch of low end punch, but the amp is more than loud enough in stock form anyway.
     
  12. MatFnEhUK

    MatFnEhUK Member

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    I think you’re right that that’s the compression reduction mod. It’s another dilemma for me whether I do this mod as well. I do feel the OD channel sounds too compressed, but I’m a little worried that doing both the gain and the compression reduction mods might be too much. Maybe there will be drawbacks that will make me regret doing both mods and it’s perhaps best not to mess with the OD channel too much. I don’t know. What I like about the NFB mod is that you can put it back to stock easily if you install a pot. There’s little harm in trying it.
     
  13. RickyLee

    RickyLee Well-Known Member VIP Member

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    I know you are having a tech do the work. But it is not that hard a job to get in there and do this simple R128 tweak I did. I used a small Bourns 1MEG trim pot and soldered it directly on the top of the board at R128. I might have found adjacent tie points in common or went directly in parallel? But I would have to see inside my own amp to refresh my memory.

    But anyway, that gives you a trim pot to adjust to taste how much reduction you like. The chassis has to be pulled and then play the amp and tweak the pot. But I think some people have just went with resistor values around the 470K area in parallel with R128 an they were happy. I think mine ended up quite low well under 100K in the setting.

    With this mod, you can leave those plate resistors as is. You could also just have the tech install a trim pot. I think some people on the JVM forum might have drilled a hole in the front panel and made a control just for this. I am still on the fence on that one as I really do not want to drill any holes. But I am sure I will always keep the amp anyway lol.
     
  14. stickyfinger

    stickyfinger Well-Known Member

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    Hmm R128 is so the relay doesn't make a loud pop when switching channels?
    There must be a better way. C66 is left not fitted on the circuit board. You can install a resistor to create a voltage divider to reduce gain. I installed a 330p cap (all I had) to help kill the top end and reduce the chances of RED squealing at high gain.
     
  15. RickyLee

    RickyLee Well-Known Member VIP Member

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    You can look at it that way. Lowering R128 is the same as lowering the grid resistor R95 470K which is going into next stage V7B. But . . . by doing R128, this change only affects OD1/2 Orange/Red.
     
  16. stickyfinger

    stickyfinger Well-Known Member

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    I get that but the concern is why the resistor is there in the first place. Does your amp have any relay noises now when switching to and from Red/Orange to say Green or any other lower gain modes?
     
  17. RickyLee

    RickyLee Well-Known Member VIP Member

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    No pops or noise before or after my mod.

    Having a load on that other side relay contact is good. And of course, you want a load on the 12AX7 grid.
     
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  18. MatFnEhUK

    MatFnEhUK Member

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    Thanks man. I didn’t realise you could make the mod adjustable. That would be ideal. I’ll talk to my tech about it.
     
  19. ricksdisconnected

    ricksdisconnected Well-Known Member

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    why would you want to reduce the gain on a Marshall?
     
  20. RickyLee

    RickyLee Well-Known Member VIP Member

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    Have you played a stock JVM 410H or the JVM2 series yet? They are crazy heavy saturation on the OD channels even with gain set around 2 lol.

    On mine, I am now able to set the gain at a higher level, let us ay around 6, and it then bypasses a bit more around the bright cap which then warms up the tone a bit.
     

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