Hello all! Need some expert advice on an issue I’m having with my JVM410. I purchased it last June from Sweetwater. In October I was playing a gig and during the fourth song the volume dropped out drastically. Almost no sound. I sent it back to Sweetwater and their tech found no issues. It was working again before I sent it back. I have been using it for several shows since, until last night when the issue popped up again…at the same venue as the last incident! Coincidence?? I hooked it up when I got home and it still has the issue. Low overall volume, OD 1 has lost a substantial amount of gain, and my clean channel 1 is mud. It’s still under warranty…is this a tube issue? Could the buildings wiring done some damage? No one else’s gear had any issues. Any help and advice would be appreciated! Already spent $300 in shipping last time all for naught…
JVM 410H Power Issue
- Thread starter Glenn07
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Len
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I had a similar issue with a different amp. It ended up being a combination of bad tubes and cold solder joints at the tube sockets.
spacerocker
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If you have ruled out all the obvious things, like guitar, guitar lead. cleaned the amp jack sockets with contact cleaner, etc - then it will almost certainly be a failed pre-amp valve! Either the first one (V6, under the can) or V7....if you've got a spare, try it in each of those two positions and see! I don't think it has anything to do with the venue - just coincidence!
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Thanks for the advice! Would one pre amp tube have all that affect on all channels? I kind of thought that each channel had its own preamp tube assigned to it?? I’m no tech though! I have some spare JJ preamp and power amp tubes. What Brand of tubes do they use in the Marshall JVM‘s now? I want to go to stock if possible…
spacerocker
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Yes - There are 2 valves containing 4 gain stages in the pre-amp section of the JVM (each valve has two gain stages). Some gain stages are on in every channel/mode of the JVM. Going up through the channels/modes from Clean>Crunch>OD generally adds more gain stages, so if one of the crucial gain stages fails, it can affect all channels and modes...
I don't know what valves Marshall are using these days- but when my JVMs were built, they used either JJ or Shuguang (or a mixture of both) for the pre-amp. V6 should have a low-microphony valve if possible. I use JJ for V6 and Shuguang for the rest, which gives a bright punchy sound. Others use all-JJ for a darker sound. Of course there are other makes available too!
Here's a diagram I put together showing how the gain stages are added:
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Thank you so much! That is some valuable information! I will try to roll some tubes tonight. Maybe that’s the issue? In the meantime I’m calling Sweetwater and I have reached out to another Marshall service center. Want to get their opinions on this as well .That diagram is awesome! Thank you so much!
spacerocker
Well-Known Member
No problem! My guess is that a soon as you change one of the pre-amp valves, the problem will go away...Let us know how you get on?
Definitely! I will try to change a few tonight. We’ll see what happens. Is shuguang a brand of tube? I’ve heard of them before…May order a few to try. I have JJs in hand.
spacerocker
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Yes - Shuguang is a make of valve - Chinese made (but very good) and original fitment by Marshall in some cases.
I would try your JJ in pre-amp position V6 (under the can) try it - and if it still doesn't work, try it in position V7 (next one along)....
So did some tube swapping and it seems the culprit was V8! Put a JJ there and the volume was back! Put the original tube back in and the volume was back down to about half! So far as I know the problem is solved!
I like the sound of the original tube over the JJ…it’s a Marshall branded 12AX7EH. Has more gain and saturation than the JJ…is this an electro harmonix tube? At least I’m back up and running!
I like the sound of the original tube over the JJ…it’s a Marshall branded 12AX7EH. Has more gain and saturation than the JJ…is this an electro harmonix tube? At least I’m back up and running!
Ufoscorpion
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When weird shit happens like that it’s generally a bad tube and definitely first thing to check 👍
spacerocker
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That's cool! Glad you got it sorted! The two halves of V8 control the drive to the FX loop, and the other half is the "cathode follower" Either half failing would give you a loss of volume! I don't think the cathode follower (or the FX driver) adds any overdrive or distortion )"gain" of its own, but the choice of valve here can change the overall character of the sound... Sounds like an Electro-harmonix..I didn't know MArshall were using them - but I'm a bit out of touch with what valves they are fitting as stock these days! Out of interest, the cathode follower is one of two valves which are subject to high voltages between the cathode and heater filament (the other is the phase invertor), so (for example) russian valves (Tunsol) are not a good idea in these positions....
Thanks! I appreciate all that info! Hopefully that was all there was and nothing more hidden! There are at least two of those tubes (V7 and 8) that say Marshall 12AX7EH on them. The JJ works but does not quite have the lushness of the other tube. I usually cut the gain on half (Orange) but I’m having to go a little higher to get the same results. No biggie…I’ll experiment.
Thanks again for all the information!
