Possible Issue With Jvm 410h?

[Iwanarock]

Yes it is a pain to have to take the chassis out of the box to bias the JVM, but it is not that hard just a pain, There re several good YouTube video's on line on how to bias it. As always you have to be careful if you have never done it before because getting shocked or worse is very possible. I did teach myself how to do it after watching the videos.

The pic above from Jethro Rocker is the layout for the pre-amp tubes. But to make it easier and to answer your question directly when looking from behind. The pre-amp tube furthest to your left is V1 under the can, V2 is next one to the right of V1 and so forth until you get to V5 which is off set from V1-V4. V5 is by the Power amp tubes and the furthest pre-amp tube to your right.

 

[Rockraver]

Yeah, i'm no stranger to working inside an amp. Just lazy. Lol! Thanks for everything so far guys. I will take her apart and bias her up right in a bit. After that, i'll start trying out different pre-amp tubes to see if I can't get her to scream some more.

 

[BeardedRetroGuy]

JVM410 Block Diagram:




Whoah whoah whoah! Wait a minute....

I think I just found an "error" in this official Marshall diagram. Isn't the Clean-Green channel set to IGNORE the Gain pot? Why the hell is this NOT included on this block diagram?

 

[big dooley]

the clean channelvolume is bypassed in the green mode... not the gain... and its only a diagram... not a full fledged schematic

 

[BeardedRetroGuy]

I guess you can tell it's been a long time since I've pulled my JVM410h out of its box and played on it, huh?

 

[MonstersOfTheMidway]

.

 

[big dooley]

I guess you can tell it's been a long time since I've pulled my JVM410h out of its box and played on it, huh?

no worries... yesterday i brought mine out for a reunion rehearsal with a former band of mine... it had been collecting dust for 9 months... my AFD is out of use for even longer...
bought a fractal unit last year and i fully turned to the digital side with a combination of the roland GR55 guitarsynth and the Axe-Fx II XL+

after a couple of months i tried the back to basic method of simply guitar > cord > amp, just for reference reasons... and honestly i didn't get excited about the sound i was getting, so my choice was the right one

 

[BeardedRetroGuy]

My JVM hasn't been collecting dust. It's inside its Flight Case. The CASE has been collecting dust, though. LOL I use it as a stand for my fan at night.

 

[Rockraver]

Just wanted to give you all a quick update. As you know, I changed the power tubes. When I went to bias them, the meter was reading 54 ma per side! I biased them down to 35 ma per side and let it run for a while. I checked back 20 minutes later, and it had risen to 36 ma per side. Just to be sure, I let it run for another hour +. When I checked again, it was still at 36 ma per side so..., i’m pretty confident it’s stable at this point. Whew!!!

Unfortunately though, a new quad of power tubes hasn’t done anything about the lack of gain. Don’t get me wrong, it sounds great and, it is definitely more open now that it’s biased properly but, I just need a bit more gain. I have 5 recently tested tubes ready to go so, we’ll see what happens when I put them in.

Lastly, the amp is still running pretty quiet noise-wise. I noticed a note that the previous owner must have put inside the chassis, and it looks like he put some nos tubes in there. I haven’t checked to verify if that is indeed the case still but, i’m gonna guess that the previous owner was into low to medium gain classic tones and hence, the preamp tubes in there are of a low gain variety. I could be wrong. Either way, i’ll find out in the coming days.

On a side note, I am trying to troubleshoot my TSL100, as it has recently developed a really bad case of noise. I figured i’d start by checking the bias, as it’s been a while since I checked that. I was completely startled to see that the bias was at around 85 ma or so per side! Luckily, I had another set of EL34’s on hand so, I swapped ‘em out. When I went to bias these tubes though, I ran into issues. One side was easy to get down to 45 ma but, the other side would not go below 90 ma! Turning the trim pot seemed to have no effect on that side.

Without a schem in front of me, i’m gonna guess that one of the bias resistors might have gone bad. I took out the chassis and, at first glance, I didn’t notice anything burnt or damaged. Any ideas on what I should be looking at? Thanks again for everything guys, and let me know if I should just start another thread about my TSL issue.

 

[Rockraver]

Oh yeah..., thanks for that pic of the layout Jethro Rocker.

 

[Jethro Rocker]

The TSL is meant to be biased at 90 per side. That's 45 per tube. I do mine slightly lower like 85 or so but NOT 45 per side. Perhaps the tubes in it were fine. Make sure they are matching pairs.

 

[Rockraver]

Wow! You're right! Forgive my ignorance. It's been nearly two years since i've done any kind of electronics work on my amps. A new house, school and a career change have stripped me of all my spare time and energy and now, it's apparent that i'm starting to forget shit. So thanks for setting me straight again. I think I had mine set to 85 per pair like you as well. That being said, I now realize I need to rebias the JVM as well, as I set it to 35 per pair, instead of 70 per pair! Lol!

As far as the noise issue on my TSL, I have some jj's, tung-sols and mesa boogie's laying around so, i'm gonna try swapping out preamp tubes first, as it could just be a microphonic preamp tube. We'll see. I'll let you all know by the weekend.

 

[Rockraver]

So I took the amp to a gig and ran it thru the ringer. Lol! I really cranked the shit out of it! It was loud and all but, the gain just wasn't what it should be. I watched some youtube videos and listened to some mic'd recordings of what this amp should sound like, and it definitely isn't up to par. Even with the Red Channel Gain dimed. So..., I ran some voltage tests and below are the results. Notice the numbers in red. I have my ideas but would like some input before proceeding. Thanks again guys.

 

[Rockraver]

You can disregard the Power Tube Heater readings for now. I just realized that I had the meter on DC at that point, and just wrote down what it read without thinking about it so, those are DC readings instead of AC. Nonetheless, is that normal for there to be 65 VDC on the heaters?

 

[Rockraver]

Just redid the voltage readings (pdf attached below). Wondering if it could possibly just be a bad tube. Though I can't imagine how I could have played it this last weekend, without issues of some sort, if that was the case. Any thoughts? Let me know.

BTW, if you're wondering what the the question marks in red (V6) mean, those readings were completely unstable and as such, pretty much unreadable. They just wouldn't stay still.

 

[Karl Brake]

Change those preamp tubes to new jj883s and fugget about it... Ive had three Marshall jvms all with enough volume and gain for my neighbors, their dogs, and third cousins. Even 50 watt jvms are loud screaming banshees!! Approaching noon on volume, mv, and or gain will take you into rebel country Mon!

 

[JCMDOUG]

There may be some mods to the amp before you got it. lots of JVM mods are made to remove gain and HISS. I keep my gain about less than 50% on the OD channels because there is so much of it. For me the tone is better with less gain and a overdrive pedal in the front.
As for volume if you have not tried this lower the channel volume to less than 75% and turn the master up to 4. The JVM has much more negative feedback then your TLS. The master volume works/helps kick the PI tube in the ass to wake up the power section. The channel volumes are better for fine adjustments between channels.

 

[Rockraver]

So, I swapped out all the pre-amp tubes and checked the voltages again (pdf below).

In short, all the plate voltages increased, ESPECIALLY on V6 but also, on V7 and V9. Cathode voltages generally dropped as well. I had the amp connected to my 4x12 to see if the noise/hiss would increase thus, demonstrating that the gain had increased but..., the noise barely got any louder at all.

Notice that the Heater voltages have not changed on V6. They are still unreadable and, the DC voltages (that I questioned you all about the other day) on the heaters has remained inconsistent with the other tubes (the others all showing a reading of 66 VDC) thus, the problem is definitely with the V6 location/circuitry.

So you know, the 1st Gain Stage tube was a 5965. I researched it and found out it was pretty much a 12AT7 so, the pinout is the same as a 12AX7 so, no problem with the tube choice itself.

I am attaching some pics of something I find odd. Having not owned a JVM before, I have a feeling that JCMDOUG may be right, in that this amp may have been modified? You'll see what I mean when you look at the pics. Let me know if that is normal. I kinda feel it isn't.

 

[spacerocker]

I am attaching some pics of something I find odd. Having not owned a JVM before, I have a feeling that JCMDOUG may be right, in that this amp may have been modified? You'll see what I mean when you look at the pics. Let me know if that is normal. I kinda feel it isn't.

The copper shielding? That is definitely not normal! I wonder what that was trying to achieve? Maybe something has come adrift there?

 

[MonstersOfTheMidway]

@OP:
Definitely modified, though I can't tell you the exact purpose of the modification, how it works, nor the name (I'm tempted to call it the Kleenex mod since it looks like you got two boxes of Kleenex in there). I have never seen nor heard of a modification that looked like that. If you had shown us the picture in your first post of this thread, that would have been my guess as to any strange/unusual sounds and behaviors with your amp.

What you want to do with it now is up to you. I wouldn't blame you if you decided to sell it so you can start over with a proper amp of some kind. Everyone is different, but if I saw that sh*t going on inside that amp before I bought it, with no regrets and no hesitations I would walk away. I hope this situation works out well in your favor.

Good luck with your amp.