Marshall JVM410C

[The Dude]

Anybody have any experience with this amp? I have one on the bench that is ridiculously noisy and will feedback if gain is too high. I can pull the first tube (which is V6 on this amp- not the typical V1), and the noise goes away, so the noise is coming from the first gain stage. I've checked or changed coupling caps, plate resistors, all the usual stuff. I couldn't find anything wrong. Tube voltages are "normal".

Then I started scouring and found that the web is riddled with complaints about how noisy this amp is. IMO, it has waaaaaaaay too much gain available, which can create problems for those who would automatically go for max gain. So, ............ it's a noisy amp. But, how noisy? What is acceptable noise? Is it fixed and just poorly designed, or is something wrong with it?

With all tones, gains, volumes, masters at mid setting and reverb down- with no signal applied, I get about .4 volts P-P white noise at the speaker output. Seems ridiculous, but if you apply a signal to it with the same settings, it's really loud and has more gain than I would ever actually use. So maybe they just made the gain go to 11? If you turn the gain and volume down to what I would consider reasonable levels, obviously the noise is less (but still audible). Schematic attached if anyone cares to look.

JVM410.zip

[nosaj]

Just a thought what about any cold solder joints? Marshall JVM 410h problem! - MarshallForum.com

nosaj

[The Dude]

I thought about that. I resoldered all of V6 and V7 (the first two tubes in) just in case. No difference. I'm relatively sure solder is not the issue. It's a double sided board and pretty well soldered. I've been over the area pretty well and chop sticking does nothing. I should note that I've also tried tube substitution. I'm inclined to believe that this is just a noisy amp at high gains and that's how it was designed. I was hoping someone could confirm or deny that.

[g1]

Ad copy states: "boasts more gain than any other Marshall to date", so some more noise would be expected, but how much?
Any local dealers where you could listen to another one for noise level?

[Enzo]

I have serviced them, but have no specific recollection of noise.

You can pull the first tube, but that doesn't narrow it down too much. I like using the grounding technique. A clip wire to ground kills the signal path, and for places like plates with lots of DC on them, add a series cap to the ground wire - I use a 0.047uf because I have a ton of them. So for example the input stage. Ground the grid, does that materially affect the noise? Doesn't matter if our clip wire adds hum, all we care about is what effect it has on the hiss. And of course that is two triodes. We can ground the grid, or the plate.

Simple and takes little time. if it exposes something or isolates a portion of the circuit, great. if it illuminates nothing, oh well, we are out 5 minutes of our time.

[The Dude]

Enzo,
I did just that- in fact one of the first things I did. I found that grounding either side of C43 even with the probe of my DVM got rid of most all of the noise. Basically a .01UF to ground. That cap is from the plate of the first tube to the grid of the second. However, it led me nowhere.

[The Dude]

Ad copy states: "boasts more gain than any other Marshall to date", so some more noise would be expected, but how much?
Any local dealers where you could listen to another one for noise level?

I tried the local GC. No luck. I'm still looking.

[g1]

I just googled "marshall jvm noisy". Sounds like this may well be normal.

[The Dude]

I did the same. Plenty of reading. I saw that lots of players are using a noise gate both in front of the amp and in the loop. I will say, if this is normal, it's not acceptable. When I dime the amp, it puts out over 4V P-P white noise! (and no, I didn't forget a decimal )

[g1]

Yeah, when I saw that about needing 2 gates, I figured they must be way beyond what we consider normal noise levels.
But it sounds like they should be reasonable at anything less than the OD1/2 settings?

[The Dude]

Yes. The clean settings are all what I would consider "normal" and useable. As soon as you punch in the OD1/OD2 and set them to "red" or max gain, it's sounds like your at the ocean. There's absolutely no way you could use this amp in the studio.

In fact, thank you for bringing me back around the game board. It just occurred to me that if the amp is quiet on the clean setting, it's unlikely there's a problem at V6 (the first preamp tube). After much chasing in circles, I don't think it's broken. Just poorly designed. Done,......I think?

[The Dude]

For anyone else who comes across one of these animals:

Besides the white noise, one of the complaints was that the amp was prone to feedback as you might expect with so much gain. I was able to find a mod that got rid of most of the unwanted feedback. Strapping a .1UF cap across R97 tamed it nicely. So, now it only makes one noise instead of two.

[Enzo]

The cathode resistor of V7a? How does that reduce gain? Wait, your description of C43 is also not working for me, is your amp the same as the first file in your zip collection?

If it is normal, then so be it. If grounding a cap in the signal path kills the noise, then we think the noise is coming from before that cap.

[The Dude]

Yes. The schematic is the same as the first in the zip file. And, you are correct. Grounding C43 would prove out that the noise would be prior to that. Sorry for the confusion in my statement, "Basically a .01UF to ground". I was referring to the capacitance in the meter when I touched my meter probe to C43- not the actual cap C43. I hope that clears it up.

To be honest with you regarding adding the cap on the cathode of V7a, I didn't really stew on it much. I read about it. I tried it. It worked. It's been a long day and my brain is toast.

[J M Fahey]

I was referring to the capacitance in the meter when I touched my meter probe to C43- not the actual cap C43.

IF adding a small cap to ground (such as your meter which by the way is way less than .01uF , think in the area of 100pF) kills "white noise" and I have seen it quite a few times, then I *strongly* suspect RF oscillation there.
The frequency is anaudible and might even be too high for a normal scope to clearly show it, but we hear the artifacts created by it overdriving some stage and *to us* that looks similar to white noise.

I suggest you scope the following stage and then repeat the probe touching act.