HUM IN MARSHALL JVM 410 JS OR GROUND PROBLEMS?

Darthslater

Member
Joined
Oct 14, 2020
Messages
42
Reaction score
19
So, i have a JS JVM Marshall and recently had a problem with a tube and noise like a hum very annoying this was never there before.
I took it in and they replaced a lead to the input switch because it was microphonic.....it helped, but did not solve the problem...
I know there are guys in here that know this amp internals and what the trouble could be it does this even when the guitar cord is unplugged from the input, it does not seem to do it
on my clean channel...the amp still sound great but when you are not playing the buzzing is annoying...the noise is like unplugging your guitar and putting your finger on the end of the cable.
Can one of you tech gurus help?
 

PelliX

Well-Known Member
Joined
Jun 16, 2020
Messages
2,644
Reaction score
5,227
it does this even when the guitar cord is unplugged from the input, it does not seem to do it
on my clean channel...

That sounds like you could have a bad contact/ground somewhere inside. Are you sure it's not doing it on the clean channel to a lesser degree as well?

Suppose a preamp valve could do that, too, indeed...
 

Darthslater

Member
Joined
Oct 14, 2020
Messages
42
Reaction score
19
Tubes are all brand new....it is not doing it at all on the clean channel we are running them pretty high volume wise.....bad thing is we are going to be recording, I can't have that nightmare going on
My thought was it could be on the input or in the loop i just do not know the internal workings of this, and being that it is so different inside than the standard 410...not many techs know them well enough...so to you guys saying the JS is almost the same...uh....NO!!! That would be like saying a cat is another breed of dog.
 

PelliX

Well-Known Member
Joined
Jun 16, 2020
Messages
2,644
Reaction score
5,227
so to you guys saying the JS is almost the same...uh....NO!!! That would be like saying a cat is another breed of dog.

The what now? :scratch:

Long story short, I presume you don't want to poke around inside?
 

Darthslater

Member
Joined
Oct 14, 2020
Messages
42
Reaction score
19
If i knew what I was doing I would I checked all the connections but....if it is a resistor or transistor I wouldnt know what to fix it with?
 

NewReligion

Well-Known Member
VIP Member
Joined
Nov 17, 2009
Messages
11,793
Reaction score
9,455
Location
Paradise
Tubes are all brand new....it is not doing it at all on the clean channel we are running them pretty high volume wise.....bad thing is we are going to be recording, I can't have that nightmare going on
My thought was it could be on the input or in the loop i just do not know the internal workings of this, and being that it is so different inside than the standard 410...not many techs know them well enough...so to you guys saying the JS is almost the same...uh....NO!!! That would be like saying a cat is another breed of dog.
PREFACE: Which type of tube went bad. Preamp or Power. If Power tube it could have left a carbon trail from an arc causing a plethora of issues. If Preamp tube there are some trouble shooting measures below.

1.) Was the amp under warranty?

2.) Why accept it from the Technician who didn’t repair it properly in the first place?

3.) Take it back and tell them to repair the amp properly. (Can we have the name of the company or individual who did the presumed poor job)? [See my point? They will not want that kind of exposure].

Trouble shooting if you choose to ignore 1-3:

A.) With a known good “instrument” cable plug your guitar in to the effect loop Return. You should get a very loud clean signal through your cabinet.

(This will cut the trouble shooting work load in half as it will tell us if the anomaly is in the preamp or power amp section of the amp. I am guessing preamp as you stated the clean channel is pristine.

B.) Regardless of being new tubes I have received bad new preamp tubes. Example: A 12AX7 preamp tube is a dual triode, that is to say two stages in one glass enclosure. I have not only had one side be noisy but one side entirely not working with new tubes.

Most people are not aware but side B of the tube (pins 6/7/8) were designed to have a quieter signal to noise ratio.

Take a known hood 12AX7 then one by one starting from one side of the preamp tubes pull one and replace with the known new good tube. If issue persists reinstall the tube, move to the next tube etc…

Once you have answered these questions for we who are willing to assist you & completed the tube rolling trouble shoot we can go from there.

It sounds as if a important ground has come loose or some has accidentally or buy human error created a ground loop.

Best Regards, David
 
Last edited:

NewReligion

Well-Known Member
VIP Member
Joined
Nov 17, 2009
Messages
11,793
Reaction score
9,455
Location
Paradise
@PelliX I will post my response to your question here as well. David

Balanced vs Unbalanced TRIODE:​


Good Read Reference:
https://www.tubesforamps.com/everything-you-need-to-know-about-12ax7-tubes.

Due to manufacturing tolerances, the two triodes in a 12AX7 tube are not always the same. The triodes can vary quite a bit in some 12AX7’s. Pins 6, 7 & 8 are side 1 or A. I can’t find my references but I know I read it years ago from Randall Aiken & in a Philips Data sheet. However if you look at most cascaded amps the high input goes straight to side 1(A) pins 6, 7 & 8 lending support to my statement. I will continue to search though the difference is negligible.

The unmatched triodes are not an issue in most circuits. In some cases, however, many believe it is beneficial to use a 12AX7 tube with matching triodes in the LTP PI (Long Tail Pair Phase Inverter). I say it is a waste of time.

For example the phase inverter, which drives the power tubes. In a phase inverter, one of the triodes drives one power tube, and the other triode drives the other power tube.

(NOTE from Me, David Hopkins) Marshall used a 82k & 100k resistor in this position in an attempt to create a balanced Phase Inverter.

It's the negative feedback that necessitates the use of the 82k/100k arrangement. When you imbalance things by swapping those resistors it does indeed have a tonal effect. It will not cause harm to the circuit.

Again I know there are guys who go out of their way to get a matched triode in the PI position.Those people do not understand imbalance has already been engineered into the circuit. This is one area where I may experiment in replacing the 82k resistor. (Trust your ears).

Some amps have a balance pot installed to either correct or induce an imbalance.

Some of the best sounding amps have an imbalance here at the PI. I work it to achieve 6-8VDC difference between the 82k & 100k.

I believe Dave Friedman told me his “his magic spot” is about a 7.5VDC difference which would mean pulling the 82k and replacing it.

You do not want a 12AX7 with two triodes that are wildly different from each other. Of course, this is not just with 12AX7 but the same applies to any other dual-triode tubes like 12AT7.

Some may offer matched triode screening options on all dual-triode tubes. Many swear by matched triode tube being ideal for use in the phase inverter circuit. But remember Marshall already sabotaged this idea by use of a 82k & a 100k in the PI section with its long Tail Pair designed PI.

The Long-Tail Pair, General The Marshall/Fender phaseinverter is commonly known as a "long-tail pair", or "Schmitt" type phase inverter, or phase splitter.

As I stated I will continue to search for the quotes I read.

Hope this helps to demonstrate how dual triodes are not balanced. Unfortunately I am having issue locating my supporting data but by utilizing pins 6, 7 & 8 first this should point to that side being the quietest.

Sorry for any typos as it is early or late depending on how viewed as I’ve been up all night.

David Hopkins
 
Last edited:

spacerocker

Well-Known Member
Joined
Feb 6, 2009
Messages
1,883
Reaction score
1,574
Location
UK
.so to you guys saying the JS is almost the same...uh....NO!!! That would be like saying a cat is another breed of dog.

You say that you don't know much about valve amps - and then you make a statement like that???? Have you examined the circuit diagrams and compared both amps?

If you did (and if you knew what you were looking at) - you would find that the circuit topology for both amps is basically the same! The JVM JS has slightly different tone shaping component values, has lower plate resistor values, has a switchable mid-shift on the OD2 tone circuit, and has a built-in noise gate instead of the 410H reverb. And that's about it! I would say that the JS model is 95% the same as the standard model from an electrical circuit point of view!

Let us know what the problem turns out to be? I will guarantee that it will be nothing unique to the JVM JS!

The difference between the standard JVM and the JS model is NOT Cat vs Dog - more like Whippet Vs Greyhound!
 

Darthslater

Member
Joined
Oct 14, 2020
Messages
42
Reaction score
19
LOOK, I AM NOT HERE TO GET INTO A PISSING MATCH I DONT HAVE TIME FOR THE BULLSHIT...I KNOW A LITTLE BIT ABOUT ELECTRONICS BUT NOT ENOUGH TO FIX AN AMP ESPECIALLY ONE WITH 15 RELAYS SANTIAGO ALREADY SETTLED THE FACT THAT THE JS IS A SUPERIOR VERSION OF THE MARSHALL JVM 410 STANDARD...IT WAS IMPROVED IT ALSO HAS MORE BOARDS IN IT...AND THE CHANNELS WERE ALL RE WORKED..SO NOT THE SAME ANIMAL AT ALL, YOU DO NOT LIKE THAT ANSWER? TAKE IT UP WITH SANTIAGO HE CAN TELL YOU ABOUT IT IN FAR GREATER DETAIL....COME TO FIND OUT THE PROBLEM STEMS FROM THE EFFECTS LOOP...SO, AND AMP TECH IS LOOKING INTO FOR ME AND IS GOING TO TRY GROUNDING THIS ITSELF...OH AND HE HAS WORKED ON THE STANDARD AND THIS ONE ISNT LIKE IT AT ALL IN HIS WORDS WHY THIS ALWAYS IS A CONFLICT IS BEYOND ME....I OWNED THE STANDARD AND GOT RID OF IT FOR MANY REASONS ONE OF THE MAJOR ONES WAS THE COMPRESSED SOUND AND THE FACT THAT YOU COULD NOT FULLY UTILIZE THE GAINS WITHOUT IT SQUEALING LIKE A PIG...NOT TO MENTION THE FACT THAT THE TONES WERE LACKING IN ALL THE AREAS I FREQUENT....OH AND BEFORE YOU SAY ANYTHING I ALSO PLAY METAL.
 

PelliX

Well-Known Member
Joined
Jun 16, 2020
Messages
2,644
Reaction score
5,227
Oooh, smooth. Caps lock is the cruise control of cool. :cool:

So, wait, it's the FX loop that's causing the problem, and he's going to ground it, correct? Can you elaborate?

I base my question on this snippet:

COME TO FIND OUT THE PROBLEM STEMS FROM THE EFFECTS LOOP...SO, AND AMP TECH IS LOOKING INTO FOR ME AND IS GOING TO TRY GROUNDING THIS ITSELF

I guess @spacerocker was right.

I will guarantee that it will be nothing unique to the JVM JS!
 
Last edited:

Darthslater

Member
Joined
Oct 14, 2020
Messages
42
Reaction score
19
YES SO FAR THIS IS WHAT HE HAS SAID, I WILL BE PICKING IT UP FRIDAY AND THEN WILL ELABOARTE MORE ONCE I HAVE A CONCLUSION I SHOULD ALSO MENTION THIS STARTED OUT WITH A TUBE GOING SOUTH, WHEN IT DID THE CABLE TO THE INPUT JACK TURNED MICROPHONIC AS I SAID EARLIER I DO NOT KNOW ENOUGH ABOUT AMPS TO FIX THE PROBLEM MYSELF WHILE I SEE MANY GOOD POSTS AND INFO ON THIS SITE I ALSO SEE A BUNCH OF BULLSHIT FROM ARGUMENTIVE JACKASS'S LOOKING FOR TROUBLE. FOR THOSE GENUINLEY INTERESTED I WILL GLADLY SHARE WHAT I LEARN FROM THIS...
 

marshallmellowed

Well-Known Member
Joined
Jun 26, 2009
Messages
5,389
Reaction score
8,947
So, i have a JS JVM Marshall and recently had a problem with a tube and noise like a hum very annoying this was never there before.
I took it in and they replaced a lead to the input switch because it was microphonic.....it helped, but did not solve the problem...
I know there are guys in here that know this amp internals and what the trouble could be it does this even when the guitar cord is unplugged from the input, it does not seem to do it
on my clean channel...the amp still sound great but when you are not playing the buzzing is annoying...the noise is like unplugging your guitar and putting your finger on the end of the cable.
Can one of you tech gurus help?
Is the hum present with nothing in the FX loop (loop bypassed)? I had an HJS, and the loop was very sensitive to the type of patch cables used in the loop.
 


Top