JCM 800 4010 Noise when plugged in to high input

[PelliX]

The tech said someone has been in here before. There was this poorly replaced capacitor. He is going to replace the caps while hes in there
View attachment 154806

Lol, C21 is missing and that dodgy top-side tack-on is actually R30; a resistor. Some tech before this one was a bit lazy, it seems.

 

[66fora69er]

Lol, C21 is missing and that dodgy top-side tack-on is actually R30; a resistor. Some tech before this one was a bit lazy, it seems.

What will the missing C21 resistor affect? "capacitor" was my wording not the tech, I dont know much about the insides of these things. Thanks

 

[PelliX]

What will the missing C21 resistor affect?

In simply terms, it could affect the high end a little bit.

"capacitor" was my wording not the tech, I dont know much about the insides of these things. Thanks

No worries! C=capacitor, R=resistor, D=diode, T=transformer, V=valve - so far these letters are all quite logical. It gets a little more confusing when you see transistors marked Q or find out that inductors use 'L', old schematics referring to diodes as 'CR', etc.

 

[66fora69er]

In simply terms, it could affect the high end a little bit.



No worries! C=capacitor, R=resistor, D=diode, T=transformer, V=valve - so far these letters are all quite logical. It gets a little more confusing when you see transistors marked Q or find out that inductors use 'L', old schematics referring to diodes as 'CR', etc.

I don't want the sound of the amp changed at all, I just want the noise stopped. Fingers crossed and thanks for the help.

 

[JosephBatista]

Hi!

Wondering if you could solve the issue?

I have a similar problem on my 2204 clone, it is not a crackling noise but a reasonably loud white noise when cable is plugged.
Already tried changing capacitors and resistors around V1, already tried installing shielded cables for grid connections, swapped V1 with many different tubes.. without success.

Thanks

 

[66fora69er]

This is still ongoing. The tech changed the input jack with no change. All Caps were changed with no change to the noise. He's still trying to figure it out.

 

[PelliX]

I don't want the sound of the amp changed at all, I just want the noise stopped. Fingers crossed and thanks for the help.

So you've been to a tech and he restored the stock layout, right? This crackling, it doesn't change if you wiggle the input cable/jack, correct? If it's only happening on the High input, not the low then it has to in be the first gain stages. Assuming that you've swapped out the V1 valve and that had or has no effect, I'd suspect the capacitors and/or resistors around V1a.



R3 could be a possible culprit, but I'd probably check/replace C1 and C2 first. This is all presuming you have A) swapped V1 and B) have normal operation on the low input channel.

This is still ongoing. The tech changed the input jack with no change. All Caps were changed with no change to the noise. He's still trying to figure it out.

Hmm, not sure how impressed I am with a tech shotgun replacing jacks like that. On a bench this fault would be easy to find. Does he mean "all" caps, or just the large filter cans? This shouldn't take more than half an hour to figure out with the right tools - which your tech should have (and the knowledge to use them). Do you have a list of what he's done so far?

I have a similar problem on my 2204 clone, it is not a crackling noise but a reasonably loud white noise when cable is plugged.
Already tried changing capacitors and resistors around V1, already tried installing shielded cables for grid connections, swapped V1 with many different tubes.. without success.

High input, low input? Is the sound otherwise normal, just loaded with extra white noise? When you say "capacitors and resistors around V1", does that include those for the input jacks?

 

[66fora69er]

Thank you Keith. I will relay this info. Chad

 

[66fora69er]

I will also try to get some clarification on whats been swapped thus far

 

[PelliX]

Thank you Keith. I will relay this info. Chad

It took me a moment to understand the confusion; just to be clear, I'm not Keith Richards. I may often grab a guitar and pretend... but I'm not. I merely use a quote of his in my signature.

You're welcome, of course.

I will also try to get some clarification on whats been swapped thus far

Yup, good move.

 

[66fora69er]

It took me a moment to understand the confusion; just to be clear, I'm not Keith Richards. I may often grab a guitar and pretend... but I'm not. I merely use a quote of his in my signature.

You're welcome, of course.



Yup, good move.

 

[JosephBatista]

High input, low input? Is the sound otherwise normal, just loaded with extra white noise? When you say "capacitors and resistors around V1", does that include those for the input jacks?

The noise only happens when I plug into the high input, even if the guitar has the volume all the way down. Actually, even if I use a wooden stick to go inside the jack and lift the ground switch that grounds C3, the noise comes. Meaning the noise is coming from the high input stage of V1.

The low input alone is dead quiet, but I never use it

The sound of the amp is amazing. The white noise is only audible when I am not playing or if I am playing with the guitar volume very low to clean the sound.
The noise turns annoying during short intervals at rehearsals. Example, when we just stop playing at the end of a song the noise is not that bad because my ears seems like "used" to the high volume during playing. But after like 3 to 5 minutes without playing the noise starts do be perceived and turns annoying.

When I say "capacitors and resistors around V1" I mean those for the high input gain stage. It means, R1, R2, R3, R4, C1, C2, C3 on the schematics posted above.

I am starting to believe that this noise is normal for high gain amps based on some posts I read on forums.. and related threads.

My context is that I am new to the valve amp universe and I am located at a region of Brazil where it is really uncommon to find valve amps or even any kind of guitar amp above 30W so I never had the opportunity to experiment factory made stock JCM800 or other high gain/high power amps. (never even have been near one..)
I am also searching about JCM800 mods and just watched two videos from Marshall where I could hear similar noise on professional videos:

- Between 0:18 and 0:22
- Between 4:24 and 4:54 as he turns the gain down the noise disappears just like mine..
- just after 0:50

What you guys think? Seems like a normal thing?

Now regarding the crackling sound that our colleague @66fora69er has, as per the experience I had trying to troubleshoot V1A, I would do the test of lifting the ground switch on high input jack with a wooden stick and see if the noise starts.. it will confirm that the noise is being generated at V1a circuitry. Then I would consider replacing R1, R2, R3, R4, C1, C2, C3. But I would focus on C3 first. Maybe it has DC leaking? But as said, the caps were already changed with no improvement. So maybe the resistors? When you say the tech changed all caps, are you sure he changed C1, C2, C3 or actually he changed filter caps?

Thanks a lot.

 

[66fora69er]

Noise still there. I'm frustrated. All these parts were replaced. After listening to the videos posted by Joseph above the noise sounds very similar to the videos. Maybe I'm chasing nothing. The weight of the old caps is noticibly different when you handle them, so likely they needed to be replaced. The rest...not sure what to think. Seems like testing prior to pulling parts would have been preferable, Thoughts? The amp still sounds good. Just noisier than I'd like. Thanks for all the input!

 

[66fora69er]

This is with the amp cranked up to 5. I rarely get it louder than 2 or 3

 

[JosephBatista]

Seem pretty similar to mine.. (PS> Ignore the humm, focus on the white noise/hiss) I screwed up with the ground scheme while doing some experiments trying to reduce the hiss. When I turned the volume of the guitar, I put it on around 1 just to have a reference of volume of guitar VS noise. If I had turn the guitar all the way up the walls of my bedroom would had been wrecked

 

[NickKUK]

So my amp is has a jcm800 front end.

I has alot of noise (hiss) when plugged in but nothing when not connectwd.
* i switched v1a gridstopper from a 68K metal film resistor to a 10K 1W film resistor. This reduced noise as resistor noise is mathematically a factor of temperature and resistance.
* I put in a silver mica 47pF from the v1a to ground. This allows unheard radio frequencies to be pushed to ground and not amplified.
* I insulated the switchcraft socket I used instead if the plasic. Key bit here is to check the correct grounding of the rotary pots to the front panel and the jack connectors are correctly grounded.
* I used grounded shielded cabling for the majority of the signal path.
* ground shielded localised B+ reservoir caps (my implementation doesn’t use muti-section capa).
* grounded shielding on the underside of the chassis.

So in short as it’s the high gain:
* nail any ground loops/lifted grounds
* reduce v1a gridstopper to reduce noise - high gain will amplify this hiss.
* accept old school carbon comp resistors are noisy, or, switch to metal film to reduce noise.
* increase wattage raiting (larger resistors) to reduce noise.
* check for dc leak of the coupling caps
* shielding is your friend.
* check your speaker cable.. and guitar shielding..

Now how much of thr mojo you kill or not kill is up to you.