Metroamp 50w kit - problem

[binito]

Hello,
I have mounted a kit of metroamp 50w.
I have high frecuency noise only when jump the channels and With all the potentiometers to the 10.
I have a beep of high frequency. The noise disappears when down the presence or down the PPIMV LAR MAR.
Without jump the channels , all is ok.I have checked everything and don't know the problem. Any ideas?
Attached video with the noise problem and attached schematic. Download the video to have better sound quality and you can hear the noise.
Thanks in advance.
http://www.dropbox.com/s/r397ux5vwnr..._1798.MOV?dl=0
http://www.dropbox.com/s/gcncjrgf93w...TIONS.pdf?dl=0

[Mick Bailey]

That MOV file is too large for my connection speed. Assuming you're talking about feedback here.

On a new build you may want to check the 'lead dress'. In particular check how the NFB connection to the speaker socket (shown in purple) is routed. The wiring to the 1st preamp tube can also be sensitive to positioning.

A check is to move the wires around while the amp is producing the noise. I use a melamine chopstick. See if it alters the pitch or whether it goes away. A picture of your build could be useful to see how it's wired.

[binito]

That MOV file is too large for my connection speed. Assuming you're talking about feedback here.

On a new build you may want to check the 'lead dress'. In particular check how the NFB connection to the speaker socket (shown in purple) is routed. The wiring to the 1st preamp tube can also be sensitive to positioning.

A check is to move the wires around while the amp is producing the noise. I use a melamine chopstick. See if it alters the pitch or whether it goes away. A picture of your build could be useful to see how it's wired.

The noise seems a problem of input impedances. And when jumping the channels make a high frecuency noise that seems a old radio tuning( all pots to 10)
I will try a change de inputs normal green wires to coaxial , and NFB violet wire.
Any moré ideas ?

[mikeboone]

Make sure your LARMAR PPIMV is not crossed on the leads going to the input grids. The diagram I worked from had them crossed and mine did what you are describing.

[binito]

Make sure your LARMAR PPIMV is not crossed on the leads going to the input grids. The diagram I worked from had them crossed and mine did what you are describing.

Oh, it's probably . I will try and update the results. Thanks.

[binito]

Make sure your LARMAR PPIMV is not crossed on the leads going to the input grids. The diagram I worked from had them crossed and mine did what you are describing.

the wires of PPIMV was in good position.
I tried to change the Green wires of HI input to shielded wire and the noise is same.
I record a new video of noise , and photos of conections.

Video:
http://dl.dropboxusercontent.com/u/2...0/IMG_1825.MOV

Photos:

[oc disorder]

Hi Binito , some very neat work there but I heard your "noise" and it is a weird one !
Perhaps you could sample it and turn it into a feature !
I have only a few things to add.
It is important that the outer terminals on the PPIMV that connect to the shield do not touch ground.
The shield has the bias supply on it.



I noticed on the Les Paul forum a similar amp with the f/b wire to the presence pot at a different angle.



It's a process of elimination. For instance try bypassing the PPIMV just as a temporary measure to make sure
that's not causing anything. You may need to add a couple of resistors just for the test.

Have you some other pre-amp tubes ??? perhaps one of your pre tubes is gurgling ? - making that weird noise !

Although this is not a solution to your current problem , most amps benefit from a thin aluminum sheet under the chassis usually attached to the bottom of the head cabinet. This shields the circuitry from underneath. A friend who works in a printers (maybe offset?) gave me a few sheets they normally discard (recycle) which were easy to work with.

I hope you have some luck with the amp. It wouldn't be always turned full volume but that noise shouldn't be there just the same.

[g1]

Disconnect the NFB wire (purple) at the speaker jack and see if the problem gets better or worse.

[binito]

Disconnect the NFB wire (purple) at the speaker jack and see if the problem gets better or worse.

I disconnected the Purple wire of speaker Jack and the noise is same.

[binito]

Hi Binito , some very neat work there but I heard your "noise" and it is a weird one !
Perhaps you could sample it and turn it into a feature !
I have only a few things to add.
It is important that the outer terminals on the PPIMV that connect to the shield do not touch ground.
The shield has the bias supply on it.

[ATTACH=CONFIG]31661[/ATTACH]

I noticed on the Les Paul forum a similar amp with the f/b wire to the presence pot at a different angle.

[ATTACH=CONFIG]31662[/ATTACH]

It's a process of elimination. For instance try bypassing the PPIMV just as a temporary measure to make sure
that's not causing anything. You may need to add a couple of resistors just for the test.

Have you some other pre-amp tubes ??? perhaps one of your pre tubes is gurgling ? - making that weird noise !

Although this is not a solution to your current problem , most amps benefit from a thin aluminum sheet under the chassis usually attached to the bottom of the head cabinet. This shields the circuitry from underneath. A friend who works in a printers (maybe offset?) gave me a few sheets they normally discard (recycle) which were easy to work with.

I hope you have some luck with the amp. It wouldn't be always turned full volume but that noise shouldn't be there just the same.

I tried Other preamp tubes and inclusive power tubes and the noise persist.
I disconnected the NFB Jack speaker and same.
I installed aluminium shield in cab and noise is better but continues the noise .
I will try disconnect PPIMV .
Other video: ( download for best sound of noise)
https://www.dropbox.com/s/qgypn3yw37...%2022.mov?dl=0

[binito]

I disconnected PPIMV and install 220K resistors. I disconnect purple wire NFB in Jack Speaker. And i've the high frecuency noise when jumping the channels and all pots to 10.( The noise is in bright cannel)
when i jumping the channels with Yellow Jacks (in the photo) The noise is greater. When jump with black Jack(in photo) the noise is less.

[oc disorder]

You will need 2 x 220k resistors to temporarily replace the shielded wires going from the sozo caps to the PPIMV .
The two resistors form a V shape and connect to the bias supply from those sozo caps.
Also you need to connect those points to the grid input resistors on the output tubes (have to undo the heat shrink )
making sure the same point from the soso connects to the same grid as it does now i.e. the new temporary wires must not swap.

So now the output tubes get their bias via the 220k resistors and the signal from the soso cap directly to the grid.


I'm beginning to think it must be around the first stage or tube. I guess if you remove the first 12AX7 the noise goes.

I will download your other video for the noise sound and have another listen.
EDIT: I missed your post .. can you plug in a shorted jack into the input where a guitar would go.. i.e. a jack where
the sleeve and shield are joined. Another way is to plug in a guitar with the volume on the guitar turned down (off).
The yellow lead might not be shielded properly? I'm going to listen to the sound mov now.
Perhaps we need a squelch control.... (joke!)

[binito]

Update:
The noise is in bright channel. If i connect one Jack and the other plug is free , make a antenna noise . If jumping channels make antenna noise with all pots to 10.
AND if Jumping channels with 3Meters cable of guitar don't make noise.

If jumping channels with this cables in other Marshall JMP dont make noise.

Finally, i make jump the grids of V1(pin 2 - 7) and havenīt noise.
Then, where is the problem of input impedance? Cables , jacks?
Sorry for my little english

[oc disorder]

Wow the sound clip is 193mb !
Listening to that clip this time it sounded like a normal Marshall...hummy !
Didn't seem to have the "squelchy bits" the previous one had.
If you turn any Marshall full volume they hum !

The short lead joining the two channels together disconnects ground from both input channel jacks
and joins both inputs to the 12AX7.

At full volume the lead will pick up noise .. there is a lot of volume there.

With a guitar plugged in the signal from the guitar will be a lot louder than the noise.

With a 1 meg or 250k load across the input from the guitar the noise will be a lot less.

[oc disorder]

Sorry for my little english

You are doing well, I (embarrassingly) only speak Australian !!!

What is your natural language ? , there are some techs on this forum who speak other languages who may help.

The bright channel has more gain so it is always noisier !!

EDIT: Have added an MP3 of the noise zipped !