HELP strange noise inside.

[12afael]

a friend bring me his plexi marshall clone (old tube guitar amp), i`m trying to eliminate a noise coming from one of the first stages.
here is the schematic of the amp.
http://www.freeinfosociety.com/electronics/schematics/audio/pictures/marshall1987.gif

it has the one wire mod that basicaly put the first  stages in cascade. I put it back at the original design but the noise is stil there.

here is how the noise look like. it have a 50Hz component but those spikes are strange to me. could it be noise induced to the ground? I have checked the b+ of the stage and it have no ripple.
the power supply use a brigde rectifier but use just two diodes of it. like a regular two diode full wave rectifier. Should I try some faster diodes?

the noise is just at that stage, I have tried filtering the anode resistor and it reduce the noise also a resistor from grid to ground reduce the noise, I have changed the tube and no difference . what I haven`t tried yet is replace the anode resistor but this noise does not sound like a bad resistor.

I`m running low of ideas so any help is welcome. thanks in advance!

Rafael

 

[12afael]

the noise is the same as this serie.

https://www.youtube.com/watch?v=SUBk_ZQPLDI

he did a brute force aproach puting DC on heathers. the strange thing is that the noise is just in one stage in my case. I have tried some snubbers(RC) but it is not easy to calculate a good RC filter without the information of inductances of the power transformer.

 

[12afael]

those positive and negative spikes can only be produced, if I'm right, from the windings of the secondary and produced by the current peaks of each diode of the full wave rectifier. it can be induced magnetically to the heaters in the transformer, it can be flying as RF if the diodes are ringing or it is contaminating the ground of the amp.

the heathers are not perfectaly clean but it does not have clear spikes, just some distortion where the spikes are in the audio signal.  not clear that the heaters are the problem  I will try rectifing them.

 

[12afael]

I wonder if a ferrite bead could be useful at heaters and the center tap of the power transformer. I 've never seen something like that.

 

[PRR]

CT of PT must go direct to bottom of first filter-cap, NOT through chassis.

Where things "ground" matters. Since you have trash on first stage, look there and also input jacks.

 

[12afael]

Thank PPR, yes the center tap is on the - of the first filter and then with a short cable to the startground very close to the IEC conector ground.

I've tried moving wires and also changing the ground conection of that stage from the input star ground to the general star ground directly with no luck.

this amp had the one wire mod switchable. when the two input stages are in serie the problematic stage is the second stage the first remain clean. in stock 1987 mode just one of the inputs have noise. and it is the normal not the bright.

 

[12afael]

I put DC on the heaters of the first tube the noise is still there, I even tried a battery.

I have changed the starground of the first stage from chassis to the main starground near the IEC conector.  nothing.

I have no more ideas . any priest in the forum to help me with an exorcism?

 

[pucho812]

I'm sure you replaced the tube... Have you checked caps? is it a noisy resistor?

 

[12afael]

yes pucho, I've replaced the tube, the plate resistor, I have checked the coupling cap and I replace the catode resistor.  but this kind of noise can't be from a bad component at the stage it should be produced in the power supply, those spikes going positive and negative are produced, if i`m right, just at the center tap of the power transformer.  this noise is not radiated by the heaters becase I already test with a battery. it come from the air as RF or it is in the ground but I don`t know how to test that. I already installed snubbers caps for the RF and I've tried different ground wirings.

I will record an audio sample tomorrow.

 

[joaquins]

Guitar amps are really high gain (especially the plexi) with unbalanced path! A really twisted mind should think this out!

Normally I've seen going to slower diodes, tube rectifiers or cap across the diodes (as the easiest way) to avoid rectification noises and sparks, more than going to faster diodes. Fender did this in some amps from factory. I've build a AC15 likely amp but never have real issues with noise at it, a cap went bad because many caps doesn't like to work at 300V even if they are rated for... bad pots, never anithing serious. But a lower gain amp mine...

Plexi isn't that high neither, a friend build something like the SLO and he did get a lot of trouble, he rewind the supply tx, he rewind the output TX and the choke... a new PS PCB if I remember right was what help the most, better choke also. first input TX was too big even for this amp and the new smaller one was better winded so it ended up working better, but not affect at noise. I don't remember supply tx changing anything. Once he end up with all this and get it sounding incredible he sold it because it wasn't the sound he wanted and build something more similar to a JCM or something like this, but that's another story...

JS

 

[walter]

Are the wires shielded from the input to the grid? You can put shielded wire from the input to the grid and connect the shielding on one end only to the plate, this will cancel the RF noise, or just connect a piece of wire to the plate and twist it a bit on the wire at the input.

 

[12afael]

the wire from the input jack to the 68k resistor IN the tube socket is shielded with just one side conected to jack ground. the grid wire from second stage/input is also shielded in one point. it make a difference if I connect the shield to ground or the plate ? it look more dangerous at the plate.

I have built some high gain amps 5150 III, engls, etc. but I never had a noise like this

here is an audio sample of the noise the amp is at full but nothing is conected to the input, it is grounded through the 68k grid stopper of the first stage.  I'm using a hot plate atenuator to test it.
https://soundcloud.com/12afael/plexi-noise

 

[walter]

I would put the 68k at the input jack. There is nothing dangerous about connecting a wire to the plate if it is insulated. That hum reminds me of when the shielding isn't contacting the chassis. Try putting a sheet of metal over the chassis opening. I assume you checked the input jack grounding switch.