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Amp static noise

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bodhibodhi Frets: 1334
in Amps
Wonder if anyone can help with this.

I have a Cornford Carrera which is making this weird old AM radio alike static noise, which gets a lot worse the higher I turn up the master volume, and seems to disappear when the master volume is maxed.  It does this even if nothing is plugged into it, and only the master volume seems to affect the noise.  None of the other controls make any difference.

I have no idea when this started, because I usually play at very low volume with the gain at maybe 3/4 and the master volume so low that you can comfortably speak over the amp.  And the static isn't audible to me at this setting (so you might say in that case it's not a problem, but on the other hand I don't want to cause any further damage if something is wrong).

At my regular setting, the amp still seems perfectly fine to me.  I only noticed the static tonight because my son fiddled with the amp and turned up the master volume by accident.

I changed tubes in April last year.  I still have various older ones so will try swapping them around over the weekend.  Switch off other appliances, check speaker and reverb tank cables, and so on.

I guess it might be amp tech time just to be safe, but I was wondering if perhaps someone could give me some pointers on how to troubleshoot properly.

It sounds a bit like this, just worse - more pulsing and crackly.  And it is quite loud - definitely not normal tube amp hiss (I think).

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

Thanks.
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Comments

  • ecc83ecc83 Frets: 1626

    Sounds  very like a noisy pre amp valve to me. If practical to do so, try tapping each one in turn with a pencil or the handle end of a very light terminal screwdriver.

    The other possibility is a resistor gone noisy, especially if the amp makers subscribe to using the daft Carbon Comp horrors!

    Dave.

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  • ICBMICBM Frets: 72242
    Sounds like it’s the valve either immediately before or after the MV control - my guess would after, ie probably the phase inverter. The reason the noise goes away when it’s up full is because the source of the noise is then effectively directly coupled to a lower impedance which suppresses it.

    It could be other things but that’s where I would look first.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein

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  • bodhibodhi Frets: 1334
    edited March 2018
    I swapped around valves this morning, and it ended up being the 8-pin 6V6 output valve.

    Much better now - thanks for the replies 

    I don't understand the technicalities of tube amps, but the valve arrangement relative to the controls are:

    Master Treble Middle Bass Reverb Gain
    8-pin 9-pin Preamp Preamp Preamp



    Not sure if that's enough to tell which ones are before or after the master volume control?  Would be interesting to know, although at this point I'm just happy that the amp is back to normal.
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  • ICBMICBM Frets: 72242
    Aha. I was both right and wrong :).

    Wrong - I forgot this amp is single-ended, so no phase inverter. The master volume is in fact directly before the power valves.

    Right - the problem was caused by grid leakage from the valve (or in this case one of them) after the MV.

    The reason it went away when the control was full up is because there is a cap in parallel with it, connected to the top of the pot - so when the control is full up this cap will then suppress the noise. If the pot is turned down a bit there is some resistance between the two so it won't. (If it's turned down to zero, the noise is grounded out.)

    Does that make sense?

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein

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  • bodhibodhi Frets: 1334
    That doesn't make one bit of sense!   =)

    But the amp is now making the right kind of noises when I play guitar, so that's good enough for me!

    Thanks!
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  • ecc83ecc83 Frets: 1626
    bodhi said:
    That doesn't make one bit of sense!   =)

    But the amp is now making the right kind of noises when I play guitar, so that's good enough for me!

    Thanks!

    Believe me, it DOES!

    Dave.

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  • ecc83ecc83 Frets: 1626

    Actually..."Changed 'tubes'* last April"...When you do this, if you can, set the amp up driving a load and delivering 1/2 rated power or so (NO amp owner should be without a bog s test meter!) after fitting the valves and run it for a good hour. If safe and practical to do so, give each bottle a little tap with a pencil from time to time.

    I used to do this after every valve change and a good 20% if the time I found a crackly ECC83 in the front end!

    Dave. *(you mean 'valves' but we shall let it go this one time!)

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