I've just got a Vox Lil night train 2W amp and am generally very pleased with it. There is however an intermittent scratchy noise that comes from it - it sounds like the noise you get when you turn a potentiometer that needs a blast of contact cleaner, except it doesn't seem to be related to any of the pots, and it happens randomly when nothing on the amp or guitar is being adjusted.
I've tried a different head with the cab, and it was fine, so I know it's not the guitar/cable/speaker cable/cab.
I'm reasonably handy with a screwdriver and a can of contact cleaner, but am a relative newbie when it comes to valves. Any suggestions as to where I start to try to sort this out?
Vox Lil night train - noise
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Re: Vox Lil night train - noise
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Last edited by matt h on Fri Mar 27, 2015 3:32 pm, edited 1 time in total.
Re: Vox Lil night train - noise
Try jiggling the tubes (gently) in the sockets. It could be some dirty connections. If that seems to be the problem, you may want to take out the tubes and clean the tube pins with some 2000 grit wet/dry (do it dry). Make sure to clean off all the dust and grit before you put the tubes back in.
It could also be a tube that is starting to go south. Try the above first.
It could also be a tube that is starting to go south. Try the above first.
What?
Re: Vox Lil night train - noise
Short version first followed with detail:
I had a similar issue with a different VOX amp. This was on a Vox AC15C1-TV. I corrected it by replacing the ceramic plate high cut cap on the top boost channel input with a 500V silver mica type.
On the AC15C1 this is C16 and is a small 120pf ceramic mounted on the preamp tube board next to V1.
The symptom was short relatively quiet noise bursts of half to two second duration when the amp was warm with input idle or unplugged. The noise sounded a little like the crackle of high voltage power transmission lines, maybe a bit lower in frequency.
The noise would go away for a short period after picking a string or strumming a cord.
The noise varied in amplitude with master volume and top boost volume but remained constant with normal channel volume. From that I concluded that the issue was upstream of the top boost pot within the top boost preamp. The top boost input tube had a 120pf ceramic cap connected plate to cathode (might be across plate resistor or plate to ground on another amp). I have been designing switch-mode power conversion since the Paleozoic era, up to 30kV and have had plenty of issue with multi-layer ceramics or discontinuities in dielectric in general. I happen to have a close enough value silver mica so...
I might normally prefer plastic films for audio filter applications but this was mounted right by the tube socket. It will tend to run hot and silver mica's deal with that well.
What was going on in the ceramic cap? Ceramic caps have plenty of internal imperfections and voids. A web search for "multi layer ceramic cross section" should bring up plenty of photos. Well - the void has lower capacitance as well as lower leakage than the ceramic. In this application with high DC bias the ceramic leakage currents discharge the local ceramic until voltage builds to the flashover point at the microscopic void. That recharges the local ceramic and the noise stops. Leakage currents then reestablish the condition and you get another noise burst. This all gets worse with temperature.
Pick a string and the peak voltage is higher, you will see flashover at the void and the ceramic takes on more charge. This lowers the stress at the void once quiet. It takes a while to reestablish the condition and the cycle continues. Being at the first gain stage there is plenty of downstream gain to make the small partial discharge audible.
Some companies pay big money for partial discharge corona testers.
Local discharge energy is so low the cap will likely never fail but that doesn't alleviate the frustration.
It's luck of the draw as to multi-layer ceramic quality and just what the internal imperfections in any individual cap. A search for "multi-layer ceramic partial discharge" will likely yield some related results.
I had a similar issue with a different VOX amp. This was on a Vox AC15C1-TV. I corrected it by replacing the ceramic plate high cut cap on the top boost channel input with a 500V silver mica type.
On the AC15C1 this is C16 and is a small 120pf ceramic mounted on the preamp tube board next to V1.
The symptom was short relatively quiet noise bursts of half to two second duration when the amp was warm with input idle or unplugged. The noise sounded a little like the crackle of high voltage power transmission lines, maybe a bit lower in frequency.
The noise would go away for a short period after picking a string or strumming a cord.
The noise varied in amplitude with master volume and top boost volume but remained constant with normal channel volume. From that I concluded that the issue was upstream of the top boost pot within the top boost preamp. The top boost input tube had a 120pf ceramic cap connected plate to cathode (might be across plate resistor or plate to ground on another amp). I have been designing switch-mode power conversion since the Paleozoic era, up to 30kV and have had plenty of issue with multi-layer ceramics or discontinuities in dielectric in general. I happen to have a close enough value silver mica so...
I might normally prefer plastic films for audio filter applications but this was mounted right by the tube socket. It will tend to run hot and silver mica's deal with that well.
What was going on in the ceramic cap? Ceramic caps have plenty of internal imperfections and voids. A web search for "multi layer ceramic cross section" should bring up plenty of photos. Well - the void has lower capacitance as well as lower leakage than the ceramic. In this application with high DC bias the ceramic leakage currents discharge the local ceramic until voltage builds to the flashover point at the microscopic void. That recharges the local ceramic and the noise stops. Leakage currents then reestablish the condition and you get another noise burst. This all gets worse with temperature.
Pick a string and the peak voltage is higher, you will see flashover at the void and the ceramic takes on more charge. This lowers the stress at the void once quiet. It takes a while to reestablish the condition and the cycle continues. Being at the first gain stage there is plenty of downstream gain to make the small partial discharge audible.
Some companies pay big money for partial discharge corona testers.
Local discharge energy is so low the cap will likely never fail but that doesn't alleviate the frustration.
It's luck of the draw as to multi-layer ceramic quality and just what the internal imperfections in any individual cap. A search for "multi-layer ceramic partial discharge" will likely yield some related results.