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Vox Valvetronix VT40+ Hum Problem


Semolina Pilchard-Player

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Hi all. I've found many references to what seems to be a problem with these Vox amps. I bought one a while back for home recording/practice sessions and haven't really used it much. However, having decided to use it a few days ago, I discover that it seems to have developed the dreaded mains hum problem. The hum exists regardless of whether any input lead is in the amp and increases in volume with rotation of the power level control (0 to 60 watts). The hum can only be defeated by plugging into the headphone jack and there is no noise when listening via phones.

 

I'm not in any way qualified to work on these things (I just play them!), but I'm guessing that this tells me that the hum is occurring somewhere in the power amp stage, my assumption being that this isn't involved when listening via phones. Either way, it makes the amp pretty unusable at any sort of decent rehearsal volume.

 

I've done a bit of research and found several references to similar problems (including YouTube clips such as this: https://youtu.be/wWcrWuvKZV), but no definitive solutions.

 

Is there anyone out there that can help please? Is there a simple/obvious fix for what seems to be a reasonably common problem? I could change the tube in the pre-amp, but I'm not convinced that this is the problem. On the other hand, anything that needs much work/component swapping is probably not economical for this out of warranty amp!

 

Thanks!

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You're right, it's unlikely that the preamp tube is causing this problem. On older amplifiers, power amp hum is usually due to the electrolytic filter capacitors "drying out" and losing their capacitance with age, but this shouldn't happen to a solid-state amp that's less than 10 years old. Like the label on the amp says "no user serviceable parts inside".

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If the headphones don't have hum then your preamp is likely OK and the hum is isolated to your power amp section. Swithing to higher watages seems to confirm this as well.

 

These amps use a 12AX7 to emulate how power tubes work. They use the preamp tube as a low power push pull or single ended amp then boost the output with a Mosfet solid state power amp. This can give the amp a tube amp tone and feel and is likely better then amps having a tube in the preamp section.

 

This tube likely sees an AC filament voltage and depending on the quality of the tube it may be radiating that AC pulse into the amplification circuit.

 

What I'd try is a large plate spiral filament tube designed to minimize AC hum. Sovetec makes a 12AX7 LPS tube that's designed to minimize hum. They have two versions a budget version which is $13 and a Black Sable that's $49. I'd try the budget version and see if there's any improvement in the hum. I'm not a super fan of the Sovtec 12AX7 tubes for tone. They aren't bad but they aren't fantastic either. Another tube like an EH cant give you strong bright tones but what good is that if it hums.

 

I'm also not sure if the circuit uses a starved voltage circuit. I doubt the tube sees a 280V plate voltage so I suspect its being run at a much lower voltage and most of your real tone if coming from SS circuitry. I've experimented in swapping tubes in starved tube circuits and the differences between tubes is a bare minimum so even if the Sovtek tube doesn't produce the best tone it will not be noticed.

 

The hum may come from other sources, shielding within the amp chassis. I'd also try the amp in different rooms to rule out house wiring issues. Dimmers, fluorescent lighting, (The new light bulbs we were forced to use) CRT Monitors or TV's, anything with a high voltage high current source may be generating EMF waves and the power amp is picking it up and amplifying it.

 

Vox circuits by design tend to have high gain in order to get that unique vox tone but as a trade off they tend to be susceptible to hum. Even the Vox Stomp Lab effects pedal I have can hum badly with certain amp emulations dialed up. Its got a noise gate which mutes the hum between notes quite well however, otherwise the pedal would be pretty useless with those gains/tones dialed up. I suspect the SS vox amps use similar circuits to get the emulated tube tones and have the same issues with hum. You wont be able to redesign the circuits but you may be able to isolate it a bit better.

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