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Old 29th April 2011, 03:16 AM   #1
hugz is offline hugz  Australia
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Default Noisy tube rectifier goes quiet when i put my hand NEAR it

I bought a preamp which uses a 274B rectifier tube.

It produces a crackle type noise through my DIY Firstwatt F2. This noise comes and goes, but is fairly constant.

I've noticed that the hum goes down quite a bit if I'm touching the rectifier tube or even if my hand is even CLOSE to it.

What's the deal here? Sounds like Voodoo.

Suggestions?

Thanks

Hugo
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Old 29th April 2011, 03:44 AM   #2
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If there's a crackle it sounds like a bum tube and perhaps best not to use it.

The hum sounds like bad grounding. Not voodoo - you could transform your amp into something like this, which uses the same principle: Theremin - Wikipedia, the free encyclopedia
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Old 29th April 2011, 04:05 AM   #3
hugz is offline hugz  Australia
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Quote:
Originally Posted by carbondated View Post
you could transform your amp into something like this, which uses the same principle: Theremin - Wikipedia, the free encyclopedia
Oh I've seen those in action.. helps to make sense of it.

I'll try get my hands on another tube.

Thanks for your help

Hugo
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Old 29th April 2011, 07:10 PM   #4
wa2ise is offline wa2ise  United States
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Quote:
Originally Posted by hugz View Post

I've noticed that the hum goes down quite a bit if I'm touching the rectifier tube or even if my hand is even CLOSE to it.

What's the deal here? Sounds like Voodoo.
Sounds like a parasitic oscillation, usually supersonic. This oscillation can load down the power supply enough to produce excessive ripple, or mix and modulate the audio to sound like hum. The rectifier isn't likely the source, but a gain stage elsewhere is. It does seem like voodoo, as it can be hard to picture what's going on. Using grid stoppers should get rid of it. A grid stopper is a resistor, usually 68K for tubes like 12AX7s or 12AV6s, or 5K for output tubes (values not critical), placed right at the tube socket terminal for the control grid. the other end of that resistor connects to the circuit that used to feed the grid directly. Idea is to create a low pass filter to attenuate supersonic frequencies, without having noticeable effect on audio. The grid stopper is the R, and the stray and Miller capacitance of the tube itself is the C of a low pass RC circuit.
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Last edited by wa2ise; 29th April 2011 at 07:15 PM.
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