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Discussion Starter #1
Hello!

AMP: Peavey 6505MH

The problem:

After few seconds, when I switch on my amp, it begins to hum (50HZ). The hum is emitted by outputs (speaker, and headphones), but not by „send”.
- The hum isn’t there above 230V, but if the voltage is lower than 230V
- The hum is stronger if the voltage is decreasing.
- At 220V the amp is useless, because of the hum loudness.
- The „resonance” level inluences the loudness of the hum.
- The switch on of the „Crunch” increases the noise too.
- The hum is not depend on volume level, gain level, and channel

https://www.youtube.com/watch?v=a0Eygw5 ... =emb_title

The amp power input is engineered to ~220 – 230V as the writing on the back of the amp.

I tried…
- to replace power tubes
- different outlet, it works fine above constant 230V
- reharsal room outlet with constant 228V, it makes the humming noise. There is some Peavey amps, but these work properly.
- in my house the voltage is fluctuating between ~220-233V, when the voltage is above 230V, it works fine

The noise is stronger after ~30-40 sec usage.

Why can’t my amp „tolerate” low voltage?
What should I do?
 

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Discussion Starter #3
I swapped all the preamp tubes and the noise has disappeared. I tapped the tubes with a pencil, they seemed good at first. You can only notice a bad tube with tapping, when the bad tube is in the nearest socket to the input jack. So I had to change the order of the "old" tubes in the sockets. The bad tube's noise only sounded from the speaker at the nearest socket to input jack.
 
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