Valve amp diagnosis

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Hello! Happy to be here and get to know you!
I recently purchased a Fender Excelsior Valve amp. My first valve amp by thw way.

It makes a lot of noise, like a humming, and it gets louder as i turn de volume up, getting to a sizzling or cracking sound.

The amp does not make any noise, at any volume, when no input jack is connected. When I connect the input jack, in any of the three channels, it starts making the noise. The noise gets louder and starts to sound like cracking as I increase the volume.
When I connect the guitar to the other end of the input cable, applying pressure to the guitar strings reduces the sound considerably.

I have played the same guitar through my friend's Super Reverb and it makes no noise.

I want to know your opinion on the issue.

From what I've read, and I'm a newbie, it seems to me that one or both of the pre amp tubes is broken and needs to be replaces.

Thank you for your time!

Cheers!
 
Check the ground connection on the front panel and input socket. W6, W7, W29 and W36. They are common connections and all should go to chassis ground.
See schematic.
 

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With the greatest respect, a valve amplifier with lethal voltages inside (which you may need to get very close to) is not the best to learn fault finding on.
With the amplifier powered OFF and disconnected from the mains source, Insert your jack lead, check for continuity between the screen connection (outer of the jack plug on the lead) and the chassis firstly. This should be zero Ohms.
 
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