Kel34 Hum

[Bickers80]

Good afternoon chaps, hope everyone is doing well and enjoying plenty of music in lockdown.

I've looked through some of the previous threads on the El34 but I'm hoping someone may be able to help me out.
I've developed a hum on the right hand channel, I've disconnected all inputs and changed valves around with the same results.
I'd already ordered some new valves ,the current ones have been on for about ten years so I thought I'd treat myself to some new(they haven't arrived yet)! It was serviced around 10 years ago too with new caps and resisters by a chap in Leighton Buzzard although I'm struggling to find his details now.
I didn't do the original build and I'd sooner not start meddling too much. So what I'd like to know is , what are typical reasons on this amp for a channel hum and also can anyone recommend someone to look at it for me? I'm based in Northamptonshire.

Thanks

Matt