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With the exception of changing the valves in your amp, We do not support self service repairs and always recommend contacting your nearest FASC (Field approved service centre) or Distributor for further assistance.

You can find their details at the following link; 

https://www.bugera-amps.com/buy.html

Please be aware that by opening your unit, you will be voiding any outstanding warranty. 
Please also be aware that if the valves are fit incorrectly which causes damage, this will be classed as misuse and also void the warranty. If you are unsure on how to fit them safely then please contact a FASC or Distributor at the above link.