Advice please on valve amp issue

[Bigbird]

Hi guys a freind has a Mastersound compact 845 amp with B&W 805d speakers.

He is slowly losing volume when the amp is on before it cuts out.
I was wondering if any of you valve experts know what the issue could be before he sends it off for repair.
He would like to try crack it himself as its mid Jan before the guys can fix it, so no music over xmas for him .
He has tried changing valves but still happens

Thanks

Karl


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[AD Audio]

At a guess I'd say it's the slow start bias circuit. If the timer circuit goes faulty vol would slowly fade as the bias reduced to the start up mode. But there's lethal voltages of well over 1000volts inside so unless he's suitably qualified he should leave very well alone. Apart from experience you need insulated gloves rated at 3kv

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[Mixc]

Hi
How long does it play for, whats the time-frame ?