help needed: were bad output tubes the cause?

[sundance_kiddsg]

a while back i played through my 2466 with a faulty speaker cable but managed to catch it just in time. i'd thought i was safe because subsequently the amp sounded fine...until the weekend when it started humming.
the hum was there once the amp is off standby, with nothing plugged in and all knobs at zero.
i decided to check the bias and had a rude shock to find that one side was down to 30mv while the other remained at 70mv. out of curiosity, i turned the bias trim down low (counter clockwise) and the hum level also decreased.
any advice on what i should do next?
is it time for a change of tubes?

thanks in advance for the help!

[SteveD]

Sounds like you have one output valve gone south on the 30mV side. If you can replace the set you can keep the good ones of the old set for spares.

[sundance_kiddsg]

thanks, Steve, for confirming.
i've just swapped out the existing output tubes for a matched quad set of gold lions and now the amp is fine: both sides biased at 72mv and no trace of any hum
cheers

[Spitfire]

Sounds like you have one output valve gone south on the 30mV side. If you can replace the set you can keep the good ones of the old set for spares.

Mr Dawson, do you have info you can share, regarding the autobias firmware.
Does Marshall intend to bring this out as a feature on any of the other standard production models, like the vintage reissues ?