My DC30 suddenly got very boomy in the low mids

ToneControl

around the 12th fret on the bottom string
suddenly all the guitars I try are mid-heavy and woolly 

is this just needing a valve replacement, or a trip to the amp repair shop?

Moe_Zambeek

Mine used to mince up EL84s regularly.

Assuming you get the same on both channels?

ICBM

Sounds like one or more of the EL84s might have become microphonic and 'loose'. They do that...

As Moe says, they eat EL84s at a rate of knots - that comes from running them at well over the rated dissipation even at idle.

Fuengi

I'm sure you're better at noticing these things than me ... but that happened to an amp of mine and it turned out to be the A string vibrating on the top of my Filtertron pickup. 

ToneControl

Fuengi said:

I'm sure you're better at noticing these things than me ... but that happened to an amp of mine and it turned out to be the A string vibrating on the top of my Filtertron pickup. 

I tried 3 different guitars, with very different pickups, very sudden too

ToneControl

ICBM said:

Sounds like one or more of the EL84s might have become microphonic and 'loose'. They do that...

As Moe says, they eat EL84s at a rate of knots - that comes from running them at well over the rated dissipation even at idle.

so replace just the EL84 set?

ICBM

ToneControl said:

so replace just the EL84 set?

Yes. If you own a Matchless it's a good idea to keep a spare set of EL84s at all times anyway .

ToneControl

ICBM said:

ToneControl said:

so replace just the EL84 set?

Yes. If you own a Matchless it's a good idea to keep a spare set of EL84s at all times anyway .

any tips on brands at the minute?

ICBM

JJs probably, of the new production. They're tougher than any of the others.