Amp trouble:

[HTH AMPS]

New valves may not cure the problem - I had something similar where the valve socket was shorting out needing a replacement.

TAKE IT TO A TECH!

From the description — blue/white sparks inside the valve — it does sound like a valve shorting out rather than an external short. However this can and does cause the screen grid resistors to expire, so a trip to the tech might be advisable.

By the way I saw the consequences of a quite spectacular external short the other day. The main problem if this happens is that a trail of (conductive) carbon is left behind that unless cleaned up will make another short a racing certainty.

It's very easy to see if there has been any external tracking by checking the valve sockets for carbon deposits/ burning.

I've seen quite a few old amps where the common heater-to-cathode short has arced the pins leaving a carbon burn and some 'tech' has cut out the burnt track with a stanley knife.  It's asking for trouble doing that imo - if the socket arcs, just bite the bullet and replace it.

Absolutely, get the socket changed.

However carbon is often deposited elsewhere in the amp and this will need removing. For example I had a DSL401 in with a toasted screen resistor that left carbon all over the PCB.

The best short out I've seen is on an AC30 where one of the valve pins melted into the socket!

absolutely, my mate's Mesa single recto output-valve PCB was so badly charred I had to rip the whole thing out and replace it.  I opted to mount the sockets directly onto the chassis and hard-wire the connections - the way it should have been in the first place.  I REALLY don't like output valves mounted on PCBs - they always cause problems.

[nilsampmaker]

Yes that´s why Marshall stopped producing their 2046 specialist, too thin pcb. I own one on which I have mounted the sockets on to the chassis.