Vibroverb 64 issue

[sbirkenstock]

Hi everybody,
did turn on my ´64 Vibroverb the first time in a year or so.
If I play the guitar very lightly, it works fine, if I do a loud single note or a chord,
the amp get´s extremely loud (louder than it should be at volume set between 2 and 3 it feels)
and it sounds very broken, like fuzzy, fartish, ugly distorted. Really bad....
I plugged an external speaker in, same thing. So it´s not the speaker.
Happens with both channels.
Touched all the tubes gently with a chopstick, all quiet.
I also left the amp on (no signal, no issue) for a couple of hours in case the filter caps would like that, but no change.

Where should I start?

Thank you,
Stephan

[LOUDthud]

Is this:

An actual Fender made in 1964 Vibroverb ?
A Fender made re-issue of a 64 Vibroverb with PCBs inside ?
Something you made ?

[sbirkenstock]

yes, the real ´64 Blackface Fender Vibroverb with a JBL D130F installed
it has an issue, but it is not an reissue

[joeboo88]

I would start with installing a new preamp tube in v1 and moving on if the trouble stays. You can also strum hard and see if any tube shows fireworks or no heater light. These are good non intrusive first steps.

[TUBEDUDE]

Touch up some connections with the soldering iron maybe.
For grins check the bias voltage change under load.

[bepone]

can be many things.. heavilly drifted bias resistors.. cold solder joint.. leaking coupling cap..