First, try installing a 7pf 1000V cap between plate and grid of the first preamp stage.
BUT, It's really hard to tell from the dark photo. Can only see about half of it.
Is the inputs grounded when you test it? Must be to make a judgement.
So, post more photos, more close ups, better light.
When you have this type of oscillation, it's usually in the first 1-2 stages of the preamp.
Then the rest of the stages amplify it.
Pull out the first preamp tube.
Does the oscillation stop?
I thought so.