My amp uses 2500 plate voltage. At start up I occasionally hear an arc, but cannot see it. Never blows a fuse. Once running no arcs. I have better than 1/4" in-between HV conductors except at each can capacitors that the aluminum case is not connected to anything and it has a plastic sleave with 1/8 clearance from ground. Caps have an 1/8" clearance from case with the 600 volt insulated wire on the negative leads only and each cap sees 325 volts. The can caps case is at least 1/8 inch from the grounded case. It uses a voltage doubler and eight 220uF 400 volt capacitors in series. Each doubler leg uses 3KV of diodes. The HV has a 1000pF 5KV capacitor to ground. PSDII shows less than 50 volts overshoot at start up. No chokes in HV circuit. I do not get it
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Breakdown potential for air at STP is ≈3,000,000 V/m. For 2500v this corresponds to approximately 0.8mm (i.e. very small). Sharp points that are producing corona can reduce this potential by almost an order of magnitude. In your case 8mm. Look for sharp points on high voltage connections which are close to ground. Your PSU may also be overshooting at startup before the power stages is fully conducting. This could raise potential substantially. Make sure your HV wiring is per BCP. High B+ voltages like this don't allow much room for error.
Wiring is HV wire at output. I believe the PSUII power supply simulator is wrong and PS has huge overshoot at start up. But, a simple voltage doubler into single capacitor stage does not normally overshoot much.
Try a stepped load, to simulate low current draw through the cold tube at startup.
The linked thread is long but after reading it, I considered it time well spent.
https://www.diyaudio.com/community/threads/the-midlife-crisis-my-833c-amp-build.232484/
I have seen arcs occur inside an OPT or power transformer usually at the point where the internal windings are connected to the lead wires. This is more common on older transformers that used fiberboard which has absorbed moisture. The arc often jumps to the end bells and usually leaves a tell - tell mark. Continued arcing will carbonize the path rendering the transformer useless.
I suspect a soft start will cure the issue. Simple to build using a 50 watt 20 ohm resistor in series with line voltage in and a relay coil fed from resistor that closes the relay contact removing resistor from circuit in a second after the HV capacitors charge.
Been there, done that. Got tired of aimlessly flying around from nowhere to newhere ...
Jan
A circuit diagram of the amplifier and the power supply might make this more interesting for everyone.