VB-2 issue - fuse blown

[jtroester]

The F2 fuse on my VB-2 was blown. I replaced the electrolytic capacitors in the high-voltage power supply according to the Youtube videos by Bassguitarist1985 (https://www.youtube.com/watch?v=WgyUNnHPOSU) and his DigiKey product list. I replaced F2 fuse by a T2A type for testing purposes. I connected a 4 Ohm/20 W resistor as a load and set the impedance switch to 4 Ohm.
First I switched on the VB-2 in standby mode. The voltages B+ and SB+ were ok, AB+ and AB++ were each approx. 20V lower than specified in the schematic. The voltage on the bias test pin was approx. 51V.
When switching from standby to power mode, main fuse F3 (F5A on 230V) blew immediately.
Does anyone have any idea what the cause could be? Could an EL34 power tube be defective?

[Bassguitarist1985]

Yes could be a bad tube, shorted fly back diodes, bad screen resistors, could be a bunch of stuff. Pull all the tubes. If nothing shorts out, then the tubes, flyback diodes, possibly the OT (very remote) are suspect.