I think you've already done that, but here it is again.
Measure the resistance of the OT primary.
1. Turn the amp off and let the filter caps discharge.
2. Set your meter to read ohms on the 2K scale. (you'll be looking for a reading of a few hundred ohms)
3. Locate the OT primary plate wires. There is a brown on one tube socket pin 3 and there is a blue on another tube socket pin 3.
4. Measure the resistance between the blue and brown wires. (no need to disconnect from tube sockets)
5. Locate the OT primary center tap. It should be red and should be connected to the junction of the first filter cap and the 5K resistor.
6. Measure the resistance between the red wire and blue wire. Then measure the resistance between the red wire and the brown wire.
Any open circuit (infinity, overload, etc.) reading in step 4 and/or step 6 means bad OT, or broken lead, or red center tap isn't connected where we think it is, or you're not measuring correctly.
For a good OT, the reading in step 4 will be twice the readings in step 6. Or another way to say this is the sum of the two readings in step 6 will equal the reading in step 4.
If the OT primary is open, the screen may be drawing excessive current and burning that 5.1K resistor. This is just an educated guess based on the fact that there is 8.2 volts on the cathodes, indicating about 30ma of current flowing somewhere. That somewhere must be the screen since the plate has zero volts on it. That 30ma must also flow thru the 5.1K and that calculates to 4.7 watts being dissipated by that 2 or 3 watt resistor.
