Hi Malcolm, I strongly recommend that you don't work on it with it powered up, and check that all the high voltage has been discharged. Once the high voltage is discharged then it is safe to work on, with the mains disconnected.
I am assuming that the 3W resistor between the two large electrolytics is at HT potential (CRC filter?). Make up an insulated lead with two insulated crocodile clips and include something like 100 ohms in the lead by incorporating a suitable resistor.
With power off you can discharge the main electrolytics by clipping onto one of the negative black speaker terminals then clip onto the 3W resistor.
If you use a multimeter on the same connections you should see the voltage dissipate or check that it has. Anything below a few volts is then safe to work on.
Give me a call if you need further assistance.
PS Bleed resistors aren't necessarily needed as when the amplifier is switched off a lot of the stored voltage will drop as the output valves continue to conduct for ashort period. However you can't rely on this to make the amplifier safe.
I love Hendrix for so many reasons. He was so much more than just a blues guitarist - he played damn well any kind of guitar he wanted. In fact I'm not sure if he even played the guitar - he played music. - Stevie Ray Vaughan