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Hi Strelok,
I was only curious if there was another workaround for this problem. As it turned out, the 470R cathode resistor eventually made the tube oscillate again, so I switched it back to the stock value of 1K without the bypass cap. There was still enough signal swing to make the reverb sound OK. I couldn't actually play with Ra as the tube drives a reverb transformer and both triodes are wired in parallel to allow more current through. The amp suffered from a number of other issues like random ground connections causing ground loops, and there was a really nasty one in the reverb circuit. It had a Vox AC30 4xEL84 power output section with one shared 56R Ck and 100R screen resistors. Each tube was dissipating ~15W of heat in this configuration.
Tomi
I was only curious if there was another workaround for this problem. As it turned out, the 470R cathode resistor eventually made the tube oscillate again, so I switched it back to the stock value of 1K without the bypass cap. There was still enough signal swing to make the reverb sound OK. I couldn't actually play with Ra as the tube drives a reverb transformer and both triodes are wired in parallel to allow more current through. The amp suffered from a number of other issues like random ground connections causing ground loops, and there was a really nasty one in the reverb circuit. It had a Vox AC30 4xEL84 power output section with one shared 56R Ck and 100R screen resistors. Each tube was dissipating ~15W of heat in this configuration.
Tomi
