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I'm repairing a 1961 Capitol amp that uses four 7591 tubes in push-pull cathode bias. It looks like the tubes have been in there for decades (Sylvania and Westinghouse). I changed the rectifier from GZ34 to a 5U4GB, and it still had 430 volts on the plates. Each tube was drawing 42.3 mA at idle, with a 193 ohm cathode bias resistor and a 50mfd bypass cap (shared between 2 tubes). There was a 16.4 volt drop on the bias resistor. I calculate the tubes were idling at 17.5 watts. I was very surprised the tubes hadn't been destroyed by running them so hot over the last 50 years. Since these tubes are rated as 19 watts, I changed to a pair of 250 ohm bias resistors, and got the dissipation down to 15.3 watts at idle. Is it normal to bias these these tubes so hot? I don't want to destroy these tubes, but their history seems to suggest they are incredibly robust. My guess is that the 193 ohm bias resistors were original.