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In the Forum: Melquiades Amplifier
In the Thread: Single-stage Melquiades vs. DHT amps

Post Subject: Driving the 45... cathode firstPosted by floobydust on: 6/22/2009

Hi Romy,

The final ratings on later ST-style glass 45s was 10 watts on the plate and 300 volts cathode to plate max. Recommended bias current is between 34-36ma max. Short answer... I prefer no more than 34ma on the 45 cathode. It's a coated cathode type on a thin filament wire and pushing them will run them down quicker. If you can manage 35ma and match the set I'd say you're good to go. The only problem is your available supply voltage. As you're "backing into" a set of fixed parameters (plate voltage supply is already defined), you might need to alter your driver operating point to get the 45 in this range and maintain optimal performance. Of course, you know better than I about the driver stage and where it's optimal operating point is. I would do some (bias) trimming on the driver stage first and see if you can reduce the 45 idle current to no more than 35ma while keeping the driver stage operating properly. If not, you might end up trimming the cathode resistor to get there.

Regards, KM

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