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I'm not sure about the 7jp4 in particular, but filaments are 100% overrated, and will burn safely up to 12 volts. I would step it up to 8 volts with a seperate transformer and see what results you get. However if the motorola has the crt in series with the rest of the tubes, you will need to find the appropriate load resistance to keep the rest of the tubes in balance or you'll toast some tube filaments
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Jordan
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