I have never done this, but this would work well with vibrator operated radios. If all of the tubes are indirectly heated, a voltage up to 7.4 volts is acceptable. If your radio uses filamentary tubes in any part of the circuitry, you must keep your voltage lower--like no higher than 6.8 volts. The battery should filter out any ripple produced by the power supply, though it may not. I have successfully used a regular 6 ampere battery charger on radios utilizing indirectly heated tubes. The load of the vibrator supply should probably make a 6 ampere battery charger quite acceptable for filamentary tube use if only there isn't a strong hum produced by the unfiltered charger. If the load does not bring the voltage down enough, a battery charger can produce up to 8 volts, which is not good for any type of radio. Usually a vibrator type radio consumes enough current, though, to drop the voltage down to about 6.5 volts.
Thomas