5ar4 are a much larger tubegz34 same tube.if it calls for gz34 you can
Put a 6106 as long as it does not sag too much in electrical demand depending on circuit, ,not the other way around currently drop over 40 volts
If it calls for a 5y3-6106 cannot go bigger to a 5ar4-gz34 something will give ,or short tube life.Bendiz best 6106 ever built like a tank.
5ar4 Mullard Blackburn .