6V Motorcycle batteries have three cells with a nominal voltage of 2.1 V/cell = 6.3V, so this looks like an excellent suggestion. Rechargeable batteries are available for less than $10 on Ebay.
However, the voltage fluctuates with the level of charge:
Motorcycle battery voltages
Lead acid batteries
A typical lead-acid motorcycle battery
The majority of classic motorcycle batteries are of the conventional lead acid variety. Whilst we think of these as being either 6 or 12v, this is a slight misnomer as the actual voltages are rarely these values if the battery is in good health.
The primary battery voltage is dictated by the chemistry of the lead acid cell. Fully charged and in peak condition, each cell has a voltage across it’s terminals of 2.1v. Motorbike batteries are made up from a combination of such cells connected in series to give the required voltage.
A ’6v’ battery is comprised of three cells giving a total voltage across it’s terminals of 3 x 2.1v = 6.3 volts. Likewise a ’12v’ battery is made up of six individual cells giving a total of 6 x 2.1v = 12.6 volts.
These voltages are not however fixed and will vary both with the state of charge (or discharge) of the battery, and also depending upon the electrical load placed upon it. The more the battery is used and loses its charge, the lower it’s voltage gradually becomes. Similarly, when an electrical load is connected to the battery this also drags the voltage down.
State of charge
It is therefore possible to determine the approximate state of charge of a motorcycles battery by measuring the voltage across it’s terminals. This is how ‘intelligent’ battery chargers work by constantly measuring the voltage and stopping charging once the desired level has been reached.
The table below shows the approximate voltages that will be measured across the terminals of a lead acid cell for various states of charge. Also shown are the total voltages for 3 cell (6v) and 6 cell (12v) batteries as will be fitted to most classic motorcycles.
Note that the voltages shown are open circuit voltages which means that there is no electrical load placed upon the battery. These readings are therefore taken with all lights etc turned off, and the engine not running.
|State of charge
* Initially after fully charging a battery, the battery voltage will be slightly higher and this will then settle down to the 100% fully charged value over the course of the next few hours.
To me the question is if a tube sounds different with voltage fluctuations within a certain range. It may very well be that tubes are designed to perform well and last long within a certain heater voltage range. Maybe the real payoff with battery power is less hum and RF interference.