|How to feed the heaters in the
First some facts. The heaters of the 6SK7 (6SJ7 if you use these) are nominally rated at 6.3v....A.C. or D.C. as it happens and draw 0.3 Amps. The heaters of the 6v6 are also rated at 6.3v....A.C. or D.C. but this time the current drawn is 0.45Amps. This valve is beam tetrode and can work hard but to do this the heaters are larger and therefore take more current.
The easiest way to run the heaters is to feed them with the correct voltage. Most mains transformers for valved equipment have a 6.3v A.C. winding just for the heaters. If your Paraset is using a 6v automobile battery to run a vibrator power pack then of course the heaters will be directly connected to the battery and therefore be operating at 6v D.C. The total heater current then required is 0.3 + 0.3 + 0.45 amps which is 1.05Amps total.
If you wish to operate your Paraset from a 12v automobile battery (which of course are more common than a 6v ones these days) then you must use electronic trickery to obtain the correct heater voltages for the valves. There are a few ways of doing this.....
Use a resistor to drop the excess battery voltage. This is simple, effective and totally reliable IF you use a resistor that is big enough to get hot (because it will) and not break. If your battery is a 12v one, then one could expect to see approx 12.6v at its terminals (varies with the charge) and valve makers know this.
The total voltage to be lost across the resistor is 12.6 (battery volts) - 6.3 (heater volts needed) = 6.3volts. We need a resistor that with 1.05 amps flowing through it causes this voltage drop.
Ohm's Law tells us that the Voltage (6.3) divided by the current (1.05) gives us the value of resistance required, which will of course be 6 ohms.
The current squared multiplied by this resistance gives us the power dissipated in the resistor and this works out to be 6.615 watts. Use the nearest preferred value resistor you can find and make it a 10W rating. Better still make it the type that bolts down to the chassis (inside your power supply) and the chassis can be used as the heat sink. Job done. This will work for ever and should be a case of "fit and forget".
Buy a large 5 amp regulator, the type that looks like a big power transistor. Make it a 5 amp one, as the switch on surge of cold heaters demands a lot of current that could wipe out a lesser current regulator. You can use a standard 5v one (they are cheap and plentiful) but of course you will need to raise the output voltage from 5 to 6.3. This is easy. These regulators have three connections. Power in, earth and power out. In the earth lead connect two diodes ( connect them so they point towards earth if drawn) such as 1N4001 or 4002 etc and the slight forward voltage drop across these two diodes will add up to approx 1.3v.....which is just what you need to raise the output voltage to about 6.3. Job done. This should work nearly for ever....I don't know what the life expectancy of one of these regulators is but it is a long time. Long enough for you get bored!
More complicated this one. Connect the two 6SK7 valve heaters in parallel and then connect them is series with the heaters of the 6V6. Now the total voltage needed to power the heaters is 6.3v + 6.3v which is of course the 12.6v of your battery. HOWEVER the current distribution is not correct and MUST be corrected or the 6V6 will be damaged. Two 6SK7 will need 0.6 Amps current and one 6V6 will only pass 0.45 Amps safely. We need to make the 6V6 pass another 0.15 Amps to even things up. Using Ohms Law again we will find that 6.3 Volts divided by 0.15 Amps needs a resistor of 42 ohms. Its power rating must be 6.3 x 0.15 = 0.94 watts. So use the nearest preferred value and make a 2w resistor to be on the safe side. Connect this resistor across the 6V6 heater connections (at the valve base) and now your heaters will run from the 12v supply. This should last for ever.