[ARC5] 6K6 vs 12A6 - filament balancing resistor

[Kenneth G. Gordon]

On 7 Apr 2015 at 10:29, Ian Wilson wrote:

> Not sure I grok this, Ken. 

Hey! Don't feel like "The Alone Stranger". I don't either. ;-) I said it was 
"...quick and dirty..."...also incorrect.

> The 6K6/6V6 is in series with a 6SR7 (replacing a 12SR7).
> The 6SR7 draws 0.3A with 6.3V.

Yup. Works out to 21 ohms. (6.3/.3)

For the 6K6, that is 6.3/0.4 = 15.75 ohms.

> So, for a 6K6 you would add a 63 ohm resistor in parallel
> with the 6SR7 heater; for a 6V6 this would be 42 ohm.

I don't understand where this 42 ohms comes from. What do you mean by 
that? At 300 mA both tubes would have to exhibit 21 ohms, or 42 ohms total, 
but that doesn't apply here.

So you want a total of 31.5 ohms, or 400 mA for both tubes, with the 6SR7 
having a resistor in parallel with its filament to divide the current.

12.6/31.5 = 400 mA

But 63 ohms in parallel with 15.75 ohms (6K6) is 12.6 ohms.

If you want both filaments in series to draw the same current, in this case 
400 mA, then you want the series-ed resistances to be equal, or 15.75 ohms 
each.

So, recalculating, one would need a 63 ohm resistor in parallel with the 
6SR7, not the 6K6.

> In both cases, less than 1W would be dissipated in the
> resistor, so a 2-5W resistor would be OK.

100 mA at 6.3 V = 0.63 watts, so a 2 watter would be sufficient, IMHO.

Ken W7EKB