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

[AKLDGUY .]

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

This raises an important point. When choosing a "2 watter", the
constructor's natural choice would probably be a carbon film or metal film
unit.

I've never seen a figure for the current-carrying capacity of such units,
and even though their wattage rating is not being exceeded in the example
quoted, it may well be that they will suffer long term damage from currents
of 100 mA. I would always use a wirewound unit in such applications.

The same applies in the case of cathode bias resistors and plate circuit
decoupling resistors for Class A audio power amplifier stages.

If carbon microphone elements deteriorate after prolonged currents of
several dozen milliamps, why take the chance with carbon resistors?

73 de Neil ZL1ANM


On Wed, Apr 8, 2015 at 7:04 AM, Kenneth G. Gordon <kgordon2006 at frontier.com>
wrote:

> 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
>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>