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

[Ron Barlow via ARC5]

My quick n dirty calcs agree with Ian's calcs.
The objective of this plan is to shunt 100 ma around the 6SR7's heater, if you are using a 6k6, and 150 ma, if you are using a 6v6.
       73 de ron 

--------------------------------------------
On Tue, 4/7/15, Kenneth G. Gordon <kgordon2006 at frontier.com> wrote:

 Subject: Re: [ARC5] 6K6 vs 12A6 - filament balancing resistor
 To: "Ian Wilson" <ianmwilson73 at gmail.com>
 Cc: Arc5 at mailman.qth.net
 Date: Tuesday, April 7, 2015, 3:04 PM
 
 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
 
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