Cheap Red LED bias on ecc83/12ax7
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 21st October 2012, 06:58 AM #21 yagoolar   diyAudio Member     Join Date: Oct 2004 Location: Warsaw, PL The idea of using an LED in a cathode is to maintain bias voltage in no dependance to anode current. See LED characteristics. LED is not a cure for heavy loads on B+. Well designed HV power supply is. What you can do is to calculate what is ΔVbias in regard to expected ΔB+. E.g. B=+302V Vbias = +2V Ibias=10mA (one can take anode current change under the load onto account, i.e. ΔB influence on anode current) ΔB +/- 5% or 30,2V (that is heavy I guess) Rbias=(302V-2V)/10mA=30kΩ Now, 30,2V/30kΩ=~1mA or 0,5mA around LED op point. Assume dynamic resistance of a LED is 5Ω ΔVbias=1mA * 5Ω = 5mV It is early in PL, so I may have made a mistake somwhere.
 21st October 2012, 07:04 AM #22 dgta   diyAudio Member   Join Date: May 2011 Yagoolar, you didn't understand his question. He wants to avoid the extra 6mA current draw necessary to put more current through the LED than the tube is passing.
yagoolar
diyAudio Member

Join Date: Oct 2004
Location: Warsaw, PL
Quote:
 Originally Posted by dgta Yagoolar, you didn't understand his question. He wants to avoid the extra 6mA current draw necessary to put more current through the LED than the tube is passing.
I was referring to post #19. The question was to use LV PSU and lower value resistor to feed LED. It is not THAT early

My intention was to ask question "what for" if that works well.

 16th May 2014, 12:51 PM #24 leftovers53   diyAudio Member   Join Date: Dec 2010 Location: NYC Im curious, has anyone since found an LED for 12AX7 LED bias that works well enought at <1ma, so the B+ bias resistor can be avoided?
 16th May 2014, 01:26 PM #25 funk1980   diyAudio Member     Join Date: Sep 2011 Location: Groningen There are loads of low power LED's that work fine at around 1mA. Just check e.g. mouser. No idea how linear they are though, but they same pretty linear at the small amount of current variation the ECC83 exhibits.
 16th May 2014, 01:31 PM #26 SY   On Hiatus     Join Date: Oct 2002 Location: Chicagoland If you have a CCS load in the plate circuit, that pretty much removes the nonlinear Z vs I characteristic of the LED. If not, the B+ resistor is still an easy option. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
 16th May 2014, 07:10 PM #27 famousmockingbird   diyAudio Member   Join Date: Dec 2010 Location: Nottingham, NH I have used low current red LED's for bias operation of ECC83's without having to use the resistor trick for added current.
leftovers53
diyAudio Member

Join Date: Dec 2010
Location: NYC
Quote:
 Originally Posted by SY If you have a CCS load in the plate circuit, that pretty much removes the nonlinear Z vs I characteristic of the LED. If not, the B+ resistor is still an easy option.
Its funny, I just last night read a post of yours from a few years ago pointing this out. But I suspect I cant use a CCS plate load in this circuit. Its two sections stacked, bottom tube a mu follower. I was going to try it at some point.

Quote:
 Originally Posted by funk1980 There are loads of low power LED's that work fine at around 1mA. Just check e.g. mouser. No idea how linear they are though, but they same pretty linear at the small amount of current variation the ECC83 exhibits.
Quote:
 Originally Posted by famousmockingbird I have used low current red LED's for bias operation of ECC83's without having to use the resistor trick for added current.
Thanks for these responses, but I was kinda hoping someone would refer to specific part number. I have tried a number of Radio Shack LEDs and so far none of them sound quite right, though the current draw has been reasonably close to the resistor/cap combo I started with. I was hoping somebody made an led that had a linear Z vs I characteristic down to .5ma.

 16th May 2014, 07:52 PM #29 famousmockingbird   diyAudio Member   Join Date: Dec 2010 Location: Nottingham, NH I have a drawer full of LED's but I think these are the ones I found to work good. Invalid Request Digikey part #1080-1067-ND
leftovers53
diyAudio Member

Join Date: Dec 2010
Location: NYC
Quote:
 Originally Posted by famousmockingbird I have a drawer full of LED's but I think these are the ones I found to work good. Invalid Request Digikey part #1080-1067-ND

Thanks! I will check them out.

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