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Cathode bias LED forward voltage drop

Posted: Mon Mar 22, 2021 12:55 am
by xraysandwich
I ordered a bag of the LED's Mark has listed in his BOM and all of the ones I received have a forward voltage drop of around 1.7V @ 5mA.

Could someone advise if 1.7V is close enough to use for the cathode bias on the driver tube (6AQ8)? If so, would I need to adjust the value of the cathode resistor to bring the bias current back to 5mA?

Or if anyone has a few spare LED's within the recommended range of 1.5-1.6V I'd happily kick you some dough to drop them in the mail.

Thanks!
Eric

Re: Cathode bias LED forward voltage drop

Posted: Thu Mar 25, 2021 5:36 am
by SLF99
I'm new at this, but at 5ma there would be .9V across the 180 ohm cathode resistor (180 x .005) leaving the balance from the LED.

Summing this derived voltage drop with the LED drop of 1.7 gets to a grid bias of 2.6 volts. Is that too far from the measured voltage of 2.7v? Unclear to me anyway.

Using that logic, using LEDs with slightly larger voltage drops (which seem to be easier to get hold of...) could be workable within some range of cathode resistor value

In any event, I don't think the cathode resistor serves to limit the bias current

Perhaps some more seasoned biasing experts could weigh in here - I am way outside my envelope....

Re: Cathode bias LED forward voltage drop

Posted: Fri Mar 26, 2021 6:19 pm
by xraysandwich
Hey, thanks for taking the time to type all that up!

I'm going to keep moving forward and see what measurements I get once it's all together on the bench. I'm sure at some point along the way I'll need to pick up some more parts and might try a different supplier to see if their batch measures any different (this batch came from Mouser).

Also found these which are nearly identically spec'd to the ones in Mark's BOM, but rated with a forward voltage drop of 1.6V @ 10mA instead of 1.6V @ 1mA.

Re: Cathode bias LED forward voltage drop

Posted: Wed Apr 21, 2021 2:54 am
by Shoreman
I have the same issue I purchased 40 of the leds listed in the Bom. I'm getting 1.67 v at 5 ma out of 32 of them and 1.68 from the other 8 leds these leds are very accurate they don't deviate much from there spec sheet not sure what to do I'm just going to roll with it and see if it measures different when in curcuit. I have been checking other leds and there VD is all over the place.I'm also going to check some other spec. sheets for other leds if there is an issue with these I will know in a week I will post my findings

Re: Cathode bias LED forward voltage drop

Posted: Wed Apr 21, 2021 2:56 am
by xraysandwich
Shoreman, where did you order yours? I got my batch from Mouser.

Re: Cathode bias LED forward voltage drop

Posted: Wed Apr 21, 2021 2:56 pm
by Shoreman
Got mine also from them

Re: Cathode bias LED forward voltage drop

Posted: Sat Apr 24, 2021 8:14 pm
by Stephe_K
The main goal is to get the grid bias with the amp powered up (measured between the grid and the cathode) at -2.6 to -2.7V. If you need to adjust the cathode resistor a little to get this voltage there, that's fine. What you are doing is setting the operating point of this driver tube.

I will say some LEDs sound better than others and this one in the BOM is sonically a good one.

Re: Cathode bias LED forward voltage drop

Posted: Tue Apr 27, 2021 9:31 pm
by xraysandwich
Thanks for the info, Stephe! That all makes sense.

I had ordered some IR LED's off of eBay to try, but I'll probably keep the ones Mark spec'd and just adjust the cathode resistor to dial in the grid bias. Though once I get everything figured out, I might swap in the IR LED's and see if I can hear a difference.

Re: Cathode bias LED forward voltage drop

Posted: Sat Jun 05, 2021 3:37 am
by Gary
Found the same 1.6ish levels on my LEDs from Mouser..
I used a 160ohm cathode resistor, which gives all together 2.67Vdc bias.
A 0.03 volt low is not a problem, amps is great.

Re: Cathode bias LED forward voltage drop

Posted: Wed Aug 04, 2021 1:56 pm
by rferguson
Question......how would one determine the current going through the preamp tube? It makes it a bit difficult since we don't know the sum of the LED and resistor resistance and doing it from the plate side is interesting because there's the plate load resistor and the 200k feedback resistor.