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Cheap Red LED bias on ecc83/12ax7
Cheap Red LED bias on ecc83/12ax7
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Old 30th April 2015, 03:16 PM   #41
rayma is online now rayma  United States
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Yes, a cathode resistor will give local DC feedback (reducing operating point variability), as well as adding local AC feedback.
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Old 30th April 2015, 04:29 PM   #42
Bigun is offline Bigun  Canada
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Cheap Red LED bias on ecc83/12ax7
I thought as much. Rules out fixed bias for dc-coupled amplifiers.
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Old 23rd June 2015, 08:14 AM   #43
soulmerchant is offline soulmerchant  Switzerland
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Quote:
Originally Posted by DF96 View Post
only simulation freaks worry about picking sweet spots where nth order distortion magically disappears.
Funny you mention this - I recall one poster who claimed that a design I was using had 20% THD (!) in his 'simulation'. However, real-world measurements showed THD to be less than 1%. Nothing replaces a measurement...

To be completely honest, I completely forgot about this thread. Since then I have gone back to simple resistor bias on that stage (accounted for in RIAA correction). I tried the low current LED's but they all seemed to color the sound more than the cheap red's. Some IR LED's were good as well, but I never found any neutral sounding low current LED's.

Battery bias (of the cathode) also coloured the sound, but battery bias of the grid is however something I still use in certain instances.

Ian

Last edited by soulmerchant; 23rd June 2015 at 08:20 AM.
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Old 25th February 2017, 02:55 PM   #44
hego is offline hego  Europe
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I use usually a 6n2p-eb (as mu-follower and resistieve load) with a bulk cheap chinoise red led (driver) and (phono) IR one. I think the activation through more mA (the resistor trick) is not necessary. It works with only about 0.5 mA and the sound is very fine. I like it.

Cheers
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Old 25th February 2017, 04:40 PM   #45
famousmockingbird is offline famousmockingbird  United States
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I have tried low current LED's with decent success but some don't work as good as others. I ended up just using a resistor off the B+ to keep the LED in conduction. I don't think it's a good idea to run the LED right on the conduction knee.
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Old 25th February 2017, 06:42 PM   #46
hego is offline hego  Europe
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You are right. All leds are not the same. Iīm surprised and perplexed with those cheap diodes and low current. Others donīt work well: knee region.

Cheers

Last edited by hego; 25th February 2017 at 06:46 PM.
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Old 25th February 2017, 06:46 PM   #47
soulmerchant is offline soulmerchant  Switzerland
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I sometimes see this thread coming back to haunt me. Anyway, in the meanwhile I have tried some IR LED's for different applications that are also not bad at all.

I also had some cheap green LED's that were fairly decent in biasing 6SN7's.

But on the whole I have seriously gone back to resistors for bias. Decent films are just fine, bypassed or not.
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Old 25th February 2017, 08:42 PM   #48
hego is offline hego  Europe
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I will try the resistor trick for work the cheap red led at >5 mA.

The IR led seems an other animal.

Cheers
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Old 15th July 2018, 06:26 PM   #49
Gnobuddy is offline Gnobuddy  Canada
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Anyone ever try using a Vbe multiplier to replace an LED? You can dial in any reasonable voltage drop you want from 0.7V upwards by using a trimpot for (R1+R2) in the attached schematic.

I would expect a Vbe multiplier to behave very similarly to an LED otherwise. Both have the same exponential current-voltage characteristic, and if adjusted to the same voltage drop at the same forward current, they will probably behave pretty similarly for small-signal AC changes as well.

The one obvious difference (the Vbe multiplier has a parallel resistance equal to (R1+R2) across the "LED" ) should make very little difference, as R1+R2 will be much greater than the dynamic (differential) resistance of the multiplied diode.

-Gnobuddy
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Old 30th July 2018, 04:03 AM   #50
gingertube is offline gingertube  Australia
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Biguns comment above that the LED is effectively a constant voltage source so the tube is running fixed bias prompts another thought. The max Rg1 value for fixed bias is much lower than for Auto (cathode bias resistor) bias. You may need to change the grid leak resistor to a lower value when using LED bias.

I'm still looking for a current controlled voltage source for cathode biasing although that TL431 circuit would seem to be headed that way.

Cheers,
Ian
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