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 Post subject: LED Biasing?
PostPosted: 09 Feb 2011, 07:22 
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Joined: 28 Dec 2010, 22:07
Posts: 352
Hi everyone,

I'd just like to ask some questions about the topic technique. I've heard people talk about LED biasing as if it were a special, magic technique. Obviously it can't be that terribly great, but there must be something to it.

I know an LED will maintain a fairly constant voltage across, which will reduce negative feedback generated in the cathode. A resistor will increase voltage as the tube increases its cathode current, generating its own NFB. As the diode has the ability to change its forward current to maintain the voltage drop, it wouldn't fight the output so much, increasing gain for small-signal tubes and power output for output tubes. It's like applying both fixed bias and cathode bias.

First, I could be wrong about everything I just said. Second, I have not actually experienced this technique and am curious to know if it's better than resistor biasing. I could always set up a tube current source as the plate load but LEDs require less calculation.

Thanks for the input
Ed

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 Post subject: Re: LED Biasing?
PostPosted: 09 Feb 2011, 07:43 
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Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
Hi,

LED biasing is a fantastic way to generate cathode bias.

First of all, you are correct in that they are constant voltage devices. So LED's can be used to generate the required voltage across a cathode. They can be strung in series to generate more bias. They can also be paralleled to provide more current handling capability. Here is an example of a popular power output stage using such a bias method:
http://syclotron.com/?page_id=3

So, why not just use a zener or a resistor? A resistor needs to be bypassed with a capacitor, because its AC resistance at audio frequencies is equal to the DC resistance for all practical purposes. The bypass capacitor must be of high quality to not introduce non-linearities. An LED is a very low impedance device when forward biased... typically under 10 ohms and practically 1-2 ohms.

A zener diode is a noisy device. It also needs a bypass cap and thermally they will drift more... not good in sensitive circuits. An LED is really low noise.

I have tested LED's in the front ends of phono stages and they will without a doubt, out perform resistors or zeners.

But like any diode, an LED can be non-linear unless a good amount of current flows through it (say greater than 5mA). If your stage can't pull that much, say a 12AX7 which perform best at 2mA or less, you can run a resistor from the cathode to B+ to keep the LED current high. But will this effect the tube it's connected to? No. Here's where the constant voltage characteristic of the LED comes in handy. The bias will be maintained, regardless of the current through it, or where it's developed from.

Different coloured LED's will generate different bias. In tests, a common 3mm red LED will give 1.85V bias and have one of the lowest impedances. Next, is an IR LED for impedance, but can have a voltage drop of up to 2.1V, like a green LED. Blue and white LED's will have a voltage drop of anywhere from 3-5V.

Hope this helps!

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 Post subject: Re: LED Biasing?
PostPosted: 09 Feb 2011, 09:24 
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Joined: 24 Feb 2009, 03:24
Posts: 562
Location: USA
All is true......but....I compared LED biasing to resistor biasing with cap Black Gate on my amp 2A3 RCA. According my criteria,
IMO, LED biasing sounds terrible, compared to resistor biasing with cap!!! For me sounding is more important, than biasing modes.....I don't advice You, IMO....

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Hi End is back proportional to the number of composite parts!
Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/


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 Post subject: Re: LED Biasing?
PostPosted: 09 Feb 2011, 10:14 
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Location: Canada
azazello wrote:
LED biasing sounds terrible,
azazello wrote:
For me sounding is more important, than biasing modes.....I don't advice You, IMO....

Hi.

I got the similar sonic impression.

Different biasing topology gives its own sound signature.

IME, I don't go for any diode type biasing, e.g. sand diode, LED, etc as I don;t think I like their sound at all. I like the sound of battery biasing if the bias voltage is over 1.2V or so.

Even battery biasing, different type of batteries give different sonic signature.

In my 1-stage tube phonostage which needs 2.3V cathode bias. I tried 1 NiMH rechargeable battery
(1.2V) plus a sand diode (0.7V) = 2.3V (after charged). But it sound sterile & dry, not musical at all. Why?

So I replaced it with a NiMH cell (2.4V) scrapped from an old cordless phone. It sound so much much better. I am amazed!! It proves to me the sand diode screws up the sound.

YES, sound comes first - our bottom line of any DIY.

c-J

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 Post subject: Re: LED Biasing?
PostPosted: 09 Feb 2011, 10:52 
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Joined: 24 Feb 2009, 03:24
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Location: USA
I'm agree....!

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Hi End is back proportional to the number of composite parts!
Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/


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 Post subject: Re: LED Biasing?
PostPosted: 09 Feb 2011, 12:39 
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BowToEd wrote:
resistor biasing

Hi.

IME, the best sounding biasing is NO resistor biasing.

For O/P power stage, we can consider fixed bias - supply the required power tube cathode bias with a separate power supply.

For phonostages, linestages & driver stages using ground cathode topologies, replacing cathode bias resistors will be much less complicted than O/P power stages. We can replace the bias resistors with sand diodes, LEDs, zeners etc etc. I find battery is most promising if the cathodle bias voltage is 1.2V or over.

But but NOT not all batteries sound good. I first tried alkaline batteries, but I was not impressed at all. Not that musical.

Finally I settled down with NiMH rechargeable batteris which sound most musical. Besides, there is no need of replacing the batteries in the future.

That said, the best cathode biasing is NO biasing. A DIY guru tried using it in his phonostage projects.

He is Thorsten Loesch, (using a Chinese pen-name : Kuei Yang Wang, always starting his posts with Janapese greetings: Konnichiwa (Good day) & ending with Sayonara (Good bye). IMO, his designs are always so advanced & so immaculate that impire me big bigtime.

Apparently like a few picky ears, he does not like the sound of resistor cathode biasing. His phonostage designs includes a lot of battery biasing to replace conventional cathode bias resistors. In some phonostages, he even used gridleak bias with cathode grounded.

I fully agree. My picky ears tell me so.

c-J

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 Post subject: Re: LED Biasing?
PostPosted: 09 Feb 2011, 14:28 
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Joined: 24 Feb 2009, 03:24
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Location: USA
I like grounding cathode and I used in my SE 300B /the best cap is missing cap!/.,....but we told about LED and resistor biasing...

_________________
Hi End is back proportional to the number of composite parts!
Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/


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 Post subject: Re: LED Biasing?
PostPosted: 09 Feb 2011, 15:10 
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Location: Canada
azazello wrote:
I like grounding cathode and I used in my SE 300B /the best cap is missing cap!/.,....but we told about LED and resistor biasing...

HI.

No way, Jose!

The last biasing I'd use would be LED biasing for 300B, considering the large current involved.
IMO, LED bias is best for small current biasing like front end tubes.

I'd worry more about keeping idling currents of 300B stable than anything else.

So to answer your question of using an alternate biasing method instead of the huge hot hot cathodle bias resistor/bypass caps, you can use an EL34 (pentode connected, grid grounded) to replace the huge cathode bias resistor hooked up downstream of the 300B.

This will for sure stablilize the idling current of the 300B & bye-bye to the cathode resistor.

c-J

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 Post subject: Re: LED Biasing?
PostPosted: 09 Feb 2011, 15:40 
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Joined: 28 Jan 2011, 11:51
Posts: 98
I guess some may like LED or diode biasing , it's certainly very easy and cheap to try...

If there is a CCS load then little point having anything more than a resistor in the cathode , unbypassed cathode resistors are my preferred biasing method but I tend to use bypassed cathode resistors in output stages mainly for bias stability . I'm quite fond of glowpath driver stages at present which allows an unbypassed cathode resistor with IT loading , this could also be used for a low power output stage . It's just a glow regulator tube in place of an ultrapath cap fed with a CCS .

BDA


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 Post subject: Re: LED Biasing?
PostPosted: 09 Feb 2011, 16:06 
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Joined: 28 Dec 2010, 22:07
Posts: 352
That's a lot of useful information.

I was talking about low-level biasing with the LED. By low-level I meant non-output tubes that conduct 15 mA or less. The output tubes swing so much that I'd worry about the stresses applied to the poor little LED. Said LEDs could double as pilot lights and therefore not waste power in a lamp or neon indicator. I don't want to blow an LED and kill my output stage.

I built a battery box with four 9v Zinc-Chloride batteries to test fixed bias on output tubes. It sounds better than self-bias. The resistor reduces output slightly by fighting the output tube's conduction tendencies. As the current increases, the bias voltage increases and the other way around. The bypass cap can help remedy this but a reactance will cause other problems.

I'm thinking of building my next amp using a differential amplifier in the preamp/phase-splitter stage. This allows me to boost and split in the same stage. I have considered using an LED or two in the cathode path but I wonder how stable this will make the diff amp. Constant voltage is different from constant current. Possible tubes will be 6CG7 or 6SL7.

Your thoughts?
Ed

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