LED choice in cathode biasing

[indaco]

Hello.
I'm trying to experiment different ways to optimize the first stage of tube amplification via cathode bias....I've read that an easy way is with the use of a LED, with certain characteristics. It has to be able to work with low current and have the lowest internal resistance, traducing in less dynamic impedance, which finally affects the sound.
Since such a LEDs are not so common and also fair expensive, is there any (valid and cheap) alternative to them? Thanks

 

[jazbo8]

Please use the search function - there are probably dozens of threads on the topic in this forum.

 

[audiowize]

I've found just about any of the HLMP series LEDs will fit the bill.

 

[gabdx]

if you want to extract all the stuff from the tube use battery bias, it is a subtle improvement from led bias.

 

[merlin el mago]

I can get 2.77V or 6V, exist?

 

[indaco]

I've found just about any of the HLMP series LEDs will fit the bill.

Yes, the HLMP-6000 series seems to be the most fitting: the problem is that it's hard to find (here). I've heard that there are some old leds working even better, one should experiment and see...

 

[GoatGuy]

Seriously… you could also and cheaply use a string of common 1N4001 thru 1N4007 diodes too. Each of them introduces about 0.77 V (at single-digit milliamps) drop. Pretty convenient as a step-ladder setup. Just put 10 of them in series, tap wherever you want in 0.77 volt increments.

Nowhere near as 'pretty' as LEDs, but darn cheap, and with very very good current-transient and dozens-to-hundreds of milliamps capabilities. Did I mention cheap? 7¢ to 14¢ at most every supplier. Buy 100. Keep em in stock.

GoatGuy (PS: you could also put a front-panel rotary switch to change the tap-point on the fly, to try out different bias voltages without otherwise having to physically modify your preamp section. HOW COOL IS THAT?)

 

[indaco]

(PS: you could also put a front-panel rotary switch to change the tap-point on the fly, to try out different bias voltages without otherwise having to physically modify your preamp section. HOW COOL IS THAT?)

Yes, really cool that...I'd have just a 10 position rotary switch abandoned in a drawer!

 

[Merlinb]

LED internal resistance is a non-issue. Bias voltage has far more effect on performance than LED impedance even could. But if you're still worried about things that don't exist, you can use band-gap diodes which have the lowest internal resistance.

 

[merlin el mago]

Yes, the HLMP-6000 series seems to be the most fitting: the problem is that it's hard to find (here). I've heard that there are some old leds working even better, one should experiment and see...

RS Amidata & Farnell have.

RS Components | componenti elettronici ed elettrici

http://it.farnell.com/

 

[dovla]

Bulk Chinese leds

See http://www.diyaudio.com/forums/tubes-valves/183329-4p1l-se-66.html#post3611427 - testing a bag of no-name leds from epay with 20 of each color showed that yellow ones have very low and flat impedance (with current typically used in tube amps).

 

[indaco]

See http://www.diyaudio.com/forums/tubes-valves/183329-4p1l-se-66.html#post3611427 - testing a bag of no-name leds from epay with 20 of each color showed that yellow ones have very low and flat impedance (with current typically used in tube amps).

Interesting table, how did you calculate the final impedance of the LEDs shown?

 

[indaco]

I answer myself: it's the slope that can be obtained from the relationship between ΔV and ΔI in the LED plot I vs V

 

[kodabmx]

Bias with a battery in series with the grid. No current flows and a carbon cell will last decades.

 

[GoatGuy]

Bias with a battery in series with the grid. No current flows and a carbon cell will last decades.

I guess the only real caveat would be to remember that the relatively big dry cells are also pretty good “interference antennas”. Like big bare wires. You can (and probably should) mount the cells in a shielded battery holder, wrapped with copper foil, connected to ground.

The “lasts 10 years” depends almost entirely on whether the packaging of the cells lasts 10 years. Of recent, I've noticed that 5–6 years seems to be the design spec. Kind of sad.

GoatGuy

 

[kodabmx]

I've found the cheapest ones work best. Go figure. Use carbon zinc cells in a paper sleeve (like the old Eveready). For that matter you can use heading aid batteries to decrease the size. My personal best was a C sized dry cell in a 1950's Eico VTVM. It was still putting out 1.54V open circuit after all those years.

 

[goldenbeer]

use a string of common 1N4001 thru 1N4007 diodes

The 1Nx types are noisy. Also, the forward voltage varies quite a bit with temperature..

 

[gabdx]

regular red emitting leds are ok.

I use a 9V battery at the phono input tube grid, no noise.

 

[indaco]

Yes...today I checked rapidly 3 leds, yellow, orange and red. After some measurement the red one has the lowest ΔV (flattest line) in a range of 4 and 8 V supplied.

 

[Magz]

Think about SiC Schottky diodes. The cleanest I've heard to date. Check out the post below.

Vf vs If for Cree Sic Schottky Diode - Measured at cathode-relevant Currents - Maxamillion - Tube DIY Asylum