LED/Diode Cathode Bias fettle-isation!

[Dave the bass]

LED/Diode Cathode Bias in place of the more usual resistor and bypass capacitor method, it doesn't seem to get mentioned much here. Is there a reason?! They have low impedance to AC so thats one bypass cap less in the amp (good some say... others less so!) so surley its a win win situation.... but I feel like I'm missing summat cos not many others here have done it and I'm out on my own, I'm scared Mummy

I've been gradually increasing current through the 1st stage 6C45p to see what effect it has on driving the Grid of a 300B via a 1:1 Interstage, yup, still fettling that Owston Amp.

A bit of reading in MJ and Merlin Blencowe books provides tables of different coloured LED's and their Fwd Voltage drop at a constant current of say 5 or 10mA. Fitting a Green or Amber LED in the cathode raised the bias voltage causing Anode curent to drop natch which is the opposite way I wanted to go but the 'sound' was nice. I then went to standard Red then Ultra bright Red, which gave me about 1.8V of Cathode bias which gave me about 19mA of Anode current with the Anode voltage held at 150V, better.

I toyed with the idea of 2 x 1N4148 in series to give approx 1.4V which will ramp up the current even more but... I found that Infra Red LED's have approx the same Fwd drop, awethumne. Ordered, fitted and lo and behold 32mA at 150V with 1.4V of bias.

The current 6C45p Anode current has ramped up quickly between LED changes from Green, Amber, Red, Ultra Bright Red and finally IR because its a high GM valve I believe. The down side is I've now pulled bias so low that the incomming voltage swing can swing the grid of 6C45 very close to the Diode line/0V line yes? Bad design by me that makes the source drive hard (I think) and possibly get upset when its really being driven hard...but.... the sound.... phwoaaarrrrr!

Thomas sparked the idea in my murky brain, I blame him in a nice way!

DTB

[IslandPink]

I'm sorry to report you are not trail-blazing, Dave
See how you feel about the sound in a few days .
I tried it a long while back, and found it not quite good enough , to my ears . On the other hand, I do tend to rely on my small stock of Black Gate NH's for critical applications on driver cathodes or in phonos . Against ordinary electrolytics it may be much closer.
The Bass ( not you - I mean the bass ! ) may be a bit cleaner/quicker ?

[Paul Barker]

Last tried it on October 25, 2001 at 22:35:12:

and said about it:

"I tried LED on the cathode of input stages is that what you did? Sounded abismal. resistors are much nicer sounding whether bipassed or not. The soundstage from led's shrank right back to the speakers, lost all dimentionality."

~Haven't tried it since. Things may have changed.

[Neal]

Yes same for me, I think the key is selecting the correct type (linear at the current you need) and passing sufficient current (~20mA) which IME is much greater than the sort of stages I need them for...if that make sense.

[Andrew]

Hi Dave,

Try a combo of resistor and diodes, you can vary the proportion to alter the amount of cathode feedback you prefer.

Andrew.

[Dave the bass]

Aaaahhh, so others here HAVE done it, cool. Ta everyone. I wasn't DIY-HiFi-ing back in 2001 Paul so I missed your sperryments.

Sound stage has deffo not shrank, I was running Ultra Red when we carried out DHT DC Heater testing the other day and the stage was mahoosive. At one point there was a tap-tap-tap on the side window... it was the hi hat asking to to be let back in the room.... Hahahahah (etc). But really, it does 'it' for me, maybe its as Neal suggests, just finding the right diode and operating that at a reasonable current. Using the IR's at 32mA is well into their operating range according to the Data sheet as they can handle up to 100mA -> http://www.spiratronics.com/data/2365.pdf

Naturally I can't see the LEDs glowing as they're IR but the CCD camera trick shows them to bright and healthy at 32mA.

Try a combo of resistor and diodes, you can vary the proportion to alter the amount of cathode feedback you prefer.

Ooo, I hadn't thought of that, nice idea MrL. Ta.

[IslandPink]

Thanks for the info Dave . Sounds like you're on to something there.
There's another experiment for me to do !

[thomas]

Hmmm well my experience has also been different! I've been using led bias for quite a while (bout a year or more), on the 'tea trollies' 6sn7 and now 6n7 input valves; initially a string of several with an extra current feed via a resistor from the HT so they passed 12-15ma, and I compared them to an 'oscon' bypassed resistor I had in before....To my ears the treble cleaned up considerably, and the bass became 'leaner but cleaner'. I much preferred them! I guess they may be a 'suck 'em and see' thing?!
I suppose I'll have to go back to the bypassed resistor now to remind myself why I didn't like this so much...