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Old 20th January 2013, 07:33 AM   #11
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What is dynamic resistance @ op point of an ultrabrightmblue LED?
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Old 6th February 2013, 04:27 PM   #12
sjs is offline sjs  United Kingdom
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On the subject of using diodes in the cathode circuit, personally I quite like SIC Schottky diodes rather than LEDs, but has anybody tried vacuum diodes?

see: http://www.chimeralabs.com/images/axiom.sch.jpg

Could be fun
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Old 6th February 2013, 04:38 PM   #13
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See Morgan Jones' Valve amp book. Red LEDs are the way to go, if you want LED bias, their AC impedance is the lowest.

Filament bias seems a good alternative. I use fixed battery bias.
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Old 6th February 2013, 05:14 PM   #14
sjs is offline sjs  United Kingdom
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Originally Posted by Pano View Post
See Morgan Jones' Valve amp book. Red LEDs are the way to go, if you want LED bias, their AC impedance is the lowest.

Filament bias seems a good alternative. I use fixed battery bias.
I have read the book, I have tried the red LEDs, and I do prefer the SIC Schottkys if I need to work in this way.

Generally I prefer battery grid bias over diodes in the cathode anyway, but my interest is whether anybody has tried vacuum diodes, as I thought it could be entertaining.

Cheers
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Old 6th February 2013, 05:46 PM   #15
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<snip>.....but my interest is whether anybody has tried vacuum diodes, as I thought it could be entertaining.

Cheers
some talk of it here...

Very interesting 300b design

and here.....

Comparison of Diode Biasing with other methods
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Old 6th February 2013, 08:44 PM   #16
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Rod

Many thanks for that.

Simon
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Old 6th February 2013, 10:55 PM   #17
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that last design by Steve Bench is interesting - he's biasing the 845 with a 0C3. I can't see the current going through the 845 - it must be in a different page. I'm assuming it's less than 40mA to be within the rating of the 0C3. Interesting - I'd like to know more.
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Old 7th February 2013, 03:01 AM   #18
Magz is offline Magz  United States
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Originally Posted by Pano View Post
See Morgan Jones' Valve amp book. Red LEDs are the way to go, if you want LED bias, their AC impedance is the lowest.

Filament bias seems a good alternative. I use fixed battery bias.
Actually, I did some measurements of the 600V Cree SiC schottky diodes, and their dynamic resistance is very low. I measured 1.3ohm between 15 and 30mA, giving ~0.9V of bias per diode. I'll be using them to bias the driver in my upcoming 833C amp design.

BTW, they sound great in a preamp as well! Excellent bass, clear treble.
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Old 7th February 2013, 09:18 AM   #19
palmas is offline palmas  Portugal
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Originally Posted by repycte View Post
I already built a DC filament supply based on a LT low drop regulator. Using this supply I can only switch to fixed bias.

I will have a closer look at filament bias. Although I already used very big film caps for bypassing.

I chose the 6C4C for testing and later replacing by 300Bs, as I have no experience building tube power amplifiers, only preamps.
If you're using DC why use the pot? Am I missing something?
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Old 7th February 2013, 01:14 PM   #20
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Actually, I did some measurements of the 600V Cree SiC schottky diodes, and their dynamic resistance is very low. I measured 1.3ohm between 15 and 30mA, giving ~0.9V of bias per diode. I'll be using them to bias the driver in my upcoming 833C amp design.

BTW, they sound great in a preamp as well! Excellent bass, clear treble.
Here's a plot I made of If vs Vf for the Cree 600V SiC schottky diodes. Very nice performance, especially at the higher currents.
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File Type: jpg Cree_SiC_Vf_vs_If_plot1 Compressed.jpg (13.5 KB, 88 views)
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