Direct heated triode (dht) cathode bypass: different options

[yagoolar]

What is dynamic resistance @ op point of an ultrabrightmblue LED?

[sjs]

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

[Pano]

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.

[sjs]

Originally Posted by Pano

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

[Rod Coleman]

Originally Posted by sjs

<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

[sjs]

Originally Posted by Rod Coleman

some talk of it here...

Very interesting 300b design

and here.....

Comparison of Diode Biasing with other methods

Rod

Many thanks for that.

Simon

[andyjevans]

Originally Posted by Rod Coleman

some talk of it here...

Comparison of Diode Biasing with other methods

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.

[Magz]

Originally Posted by Pano

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.

[palmas]

Originally Posted by repycte

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?

[Magz]

Originally Posted by Magz

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.