What is the RIGHT way to calculate the Value of the cathode resistor?

[kevinkr]

I'll skip over the the perceptual pitfalls, as that seems only to apply to other people.

But, there is a physical difference that might be audible. A 2.5V filament should have less intermodulation with an AC supply, than a 5V one, all else being equal. The electrical relationship between filament, plate and cathode, would also be different for DC, reflecting the difference in the voltage gradient across the filament.

Sheldon

Yes, a theoretical advantage I have yet to hear conclusively demonstrated. (Like you?) What is really nice about 2.5V filaments is that you can heat them with AC and not have a major issue with hum on the output of the amplifier - not my experience with the 300B unfortunately..

It is even harder to explain why the 50 is revered (wrt filament voltage gradients) as are several other types with 7.5V filaments..

Now I am working on a 211 amplifier, no AC heating there unfortunately unless you count the ripple from the 10V/16A Vicor resonant switchers I will use for the filament supplies..

 

[Sheldon]

Yes, a theoretical advantage I have yet to hear conclusively demonstrated. (Like you?) What is really nice about 2.5V filaments is that you can heat them with AC and not have a major issue with hum on the output of the amplifier - not my experience with the 300B unfortunately..

It is even harder to explain why the 50 is revered (wrt filament voltage gradients) as are several other types with 7.5V filaments..

Now I am working on a 211 amplifier, no AC heating there unfortunately unless you count the ripple from the 10V/16A Vicor resonant switchers I will use for the filament supplies..

I purposely did not use the word "advantage". And nope, I haven't compared the two. But I have made a couple of amps based on the 801A, with 7.5V filaments. Hard to get rid of hum there. I tried Steve Bench's cancellation techniques, and it did eliminate most of it. But I didn't have a scope at the time, so couldn't chase out the last bit. I went to current regulated DC instead. I also converted that initial amp to a Loftin White design, ala Darius (oldeurope - who apparently was a bit too rigid for the noisy open air market here). I use it as a headphone amp, and it's quiet as a grave.

I seen some claim that they could hear a difference depending on which end of the filament was grounded (fixed bias). But in that case there is a bias difference. If I readjusted the bias accordingly, I couldn't hear a difference.

OT a bit, but I did have success with another of SB's ideas. I run the 801 filaments at about 6V for a significant overall reduction in distortion.

Sheldon

 

[robot]

I think that is not good advertisement for the manufacturer which said that the 300B and 2a3 had same sound...