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

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#21
Sheldon said:


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
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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.. :devilr:

:xmastree: :xmastree:
 
#22
kevinkr said:
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.. :devilr:

:xmastree: :xmastree:
Click to expand...

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
 
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