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In Reply to: RE: Spice- I think I got it but..... posted by Tre' on November 23, 2014 at 17:45:11
Here is is with the 185uf.
Did you do that calculation manually? If so, whats the formula?
In the past I have used Tube Cadd but unfortunately it wont work on a 64bit machine!
Seemed like a little less phase shift with the 74uf cap....I usually end up tweaking this by ear but try to at least get it close.
After TK's trick with the scale(Thanks Triode Kingdom), it doesn't look as bad. I'm going to try replacing the front end tube and see if I cant get this a little more linear. I'm getting about 28 watts (according to spice) with just under 1v input so I have a little room to play with a lower mu tube on the front end. Just a little confused on how the 211 behaves driving it this hard, spice shows about 763ľA at the 211 grid. The current wave form starts looking nasty at about 17watts (waveform below is at 28watts). I'm guessing this isn't going to sound good....
I agree with you about using SS for bass but have not found a electronic crossover I can live with. Got my moniker by playing high power SS thru these Khorns in the past.
The attached sim was the output at the 10K transformer
Follow Ups:
"Did you do that calculation manually? If so, whats the formula?"Yes. The formula is in a Excel spreadsheet linked but it's just the cathode impedance (of the tube itself) in parallel with the cathode resistor value, then the normal math for the -3db point of a single pole filter.
Calculating the impedance of the cathode of the tube takes into account the mu of the tube, the plate load impedance and the value of the cathode resistor.
The main thing to understand is it's not just the value of the cathode resistor that need to be bypassed but the cathode resistor value in parallel with the impedance of the cathode itself.
BTW TubeCad calculates the value of the bypass cap assuming a -3db point of 20Hz when 2Hz is more correct. So just add a zero. If TubeCad says 20uf, use 200uf if you want the cathode to be bypassed to 2Hz.
It takes 2Hz to get a flat and phase shift free 20Hz.
"I agree with you about using SS for bass but have not found a electronic crossover I can live with"
I use a Sallen-Key op-amp based low pass filter for the bass and just a single series cap for the highs.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 11/24/14
when i used the tube cadd, I always ended up with a larger cap than calculated in the program. It just sounded better.
Thanks for the calculator, takes out the guess work now!
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