Design of output section of rack tube preamp?

[Chuck H]

Yes. Something hinky about how I have things set up because previously when I've tried extracting files to the same folder as all my spice stuff it has never worked. This time I just dragged everything onto the desktop. From there I just created a new folder and put everything in there. Working great so far and thanks again.

[Dave H]

I didn't use a tube model for my simulations. Just an AC voltage with the series resistance approximated at 1K. Do you expect that the performance of an actual tube model would be different in the circuit I proposed?

That won't tell you the large signal performance.

I don't know how accurate this is (I'll have to check it with Spice) but the standing current in the CF is typically 1.8mA (Bassman) and that's the max peak output current in the negative direction (when the tube cuts off). Your tonestack input impedance won't be much more than the slope resistor at mid/high frequencies say 4k. 1.8mA x 4k = 7.2V peak. The output of the stack will be about 1/10 of that or 0.7V peak at clipping and probably badly distorted well before that.

[Chuck H]

I just simulated it and I was surprised at just how much larger I had to make the series resistance to get to 0dB nominal. So 15k for the series resistance is too low/small. I must have missed something in my original guestimates. But with the models provided it seemed to work fine.?. I didn't see any distortions other than what a tone stack typically does and the frequency response looked right too.

[Dave H]

I get this for a CF driving a 4k resistor (no tonestack)

[Chuck H]

So this would apply to any frequencies in the mid band, right? LF and HF have the benefit of additional resistance affecting the load I think.?. And the mid band would also be of a lower amplitude. I didn't save the circuit after running my sims but I'm going to do it again and give it more thorough scrutiny. Not that I don't believe your assessment of the circumstances, I just like playing with my new tube models I already have one of my full amp design circuits plugged in Thank you!

[ACHIEVEIT]

Back to the drawing board..
Indeed the pad doesn't perform satisfactorily. I would like to give MOSFETS a try for this purpose. I tried to gather as much info as possible and tomorrow I will get my hands on some IRF820s (can't find LND150s locally - will have to order them). I don't have any experience with them so once more I could use some guidance. Would something like this work? It is also not clear whether the attached circuit would work with both LND150 and IRF820 or I would need to provide a means of biasing the gate in case of IRF820. What about the resistor and capacitor values?
Thanks again for the help!