TSE interstage coupling cap

[jrenkin]

I need some help understanding the coupling capacitor in the 300b TSE circuit.
George has chosen 0.22u value.
what is the impedance of the stage used to calculate this cap value and the -3db frequency? Is it the grid leak resistor on the output tube? If it is, the 20K ohm lead to a 36Hz knee and what seems like a high value, but I don't know how to account for the Mosfet.
I can build the circuit as it is, but I would prefer to understand it better. Thanks!

[tubelab.com]

but I don't know how to account for the Mosfet.

The mosfet is a buffer. Look at it as a cathode follower with near perfect characteristics. The Gm is very high so its gate looks like a 6pF capacitor with infinite input impedance. The load on the coupling capacitor is R15(121K) in series with the combination of the bias pot and its two resistors (about 60K depending on bias setting). So you have a .22uF cap and about 180K of resistance. This puts the 3db frequency in the 5Hz range. You can use a larger cap. I have used some 1uF "Wonder Caps" because I had some, but the extremely low corner frequency makes adjusting the bias tedius since the response time of the bias pot is several seconds.

[jrenkin]

Thanks, that helps a lot. I wasn't sure where to find the load value.

Does the R15/pot circuit also set the bias current through the mosfet?

A more general question, What are the advantages/disadvantages to a higher or lower -3db frequency? If it is low and the OPT is undersized I could see an issue of saturation. Unlike speakers, though, other than that and the bias adjustment lag, a low cutoff should be fine, right? Or does it eventually affect the amp's responses to clipping and recovery (and are those managed better due to the mosfet?).
Since my system performance in the room drops off around 50hz due to awkward room size, shape and openings and the limits of my speakers, would there be an advantage to increasing the -3db frequency with a smaller CC since I am likely to be using separately powered subs crossed at 150hz - 200hz or so.

Since my brain is working and my patients are not coming in due to a snow storm, I will ask one more general mosfet question.
The mosfet is to prevent problems at clipping when the designer of the circuit drives it too hard as he says he is prone to do. but I don't. I tend to listen to classical and jazz and rhythm, with a little rock, but not hard rock or metal at all. I also don't crank it much, hitting average listening at say 74-80 db (not transients). Is the mosfet doing anything at all for me? Am I better off without it, does it not matter, or should I keep it. If I take it out, I just need to revalue the 300b grid leak resistor per the data sheets, I assume.

Thanks for helping my brain! I am close to getting the first sounds out of my TSE!

[jrenkin]

No thoughts over the holiday?

happy holidays everyone!

[boywonder]

So the question is....do the mosfets help when in AB1 (not driving grid current)?

Here are my (semi-newbie) thoughts,.......George probably has additional insight... the mosfet provides a very high impedance and relatively low input capacitance (easy to drive load) for the 5842's so they don't have to deal with the miller capacitance of the output tubes (triodes), as well as providing the current when driving + grid voltage/current (AB2).

The driver tubes just have to provide enough voltage swing since the current demands are handled by the mosfets.