Cathodyne vs Cathodyne

[navdave]

Which version will create more power tube output swing? The AC or DC coupled version?
I also read about something called the Paul C mod that is supposed to give this type of inverter more head room.

[drgonzonm]

Do a search on cathodyne Paul C mod, and you can find a discussion on the Paul C modification as well as two others.

[HotBluePlates]

Which version will create more power tube output swing? The AC or DC coupled version?

That's a trick question. They're both split-load inverters, using the same-valued load resistors, and presumably the same supply voltage. Fixed-bias (the d.c. coupled version) instead of self-biasing might save you a volt or two. So the output swing is essentially unchanged between the two versions.

Without looking at the complete circuit, it's hard to say why the designer chose a pair of 100k resistors for the loads; that's equivalent to a 200k load for a normal plate-loaded stage. You'd need to compare particulars to see if that change actually provides greater output than 56k resistors for plate and cathode.

Do a search on cathodyne Paul C mod, and you can find a discussion on the Paul C modification as well as two others.

Bottom-line up front: the most important change in the Paul C mod is that the B+ feed for the phase inverter is moved to a higher-voltage point in the power supply. That change is what enables a larger output swing from the split-load.

[Fresh_Start]

Do a search on cathodyne Paul C mod, and you can find a discussion on the Paul C modification as well as two others.

Bottom-line up front: the most important change in the Paul C mod is that the B+ feed for the phase inverter is moved to a higher-voltage point in the power supply. That change is what enables a larger output swing from the split-load.

With great respect, the "Stokes Mod" is the one that raises the B+ supply for the inverter.  The 'Paul C. Mod" changes the bias method.

http://www.thegearpage.net/board/showthread.php?p=3671603

http://music-electronics-forum.com/t14923/

http://www.el34world.com/Forum/index.php?topic=7876.0

Cheers,

Chip

[tubeswell]

If you are using it to push 6L6s, then a 12AX7 with 200k load is a good idea, because 100k above and below will get more swing on the 6L6 grids (if you have enough swing in front of the cathodyne). I just did this with a 6L6 amp today. The 56k above and below just didn't have enough oomph, but the 100k is the ticket. (Also helps if your HT is between 350 - 400 for the cathodyne supply, and you need 2-3 inverting gain stages in front).

[HotBluePlates]

With great respect, the "Stokes Mod" is the one that raises the B+ supply for the inverter.  The 'Paul C. Mod" changes the bias method.

Thanks for the correction Chip!

Reading one of the links, it looks like Paul C said the point was to change the bias point, as well as changing the bias method.

If you are using it to push 6L6s, then a 12AX7 with 200k load is a good idea, because 100k above and below will get more swing on the 6L6 grids (if you have enough swing in front of the cathodyne). I just did this with a 6L6 amp today.

I'll buy that!

Just remember that whatever load you use, you probably want something near 1/3 of the B+ across the tube (leaving 1/3 across each load resistor). Assuming your B+ isn't very high, that should leave enough voltage across the tube to function properly. So the thinking then is if you need to raise the output swing, you increase the load resistors, but you also need to increase the supply voltage to leave enough across the tube.

Eventually, this approach starts to fall flat (you run out of supply voltage, or the tube is running with too little current), and you should switch to a long-tail inverter (or, gasp, a paraphase inverter).

[tubeswell]

Also, if you're going to use the 200k load (100k + 100k) with the cathode-biased option, you'll probably want a bigger bias resistor than 1k5. (I went with 3k3 - fully bypassed, and it sounds pretty damn good.) I also have 400 HT voltage.