Cathodyne PI?

[fusionbear]

I have a general question about the cathodyne PI. Why is it not used in most modern amp designs? I really like the way it sounds in my S100 inspired clone. I am still very much a beginner and would like to gain a little more theoretical insight as to the workings of this PI vs. the common LTP...

Thanks!

Any reading recommendations would be greatly appreciated...

[tonewood]

The Merlin Blencowe book on pre amps has good info on the PIs.

[Gaz]

I just read in an Ultimate Tone book that Kevin O'Connor uses them in almost all of his London Power amps (and I'm sure he's tried them all).

I know the newish VHT Deliverance uses one, but that's the only 'modern' amp I know of, that's not a vintage inspired amp.

[rdjones]

It doesn't clip as gracefully as the LTP, and doesn't drive heavy loads easily.
There's impedance issues. The low side has a lower source impedance than the top.
At low levels the balance between the outputs is near perfect but at high levels (near clipping) things get ugly fast.
Merlin has a page on the "split load" inverter that shows various behaviours.

http://www.freewebs.com/valvewizard/cathodyne.html

As he points out, most of the issues with the cathodyne can be overcome with careful design.

I've got several amps with the cathodyne inverter including a Princeton and a Fender Champ II.

rd

[Milkmansound]

The issues are what make it sound so good

[rdjones]

The issues are what make it sound so good

On a Sunn I rebuilt I added a largish grid stopper on the inverter.
It sounded pretty good.

rd

[Milkmansound]

Good move - I always put a large grid stop in there to help tame the distortion. You don't always need it, but it's a good idea to add it in just in case

[fusionbear]

It doesn't clip as gracefully as the LTP, and doesn't drive heavy loads easily.
There's impedance issues. The low side has a lower source impedance than the top.
At low levels the balance between the outputs is near perfect but at high levels (near clipping) things get ugly fast.
Merlin has a page on the "split load" inverter that shows various behaviours.

http://www.freewebs.com/valvewizard/cathodyne.html

As he points out, most of the issues with the cathodyne can be overcome with careful design.

I've got several amps with the cathodyne inverter including a Princeton and a Fender Champ II.

rd

Thanks, that article is great...

I love this place....

[megawhat]

The VHT does indeed use a cathodyne.
In the 60watt 2xKT88 there are no buffers, but in the 120watt version I believe there are cathode followers between the PI and the KT88's. I also think the Pitbull uses a cathodyne too, but has no followers between the PI and EL34's. I might be wrong about that though. The VHT also doesn't have the grid stop on it that merlin suggests in his book/website. It is a very tight and kind of dry amp.

Randy Fay's Daisycutter uses one.
Pretty sure the Vox Night train uses one.
And I think the Peavey Windsor and Valveking both have cathodynes.

[Milkmansound]

Sometimes they don't need the grid stop. I built a tweed deluxe from a mojotone kit a few years back and the breakup on it is perfect without the grid stop in there. Never got around to putting one in either and it's still a great amp! I got to use an original the other day and my little cheapo clone sounds pretty damn close - that probably does not have the stopper either.

[Gaz]

Orange/Matamps can definitely benefit from the grid stop, but then again, the intense blocking effect can be very cool for certain styles.

[azatplayer]

Vox nighttrain has one as well. Killer little amp.

[gingertube]

One of the best kept secrets of guitar power amp design.
The cathodyne (or split load or concertina or whatever you want to call it) is stunning.
I've stopped using the stacked schmitt (diff amp) splitter entirely and now only ever use the cathodyne.

I'm with milkmansound - I think that it is its "warts" which make it sound so good.

Cheers,
Ian

[tubeswell]

You can smoothen the overdrive clipping to some extent with a largish value (470k - 1M) grid stopper at the cathodyne grid pin. This makes the onset of distortion a bit smoother.

[roberto]

Concertina works best with EL84 and 6V6, because usually it starts saturating just after those valves. It works very good with EL34 too, expecially with the cited 470k-1M grid stopper. I don't like it with 6L6 equipped amps (maybe a 12at7 will work better, never tried).

It works quite good when dc coupled with something like Rp=220k Rk=470R on the previous stage, 470k grid stopper and a 47n coupling cap to 150k 4k7 voltage divider as tubish DI.