Merlin's Switchable PI: LTP, Cathodyne, and Paraphase

[danhei]

I believe I saw posted or linked to here a clever phase inverter implementation by Merlin which was able to switch between LTP, cathodyne, and paraphase configurations.

I haven't been able to find it again on this site nor was I able to find it in Merlin's books.The closest thing I've found was this post over at DIY Audio but this seems more complicated than what I remember with more switching positions. Pretty sure the Merlin one was just a three position switch.

Does anyone remember this? Or have a page reference in one of Merlin's books?

[sluckey]

I read every post on this forum and I don't recall ever seeing such a post.

[Colas LeGrippa]

A switch and a pot  in the path of a LTP to select between resistors values would give you a large variety of tones (sometimes subtle though) and much more simple than switching to a paraphase  not so much tonally different.


Colas

[VMS]

i think this is the thread you were looking:


http://ax84.com/bbs/dm.php?thread=425347


Merlins simpler way is down the thread

[danhei]

i think this is the thread you were looking:


http://ax84.com/bbs/dm.php?thread=425347


Merlins simpler way is down the thread

That’s it! Thanks a bunch VMS.


Wrong forum; my apologies as I had this mental image that I’d seen it over here not AX84 even though I’ve been a member there as well. 

[kagliostro]

Is this the schematic you are looking for ?





Franco

[danhei]

Is this the schematic you are looking for ?

...

Franco

Yes, thanks Franco

[PRR]

> Is this the schematic you are looking for ?

Thanks for finding one with values.

The issue with this is the gain varies a *lot*.

From the one input to one output:
Par ~~ 50
LPT ~~25
Cat ~~1

I suspect the large differences in gain will change the "feel" of the amp much more than the type of driver can.

[shooter]

+1
that's a big change, thinking it could be an "answer" for my amps to loud for my apt 
built in attenuation AND flavor 

[danhei]

I suspect the large differences in gain will change the "feel" of the amp much more than the type of driver can.

Later in the AX84 thread Merlin posted this version using a 4P3T that does includes gain stage + cathodyne (rather than the paralleled cathodyne in the picture Franco posted. I think this would solve the gain differences between the cathodyne and paraphase, but still have the ~1/2 gain issue for LTP.

[PRR]

I screwed-up. (assuming 12AX7) The Paraphase gain is near 25 due to no cathode bypass.

The LTP, each cathode bypasses the other, gain is near 25.

The latest form with gain-cathodyne, as shown with no cathode bypass, is gain near 25.

This leaves a question of where to inject NFB (if desired). The unbypassed cathodes offer an obvious spot which will work about the same either way. The LTP has the far grid, which can probably use a similar network. Too sticky tonight to think on it.

Here's the latest form stripped down to one mode so we can see the gain and cathodyne.

[DummyLoad]

I screwed-up. (assuming 12AX7) The Paraphase gain is near 25 due to no cathode bypass.

The LTP, each cathode bypasses the other, gain is near 25.

The latest form with gain-cathodyne, as shown with no cathode bypass, is gain near 25.

This leaves a question of where to inject NFB (if desired). The unbypassed cathodes offer an obvious spot which will work about the same either way. The LTP has the far grid, which can probably use a similar network. Too sticky tonight to think on it.

Here's the latest form stripped down to one mode so we can see the gain and cathodyne.

perhaps use the fourth pole of 4PDT switch to connect a 1-10uF bypass cap when in cathodyne mode?

you'd have AV of ~50, ~25, & ~25. so 1/2 gain switch basically.

--pete