Phase Inverter Choices

[JJH0906]

Greetings, fellow builders!

What are the members opinions of the best choice of a phase inverter for a Push-Pull AB driver for two 6L6GC output tubes? The Cathodyne configuration looks fairly simple but I hear a lot about the AC or DC Long Tail Pair. I've tried to make sense of some info I've found online but thought it might be good to experienced builders thoughts.

I am not looking to overdrive the output tube much, if at all, just simply trying to drive the output tubes to get some volume out of them.

Thanks for your ideas!

[pdf64]

Follow the evolution of Fender etc circuits through the 50s. Cathodyne is fine for cathode bias, but for cooler fixed bias, LTP rules. Reason being that cathodyne requires a high HT feed to fully drive grids (ie up to Vg-k = 0V) that have got a lot of bias voltage on them.
And if NFB is desired, cathodyne may require an additional coupling cap in the loop, thereby limiting the degree of NFB that can be applied.

[JJH0906]

Thanks for that!

That was sort of the direction I was leaning, anyway. The Cathodyne seems a bit too simple and I noticed the HT+ being used as you mentioned, which looked a little sketch. I've heard alot about the LTP being a superior circuit so I'll look into that more fully.

[wpaulvogel]

Don’t forget the paraphrase inverter. It’s nice in a guitar amplifier because it adds a little distortion ( not clipping) that is pleasing. It’s also got plenty of gain to drive big bottles.

[Roe]

paraphase?

[martin manning]

paraphase?

Paraphase. Spell check fail. http://www.valvewizard.co.uk/paraphase.html

[JJH0906]

I had seen the paraphase inverter with Merlin's website, and a couple of other sources, but my understanding was that it was kind of a dated way to do phase inversion. The LTP appeared to be coming out on top.

What I'm going for is to have as little distortion as possible, but meaning as far as clipping, not necessarily harmonics. This build is intended for a pedal platform so I'm just trying to keep things as clean as possible from the beginning to the output section.

[pdf64]

I think paraphase typically involves an additional RC coupling over an LTP, so the degree of NFB that can be applied may be limited by that.

[Phil_S]

Look at Ampeg schematics for paraphase examples. LOL, I was going to mention this and use of an interstage transformer, as well. Then I thought, what's the point? I dunno... There is one more...self split, but I've only seen that used with triode output stages.

[Andy Le Blanc]

LTP came out the need for stability, from oscilloscopes. Self stabilizing and resistant to tube characteristic changes that come with age. Solid, but it operates using feedback, and it cancels even order harmonic distortion. They can sound a bit "hard" or "cold". Good thing is that you can be critical about the tail resistor to tweak the feedback to taste, before applying global feedback.
Cathodyne can be made nearly ideal, you can control local feedback and balance, but at a point they fall apart when driven into distortion. I really like the various paraphase for they're tone color, the inconsistencies and instabilities make it, but it can be a "never clean" tone. There are plenty of examples of paraphase using 6sn7 and 12au7, to accomplish phase inversion with low distortion.
One caution with many old paraphase designs, many were very simple and ment to drive 6v6 and 6l6, "EL" family can have enough of a grid current to upset the invertor tubes bias. They need an additional coupling cap and resistor to prevent it, ala ampeg and vox. If your up for it, leave enough room on your pcb to try more than one invertor, the fender layout for a LTP can be reworked for cathodyne and others.

[JJH0906]

Wow that's alot of info! Thanks!

The intention is to have/drive as little distortion as possible and also to drive a pair of 6L6 tubes, so everything is a bit more "modern".

I did like the paraphase but the LTP seemes to be the preferred choice and I figured I'd try to stick with something that was easy to reference.

Has anyone ever seen/used a CT transformer for the phase inversion? I saw it in a book by EJ Jurich book but he didn't explain the circuit in much detail.

[thetragichero]

Has anyone ever seen/used a CT transformer for the phase inversion? I saw it in a book by EJ Jurich book but he didn't explain the circuit in much detail.

something like this?
https://schematicheaven.net/fenderamps/ ... _schem.pdf

[WiderGates]

https://www.tube-town.net/cms/userfiles ... ematic.pdf

[sluckey]

Plenty of old Gibson amps. Look through the GA series...

https://el34world.com/charts/TubeAmpSchematics.htm

[JJH0906]

Those are some interesting schematics!

Forgive my ignorance, but is there an advantage or disadvantage of using a CT transformer to do the phase inversion, versus a tube? Cost? Ease of use? Sound quality?