Phase Inverter

[C Moore]

Is there any reason(s) you cannot change the PI in most amps.?
For example.....
to just arbitrarily change from a LTP to a Cathodyne. Are there considerations other than just swapping one PI for another.?
Thank You

[Phil_S]

I suppose you can do it, but you might not like the result at all. They do different things to the signal. I'm not a technical person, but here's how I'd sum up the three basic types:

Cathodyne: this is a current (ampere) device, feeding differential voltage but equal current to both side of the PP PA. It has unity gain (zero gain) so it tends to be very transparent to the pre amp signal.

LTP: this one provides some voltage gain (signal increase) as it splits the signal to feed the PA. Because of that, you can use it to goose a signal. In a sense it will make up some for tone stack losses and cover some other sins.

Paraphase: This one is a voltage based topology, too. It operates like a see-saw. The output from one side is used to feed the grid on the other side. Ampeg used this and that company wanted to make guitar amps that minimized distortion. For a good laugh, see if you can find some of the early sales literature. The didn't envision Cream or Hendrix. They were thinking about jazz players.

I built an amp a while back for someone. She asked for no distortion (don't ask.) I started with a paraphase, but the voltage swing was too much and it sent the PA into OD too soon. I used a lossless tone stack and sent the signal into a cathodyne. It was nearly impossible to get into OD with a pair of 6L6 (if you can imagine.) She loved the amp. Go figure.

Choose the PI that works best with your preamp and your intentions for the PA.

I know folks are going to tear this post apart. I don't claim to know that much.

[martin manning]

Good answer, Phil! I'll only add that I'm prety sure that the paraphase can have higher gain and more headroom than the LTP.

[Firestorm]

That was nice summary by Phil. I'll just throw in the need to decide how much signal you want at the output tube grids. Different tubes want different drive. If you have a MV it doesn't matter so much. Also if you are scaling the output.

[Firestorm]

Damn cellphone hiccups. I'm not going to post anything again until all my computers are back online. Sorry. Probably a relief for some of you to shut me up.

[renshen1957]

[quote="Phil_S"]I suppose you can do it, but you might not like the result at all. They do different things to the signal. I'm not a technical person, but here's how I'd sum up the three basic types

Paraphase: This one is a voltage based topology, too. It operates like a see-saw. The output from one side is used to feed the grid on the other side. Ampeg used this and that company wanted to make guitar amps that minimized distortion. For a good laugh, see if you can find some of the early sales literature. The didn't envision Cream or Hendrix. They were thinking about jazz players.

/quote]

Hi,

Very good explanation on Paraphase PI as used by Ampeg. Before Hull sold out the company, Trainwreck's Ken Fischer worked for Ampeg. Hull was totally into catering to the Jazz Musicians (to the point ignoring Rock and Roll).

Best Regards,

Steve

[C Moore]

Thanks for the responses.
Guess what I am really wondering is how effective a PI change might be in an older amp (5E3 just for example) if you were to install a LTP.
Or the other way around installing one of the "older" PI in a BF Fender amp.
If you like the amp, but wanted wanted the amp to have More/Less gain and/or headroom for example
Just kind of curious.
Thanks Again

[roberto]

If you go to a LTP PI to a 5E3 amp, don't add an extra tube.

6V6 should be biased around 18V, so a 36Vpp will overload the PA, that means around 1,5Vpp just before the PI can overload the PA.
So with gain maxed and around 60x gain on first stage (maybe a 12ax7 instead of a 12ay7) you can overload the PA with 30mV in the input of the amp.

[Phil_S]

Gosh, thanks for all that positive feedback!

Hired: I guess you know, if you change the PI in an amp, whatever it started out as, well, it isn't that anymore! There is nothing like doing it to understand it. If you have the bug, give it a try.

[printer2]

PI part values are tweaked to work with the input requirements of the power tubes and the overload characteristics desired. While this circuit is not optimized for any amp it does show the different PI's can be a step away from each other. Needs a four pole three position switch.

[img625]http://i406.photobucket.com/albums/pp14 ... edPI-3.jpg[/img]

Did a number of other combinations, no time to look them up now. Built an amp with switched cathodyne and paraphase. Gain is not that much different, don't forget the gain stage before the splitter triode is part of the PI. One day I will get back to experimenting with these circuits again.

Fender Deluxe 5E3 plus 5D3

[img657]http://i406.photobucket.com/albums/pp14 ... o5E3-4.jpg[/img]

Think I did a better version after.

Cathodyne to LTP with added switches to go to Fender Vox and Marshall values.



[img781]http://i406.photobucket.com/albums/pp14 ... TP-FVM.jpg[/img]

[C Moore]

Gosh, thanks for all that positive feedback!

Hired: I guess you know, if you change the PI in an amp, whatever it started out as, well, it isn't that anymore! There is nothing like doing it to understand it. If you have the bug, give it a try.

Hey Phil -
Yeah, exactly why I was asking, but you rarely hear it discussed. Guys make ALL KINDS of changes to their amps, but you never read a bout a "simple" PI change.
As you say, the best way is to just do it.
Again, I so rarely hear it discussed, I was wondering if there was an Electronic/Electrical reason not to do it.
Thanks Again

[C Moore]

Printer2 -
Thanks for your efforts.
I appreciate it.
best