cathodyne phase inverter has no gain?

[blindopher]

Hi,

 

I must be missing something here. A cathodyne PI looks to me quite like the circuit for a normal cathode biased stage -  plate\cathode resistors and all. So why does it have near unity gain? Why can it not amplify? In other words, how is it different than a normal gain stage?

 

thanx!

[WRGKMC]

Its not supposed to amplify, its supposed to create a reverse phased copy of the input signal. This is essential in making the push pull power tubes operate.

If you look at this diagram you'll see one AC wave going positive and the other going negative at the same time. 

 

In your power tube section each tube does half the job of driving your speakers. The positive going wave will drive one tube and the negative going wave will drive the other tube.

 

Think of it like spark plugs in an automobile. One spark plug fires causing the gas vapors in the cylinder to explode pushing the piston down the tube. As the piston reaches the bottom, the second piston is at the top. It fires and drives the first piston back up. Thats the essence of how a two stroke engine works. The Phase inverter is kind of like the distributor that supplies the small spark that makes the cylinders fire on time so they work together to drive the crankshaft, transmission and then turn the wheels. 

 

Dont know if that discription helps much but it works to give you a basic idea.

Tubes are like diodes. They can only pass electricity in one direction. zero to a positive direction. The signal is reversed so when the wave is going negative it drives the second tube to condict as the first tubes input signal is going negative. The result is you speaker pushes outward and gets sucked inward in a bi directional movement.

 

There are other ways of amplifying with tubes. A class A amp will work between zero and 100%. The tube is biades for a zero reference is 50%. When the tube is fed the positive side of the ac signal, the tube will climb to max of 100%. When the signal goes negative it goes down to zero. This is kind of like a one stroke engine with a single cylinder and single spark plug.

 

It has less power but the motion of the cycle is consistant and doesnt require balanced timing like two stroke or more engine does. Push pull amps need to have the signal copied and reversed. The input signals must be equal clones or one tube may not be a mirror image of the first. The tubes must also be a matching pair so they produce equal amounts of power. Biasing is like adjusting the spark gap so each side powers its half equally and smoothly. you know what happens whan a spark plug doesnt fire well, the engine shimmies. Same thing with power tubes. The bias is used to make both sides power equally.

 

thats about the best I can do in laymens terms. The rest requires a bit of study. How it works is actually allot simpler than most think. sa polarities reverse so does the electromagnet of the speaker and its the ley to its movement, quickly changing mahnetic fiels that are either attracted or repelled against its perminant magnet.

  

[blindopher]

thanx, but to be honest, it still doesn't answer my main question which is: why isn't it amplifying? For all i see it should by its very topology, at least where the signal taken off the plate is concerned. I mean, the signal at the grid of the PI is modulating the HT present in the tube, isn't it? That would mean amplification. I mean, it appears (aside from the fact that another signal is taken off the cathode and is in-phase) just like a normal gain stage.