Phase splitter

[dscottguitars]

Hi, I am wondering how this works in the Fender Bassmans and other schematics I've seen this pattern. The circuit I am working with is from schematic AA864. With a 12AT7 it has an input line into the grid (pin 2) and a 470 ohm resistor coming off the cathodes into a junction of 2-1 meg grid resistors and a 23K resistor. The 23K is then connected to a 0.1uF cap and an 820 ohm resistor from the feedback line off the OT. The other end of the cap goes to the other grid (pin 7).

How does this work?? And how much room is there to work with wrt the values used? I have a 24K resistor and wonder if it will work for the 23K and what changes it makes. If not much, how much change would be noticeable and what will happen. This is one of the only parts of amps I can't figure out what is going on. Thanks

[defaced]

That's a standard AC coupled long tailed pair phase inverter. The value is actually 22k, and is not terribly sensitive. Use the 24k you have with confidence.

Here's some reading on it if you're interested: The Valve Wizard -Long Tail Pair

[jmaf]

If you want to research more, it's called a Schmitt or long tail PI.

See also: The Long-Tail Pair

[dscottguitars]

thanks...

Thanks... knowing the name of it really helps in finding info. And, my schematic is fuzzy and it looked like a 23K, I have a 22K also. But, now I see I have a little room. Thanks again.

Which is better the AC or DC coupled? One is definitely simpler.

[jmaf]

Thanks... knowing the name of it really helps in finding info. And, my schematic is fuzzy and it looked like a 23K, I have a 22K also. But, now I see I have a little room. Thanks again.

Which is better the AC or DC coupled? One is definitely simpler.

DC coupled is better, because it doesn't distort and insert phase shifts in the sound in any way, but it's also a lot more difficult to achieve, as you must adapt the DC levels of two triodes so they work together. On most guitar amplifiers the phase inverter will be AC coupled. You'll normally only find DC coupled pairs driving the tone stacks in most guitar amps(that's what Marshall/Fender fender usually did).