Looking for PP design when I already have the OPTs

[Robert Weaver]

Not a very stable solution to get the opposite phase.You have to adjust the trimpot on the brink of oscillation to get the amplitude right.If the voltage divider on the OT is half way there is no output to the grid from the potm.Adjusting one direction givers NFB , the other PFB !
With the lower triode grid on gnd level and the 150ohm replaced with a (low drop) current source it could be possible.
Mona

Actually, there is no problem with instability. The proper adjustment is well into the NFB region. As I recall, I don't think I was able to get it to oscillate at all with any pot setting, although it is limited in range by the two fixed resistors. If the bottom leg of the pot were connected to ground rather than the bottom lead of the OPT, then there could be a stability issue. However with the current arrangement, and if you take into account the autotransformer effect, there is considerably more NFB than may appear at first glance. I've been using the amplifier daily for 3 years, driving my computer desktop speakers, and have never had any stability issues with it.

However, I do agree that this is generally not a good method for phase splitting. I did it this way because I was challenging myself to design a complete one tube push pull amplifier. After accounting for a preamp stage and the push pull output, there were no tube sections left for a phase splitter. So, unless you are setting for yourself some strange design constraint, there are much better ways to implement the phase splitter.

 

[Ketje]

Actually, there is no problem with instability. The proper adjustment is well into the NFB region. As I recall, I don't think I was able to get it to oscillate at all with any pot setting, although it is limited in range by the two fixed resistors. If the bottom leg of the pot were connected to ground rather than the bottom lead of the OPT, then there could be a stability issue. However with the current arrangement, and if you take into account the autotransformer effect, there is considerably more NFB than may appear at first glance. I've been using the amplifier daily for 3 years, driving my computer desktop speakers, and have never had any stability issues with it.

However, I do agree that this is generally not a good method for phase splitting. I did it this way because I was challenging myself to design a complete one tube push pull amplifier. After accounting for a preamp stage and the push pull output, there were no tube sections left for a phase splitter. So, unless you are setting for yourself some strange design constraint, there are much better ways to implement the phase splitter.

If you take away the OT there is a point where you get the opposite phase (a x-1 amp).But here you get never perfect balance, that's the x0 point you have to stay away from.
Even with one side doing nothing the amp function, the other tube just give some extra power, not up to 50/50.
In the mean time a simple idea in style with the amp in the other threat of the TS.
Mona