Has anyone dabbled in AB2 ?

[es345]

What does the driver waveform at the power tube grid look like - stay unclipped?

Here to scope pictures in overdrive showing the output and the signal at G1, one with and one without NFB

- without NFB the G1 signal is not clipped
- with NFB it is even going higher. the reason is that at the moment of clipping the NFB signal is clipped. At that moment you get the open loop signal at G1 - which is higher than the closed loop signal. The second thing you can see is that the clipping is getting unsymmetrical. From my understanding this is caused by the LTPI no longer beeing symmetrical.

[es345]

Are you getting 200W out of a pair of KT120's, or a quad?

It is out of 2 KT120: Anode at about 800V, G2 with separate OT winding at about 600V. This is clearly the limit what is possible, and I do not recommend this operating mode for Overdrive applications.

[JazzGuitarGimp]

Hi Hans,

Yes, I had thought of the dual-gang pot trick. I meant to mention it in my last post, and forgot (getting old....). I imagine if you were to use a pot of a high-enough value, it would take you all the way from AB2 to AB1.

Though it is only 20% cathode bias, my intent with R13 and C5 is to do a "combo bias". I am trying to get just a bit of the lovely compression from full cathode bias. One of the requests form my friend, for whom I am building the amp, is "fast transient response" - which to me implies fixed bias, but I also wanted a bit of that 5E3 compression, so I only used 20% cathode bias. We'll see how that goes.

Having the bias controls on the grids of the 12AU7 occurred to me, but I really didn't know if it would be the better way to go. Thanks for pointing me in the right direction.

When I copied your schematic to my desktop and opened it, it is very low resolution - so low that I cannot see what's going on. Did you omit the 100R build-out and 330R grid-stopper resistors?

Many thanks,
Lou

[es345]

Lou,

I have just checked the resolution, it should be ok. Try to do a "save as" and then open it with a viewer.

Regarding the resistors: I assume you mean R95/R8 and R96/R14. They are replaced by the pot and the 47 Ohm resistor.

Hans- Georg

[pdf64]

Hans,
excellent, thanks for that info and screen shots.
Pete

[Structo]

Here it is in JPEG format.

[JazzGuitarGimp]

Here it is in JPEG format.

Excellent! Thank you, Tom.

Lou