clipper

[Andy Le Blanc]

Anybody measure a 12ax7 stage with a 10k Kr (ala.marshal) when its over driven?

I assume that its bias point is pushed up to where one side of the wave form
is clipped.

[markr14850]

That 10k is usually not bypassed. I'd imagine that local NFB has some noticable effect on the character of the distortion.

[jjman]

Top curve is a 12ax7 with 100k plate and 10k cathode. 246V on plate. Bottom curve is the 12ax7 driving it. The top curve is plate voltage but inverted on the scope, so it's phase is actually the plate current. It's reaching cutoff but not saturating.

[Alexo]

Hey JJ, are those traces both set at the same volts div.?

Just from some time spent tweaking, it almost seems like the 10K stage is close to unity gain, though I haven't gotten the chance to check it out for sure.

[VacuumVoodoo]

Hey JJ, are those traces both set at the same volts div.?

Vertical scale and time base settings are displayed in clear text on screen in the photo.

[PCollen]

Hey JJ, are those traces both set at the same volts div.?

Just from some time spent tweaking, it almost seems like the 10K stage is close to unity gain, though I haven't gotten the chance to check it out for sure.

Express clipper stage unbypassed voltage gain is approx 8.4 (18 dB) .

[Alexo]

Hey JJ, are those traces both set at the same volts div.?

Vertical scale and time base settings are displayed in clear text on screen in the photo.

I'm still new to digital scopes, thanks for the help, though.

[Alexo]

Hey JJ, are those traces both set at the same volts div.?

Just from some time spent tweaking, it almost seems like the 10K stage is close to unity gain, though I haven't gotten the chance to check it out for sure.

Express clipper stage unbypassed voltage gain is approx 8.4 (18 dB) .

Thanks PC, obviously not unity, but quite low, food for thought..

[tubeswell]

Hey JJ, are those traces both set at the same volts div.?

Just from some time spent tweaking, it almost seems like the 10K stage is close to unity gain, though I haven't gotten the chance to check it out for sure.

Express clipper stage unbypassed voltage gain is approx 8.4 (18 dB) .

Thanks PC, obviously not unity, but quite low, food for thought..

As markr said - probably cathode current feedback

[Andy Le Blanc]

thank you.... soldano uses one in the same stage in the gain structure
as an express but uses a 39k, before another gain stage.
Even closer to unity, seems it may function more as a buffer.
Anyone play with the value in a wreck?

[PCollen]

Hey JJ, are those traces both set at the same volts div.?

Just from some time spent tweaking, it almost seems like the 10K stage is close to unity gain, though I haven't gotten the chance to check it out for sure.

Express clipper stage unbypassed voltage gain is approx 8.4 (18 dB) .

Thanks PC, obviously not unity, but quite low, food for thought..

Someone pointed out that the Soldano SLO uses 39K vs. 10K. In an Express, that would give an unbypassed voltage gain of about 2.4 (8dB)

[Alexo]

...looks like as Rk approaches Ra, gain approaches unity. ..? Guess that makes sense, considering split-load pi's.

[dave g]

The alternating cold/hot bias found in a lot of high gain preamps is where you get a lot of the sustain and compression. For example, in the SLO the 3rd stage with the 39k Rk clips early at cutoff, while the next stage is biased hotter (2.2k Rk) and will clip mainly via grid conduction. Because the 3rd stage is inverting, however, the same side of the waveform ends up being clipped by both of these stages. The 3rd stage clips much harder and in a much less pleasing way than the 4th stage, but you actually don't hear it because the 4th stage soft clips the already hard clipped side of the wave, if that makes sense. The 3rd stage then ends up acting like a compressor, preventing the 4th stage from going into hard clipping (since the amplitude of the waveform is limited by the 3rd stage) while simultaneously providing you with much more sustain than if you had just clipped the 4th stage alone.

[Andy Le Blanc]

using that rational...

do you think that its voltage limiting or current limiting?