Hot and cold gain staging

[Mr.Death]

Hypothetical question.. If I had an  amp with the first 2 gain stages " hot" and the 3rd cold what would happen to the signal. Would you get blocking distortion? Thanks.

[pdf64]

There’s a certain degree of correlation between a stage’s bias and how prone it is to bias shift / blocking. But it may not be significant, and may be more related to the cathode being bypassed or not.

[tubeswell]

More coupling capacitance increases likelihood of blocking distortion - all other things being equal. And low frequencies charge up big coupling caps more than high frequencies do (because of the longer wave lengths) - hence what pdf64 was saying about bypass caps. But you also need a decent amount of gain, so blocking tends to occur more, the further along the signal chain you are.


However, you 'can' reduce the onset of grid current with a decent bit of grid stopper resistance.


Or you can use DC-coupling (no coupling cap), or an interstage transformer, to side-step blocking distortion altogether.

[dwinstonwood]

Hi Mr.Death, I've actually built an amp that's similar to what you're describing. But, my first two gain stages are more middle-biased with 1.5K and 2.7K cathode resistors. The third stage is cold biased with an un-bypassed 10K (Trainwreck/Marshall). I have a 500K gain/mv pot in front of the third stage.

I've also built the same basic amp using an un-bypassed 1.5K cathode resistor on the third stage (Fender AA864). I like the amp with the cold-biased stage better.

The cold bias point allows the guitar signal to more easily push the negative grid voltage so far negative that the tube approaches cutoff (to the right on a load line). I like the sound, and I've seen it described as a soft type of clipping.

As usual, I'm probably overlooking something important, but it seems to me that it would take a very big guitar signal to swing a cold-biased preamp tube's grid voltage so far positive (to the left on a load line) that it started conducting current. Am I right in thinking that blocking distortion occurs when the grid is so positive that it begins to conduct current?

I modified one of Rob's graphics to show the "possible" swing of the guitar signal around the bias point.

But, I guess that if the first two hot stages increased the signal gain by a lot, it could swing the grid to 0V. That's maybe one reason why it's nice to have a gain pot in front of the cold stage. 

[Mr.Death]

Thanks for the input guys. Love this site for all of the knowledge.