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Cathode follower drive to output tubes

1/10/2000 4:19 PM
lion	Cathode follower drive to output tubes This interesting issue came up in another thread. Randall Aiken: >The only way to drive the output stage into class AB2, or grid current, operation is to provide a low impedance drive source capable of sourcing and sinking the grid current. The best way to do this is with a DC-coupled cathode follower drive to the output tubes. Put the cathode follower after the bias feed resistors, refer the cathode resistor to a large negative voltage (such as the raw bias supply), and ... This will completely eliminate the grid clamping, and allow the full excursion from the PI to drive the output tube grids. The result? More output power, and no blocking distortion. This is my preferred method for output tube drive.< I’m considering giving this mod a try in my DIY amp. (2 x EL34 – no NFB) The reason is that I’d like a little more clean power. I’m not sure that it’s grid blocking – but with the amp cranked the output waveform is clipped (of course) - BUT also has humps at the zero crossings (looking just like crossover distortion caused by wrong bias adjustment). So before I decide on the mod I’d like to get a few things cleared up. First off – it would be nice to have the resident experts opions on whether or not adding CF drive to the output tubes would be an improvement worth considering. Second – some practical problems: I’ll have to add a tube – which should be possible – but how much extra load will a 12ax7 put on the PT – and will it be OK? The physical placement of the tube isn’t that critical – no gain being involved, right or wrong? The plate voltage to the CF – how much would be max (with the CF cathode resistor refered to the raw bias = - 50V)? I’d be more comfortable working from a schematics – so if anyone could help me out here I’d be more that happy? Any comments, advise – or help otherwise would be appreciated. regards Lion

1/10/2000 4:19 PM

lion

Cathode follower drive to output tubes

This interesting issue came up in another thread.

Randall Aiken:
>The only way to drive the output stage into class AB2, or grid current, operation is to provide a low impedance drive source capable of sourcing and sinking the grid current. The best way to do this is with a DC-coupled cathode follower drive to the output tubes. Put the cathode follower after the bias feed resistors, refer the cathode resistor to a large negative voltage (such as the raw bias supply), and ...
This will completely eliminate the grid clamping, and allow the full excursion from the PI to drive the output tube grids. The result? More output power, and no blocking distortion. This is my preferred method for output tube drive.<

I’m considering giving this mod a try in my DIY amp. (2 x EL34 – no NFB)
The reason is that I’d like a little more *clean* power.

I’m not sure that it’s grid blocking – but with the amp cranked the output waveform is clipped (of course) - BUT also has humps at the zero crossings (looking just like crossover distortion caused by wrong bias adjustment).

So before I decide on the mod I’d like to get a few things cleared up.

First off – it would be nice to have the resident experts opions on whether or not adding CF drive to the output tubes would be an improvement worth considering.

Second – some practical problems:
I’ll have to add a tube – which should be possible – but how much extra load will a 12ax7 put on the PT – and will it be OK?

The physical placement of the tube isn’t that critical – no gain being involved, right or wrong?

The plate voltage to the CF – how much would be max (with the CF cathode resistor refered to the raw bias = - 50V)?

I’d be more comfortable working from a schematics – so if anyone could help me out here I’d be more that happy?

Any comments, advise – or help otherwise would be appreciated.

regards
Lion

Replies:

Randall Aiken Those "humps" that get worse as you... -- 1/10/2000 5:09 PM
Bruce Before I venture a comment, I'd lik... -- 1/11/2000 8:04 AM
Stephen Conner just play quieter -- 1/12/2000 3:14 PM