6H6p cascode differential driving 2A3 PP

[panos29]

Right, after much search and experimentation I managed to built the following amplifier aroung 6H6p cascode differential driving 2A3 PP. Maximum output is around 12W at 0,3% distortion.

Having this on the test bench, I disconnected the output stage and took measurement of the driver, to my astonishment distortion of the driver driving only grid leaks (330K) was 2%!!!

I then trimmed the various driver parameters, as total stage current and top grid voltage and arrived easily at 0,2% distortion at same drive voltager as previously.

Wierd as it may seems output distortion (with the output stage on) went to 2+%!

I really believe its a matter of distortion canceling, but what is more "correct" sound-wise? I auditioned the low total distortion option and sound was quite nice. I will listen to the other option shortly, but I just thought I might try to ask for opinions on the subject.

[SY]

Is your measurement single-ended or differential? If it's single-ended, you can pretty much ignore the second harmonic component (which dominates THD) and optimize to lower third and fifth- the second will be canceled by push-pull operation.

If your measurement is differential, you've got some imbalance issues to nail down.

[analog_sa]

Hi Panos29

I have no opinion regarding the distortion cancellation but have other suggestions/questions. Have you considered replacing the cathode bypass cap with a WE Ultrapath? To my ears the subjective improvements are very worthwhile.

It's great you use a separate B+ for the output stage but why is the i/p-driver fed from a SS rectifier?

IME the driver stage sonic character is more obviously influenced by the PS quality. What i mean is that i'd rather have the drivers PS tube rectified than the outputs.

Recently i tried Crees in my PP 2A3 (for the sake of reducing weight and heat, not better sound) and the output stage sounded as an acceptable downgrade from an 83. The driver stage,otoh, produces some of the fiercest highs i've ever heard from a tube amp when the cree diodes replace a GZ34.

[panos29]

All measurements were done SE and ideed the second may ccancel due to PP circuit. However, I really believe it is an harmonic canceling effect and my question is more of aphilosophic character. Is it better to have harmonics cancelled which leads to lower total distortion or is it better to not to cancel it? I am also thinking of adding a costant current sink at the cathodes of 2A3s with a capacitor in style of WE ultrapath as analog_sa says.
What are your opinions on this?

[SY]

Well, that depends on whether or not you want your amplifier to be an effects box or whether you want it to deliver the signal unvarnished. For the latter, minimize third/fifth/seventh and don't worry about second. Cancellation works and has no penalty.

[Wavebourn]

Did you try MOSFETs instead of upper halves?