SE class A2

[valderama]

http://oi67.tinypic.com/708b4n.jpg

I have a project and do not know exactly how to define the working class for the final stage("class A2 means that the grid is driven MORE POSITIVE than the cathode for part or all of the waveform")
Your opinion counts
(sorry for my English)

 

[jazbo8]

The schematic shows a screen driven output stage, the grid is still negatively biased, since no component values are shown on the schematic, I'm not sure if it is class A2...

 

[Parafeed813]

The picture doesn't show up on my phone.
Please upload it to the forum.

 

[jazbo8]

 

[valderama]

the command-grid(g2-6П36С) is always positive static or dynamic (for all of the waveform).
I can connect to 0V g1 (Ug=0V)

 

[jazbo8]

If that's the case, why are you asking if it is class A2?

 

[valderama]

I want to know your opinion , i may be wrong to say A2.

 

[jazbo8]

FWIW, this seems to be a convoluted design, if output power is a concern, why use screen drive + A2? Couldn't the same be achieved with normal grid drive in A1?

 

[ballpencil]

class A2 means that the grid is driven MORE POSITIVE than the cathode for part or all of the waveform

Jumping into to the mud of amp class definitions, in my opinion the traditional sense of Class A2 refers to the control grid (g1) being driven positive as A2 classification can be applied to triode as well. So your statement is incomplete.

Screen grid (g2) is always more positive (not always but usually is) than the cathode so it makes no sense to classify it as Class A2 just because you're driving the screen grid. Call it Class A3 if you wish Bonus point: drive G1 as well and you have Class AAA or what have you.

 

[valderama]

I did not realize still want to see more opinions(A2 or A1...)
in this connection I wish to work (It is interesting for me)
in this particular case control grid is g2(I can connect to 0V g1)

 

[valderama]

I try to put some values, may work well

http://oi66.tinypic.com/vnpnwn.jpg

 

[jazbo8]

With such low idle current, how can it be A1 let alone A2?

 

[AV8R]

I developed this method in my EAR859 SET amplifier. I still define it as Class A1.
Since at no time does the screen grid transistion to negative, that is current is always flowing in the screen grid. Grid 1 is tied to cathode. therefore is not functioning in a valid way.
Class A2 defines a drive that transitions from zero grid current in a negatively biased grid to a positive grid with grid current for part of the cycle. consequently the power drawn from the power supply is not constant or linear.

 

[jazbo8]

I developed this method in my EAR859 SET amplifier. I still define it as Class A1.
Since at no time does the screen grid transistion to negative, that is current is always flowing in the screen grid. Grid 1 is tied to cathode. therefore is not functioning in a valid way.
Class A2 defines a drive that transitions from zero grid current in a negatively biased grid to a positive grid with grid current for part of the cycle. consequently the power drawn from the power supply is not constant or linear.

This definitely falls in the "power drive" catagory... what are its advantages?

 

[Parafeed813]

This definitely falls in the "power drive" catagory... perhaps a tad more linear than the normal grid drive, are there other advantages as well?

Screen drive is a lot more linear than normal g1 drive while retaining the pentode efficiency.
The downside is the high output impedance of pentode mode is retained as well and you need some power to drive it.

A lot has been written here on screen drive and more recently on its 'successor': crazy drive.

 

[jazbo8]

I guess $64k question is normal grid drive with local and/or global feedback sufficient to offset the screen/crazy drive's improved linearity.

 

[AV8R]

I called it "Enhance Mode Triode" that was 25 years ago now.
The screen drive is easy from a simple cathode follower. As if driven from an effective zero Ohm source the output is very linear. A family of curves is about half way between a perfect pentode and a true resister or basic triode line. As in all these things, drive impedance and linearity are important with any output circuit whether conventional G1 drive or even grounded grid drive.

Finally there are several thousand of the EAR859 amplifiers out there world wide.

 

[AV8R]

Pentode efficiency if by that you mean the traditional near constant current curves rather than a poor resistor curve. But in my Single End circuit as true Class A it can only achieve a theoretical 28% efficiency. or 30% if you accept 10% distortion as a no feedback criteria.

 

[artosalo]

I have a project and do not know exactly how to define the working class...

(A or AB)2 as an operating class indicates that the drive requires current i.e. power.
As in this case the g2 is used for driving the tube, then I would say that it clearly is in A2-class.

 

[45]

Class A2 defines a drive that transitions from zero grid current in a negatively biased grid to a positive grid with grid current for part of the cycle. consequently the power drawn from the power supply is not constant or linear.

Or the contrary as for example in a positively DC biased valve like the 811a where it swings into negative grid voltage only when approaching max power. However, even at lower grid modulation such that the grid is always positive respect to the cathode the grid current draw will change with applied signal. The fact that the power requirement is not constant or linear doesn't necessarily is a bad thing. A actively loaded cathode follower or a step-down inverse mode inter-stage transformer using proper triodes will handle it perfectly.