Anyone please explain"What is an A1,A2 in SE amp?

[Triod lover]

Anyone please help me out what is an A1,A2 bias in tube SE amp?

 

[ray_moth]

A1 means the control grid is always negative wrt the cathode. No significant grid current flows. A2 means the grid can be driven positive for some part of the duty cycle and grid current will flow while it is positive. A2 can give much more power than A1. The input impedance drops dramatically from, say, 1 meg when the grid is negative to less than 1k when it is positive. It acts like a diode.

To use A2, you need an output tube that can safely draw grid current without the grid deforming or melting and a driver circuit that can supply the grid current, usually either a direct-coupled cathode follower or an interstage transformer.

 

[dhaen]

From Ray Moth:

The input impedance drops dramatically from, say, 1 meg when the grid is negative to less than 1k when it is positive. It acts like a diode.

So to be of any practical use, the source impedance must be very low, or else there is a sudden rise in distortion with grid current.

To use A2, you need an output tube that can safely draw grid current without the grid deforming or melting and a driver circuit that can supply the grid current,.....

I differ from Ray in this opinion. Although it's true that there are particular valves that are designed to survive large grid current, in an audio circuit that is A1 most of the time, and A2 on peaks, these small pulses of grid current are no problem for most valves.

 

[Triod lover]

Thank you you both for information,am planing to build an 833 SE amp and wonder how much for grid voltage and current can be used on this tube say 20 volts is it safe?
Triod Lover.

 

[Tweeker]

You can use lots of grid current with this tube, it sees B2 and class C use. Absolute max grid current rating is 100ma, more than some plates.

 

[EC8010]

If you're not clear as to the difference between A1 and A2, should you really be building an 833 amplifier with all the danger that implies? It only takes one slip...

 

[Triod lover]

Thank you so much,anyway,i plan to build this 833A in the near future but not use as high as 1000 v.,will use around 700 v.,anyone could me the schematic to study.

 

[ErikdeBest]

As a Dutch DIY'er says: "this project, based on 211, uses 1000V on the plate, which requires 120% attention...that is, the other projects with 400VDC still require a full 100%."

There is a truly long thread on an 833 amp by a Dutch member, with complete schematics and all the other blabla

Erik

 

[Tweeker]

You might try a GK71 connected as triode in A1 for easier way to a bit more power, these have an operating point more like 845 than 833.

Driving an 833 into grid current will either take a full power stage before it or SS.

Steve Bench article on A2 featuring a positive biased 841 amp.