A2 Questions

[Svetlana]

When a tube grid is driven in such a manner that the grid starts to draw current, & if it is using a fixed bias supply, will the tube grid (try to) draw the current from the bias supply?

If so, what effect will this have on the driver?

If the current is available from the bias supply, will the driver tube only need to swing the voltage?

[Tweeker]

How is the previous stage coupled to the grid?

[tubelab.com]

Like Tweeker said, it depends on how the driver is coupled. The grid of an A2 output stage will draw current when it goes positive.

If the stage is driven by a follower (tube, mosfet or bipolar) the current will be supplied by the positive supply connected to the plate (drain, collector) of the follower. The negative (bias) supply does not necessarily need to posess a low impedance since no signal current flows through it.

If the stage is coupled via an interstage transformer all of the grid current is supplied by the driver tube. In most cases the driver transformer secondary is connected to the output tube grid. The other lead is connected to the bias supply. The signal current flows through the bias supply (or its bypass capacitors). The bias supply must have a low (and constant) impedance for all audio frequencies.

Since the bias supply is negative, and the grid will only draw current when it is positive the current does not come from the bias supply directly.

[Brian Beck]

If the driver is supplying the voltage, it must supply the current too. That's the definition of any voltage source.

(sorry, just saw George's response)

[CLS]

How about positive bias supply?
Like Shishido's 801 & 811 designs.
They are transformer coupled, with 317-based voltage regulator to provide the positive bias, about 20-some or 30-some V.

[Miles Prower]

Originally posted by CLS
How about positive bias supply?
Like Shishido's 801 & 811 designs.
They are transformer coupled, with 317-based voltage regulator to provide the positive bias, about 20-some or 30-some V.

Same deal: the driver supplies the extra grid current, and the positive bias supply is part of the signal's AC return. Therefore, it still needs good regulation and the lowest possible AC impedance. 317-based voltage regulators can manage that.

[Svetlana]

What operation points does he use?
What driver config does he use?
Is there any special requirements from the OPT?
Will A2 work in Push Pull?

[Tweeker]

A2 could be done in pushpull, but itd be a kinda weird operating point to choose. PP AB2 and B2 is common in modulators.

The 811 was intended for 0 bias class B use, it can be used in class A by using positive bias or other adjustments. Nothing special for the OPT. No idea on the driver.

[coresta]

look at the Altec Lansing PA using a pp of 811 . There was a very interesting study in Sound Pratices . The PP driver is a PP of 6W6 cathode followered with a beefy centertap choke ... 170watts of pure class B but zero biased . Get the B+ low and you have a wonderful A2 PP DHT amp !

[Svetlana]

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