Grid Current

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pre65
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#16

Post by pre65 »

Paul Barker wrote: I haven't spent any time thinking about the filament, it would surely be better as two individual supplies but I expect parallel would be fine. Series is naturally out of the question.
I didn't realise the filament were not 6.3v. :(

So 2.1v or 4.2v depending how you wire them.

4.2v @ 0.325 would mean we could run a pair from one of Andrews modules with a resistor added ?
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#17

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As the 811a only needs 80ma, would I need to bias the 4P1L pair differently ?

And if using cathode bias on the 4P1L pair would one bias resistor do both valves ? ie 9v @ 80ma = 112.5R
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#18

Post by Paul Barker »

here is the 300b driver. Because it is effectively auto bias it can tolerate 100mA.

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#19

Post by Paul Barker »

I can adjust it for the 811a as the output valve. but I designed it for the 211. So will get back to you.
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#20

Post by Paul Barker »

811a doesn't lend itself to the design because it has to be in positive grid bias.

I'll look at 801a for you.
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#21

Post by Paul Barker »

801a doesn't lend itself to the simplicity of the design either.

If you run it at 370v it would pull 10v negative bias at 50mA and you would get 4 watts power output in A2.

The single 41Pl at 50mA would match it perfectly.

So it would probably be quite a nice low powered amplifier.

But the 211 just happens to fit the criteria best with a healthy 25 watts power output.
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#22

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Paul Barker wrote:811a doesn't lend itself to the design because it has to be in positive grid bias.

I'll look at 801a for you.
Can one not still bias the 811a grid to +40v so it's still at +20v relative to the -20v cathode ?
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#23

Post by Paul Barker »

Yes you can but there will be grid current so you loose the serendipity of my design.

Once you actively bias the grid of the valve in Class A2 you are in the ball game of high powe rlow impedance grid bias design.
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#24

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Ah, so is the 211 in your diagram in 360 degree A2 but without drawing any grid current ?
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#25

Post by Paul Barker »

No it is only in A2 less than 50% of the time because it is negatively biased, due to the autobias across the interstage of +20v at the cathode which translates to -20v at the grid.

Yes there is grid current but the serendipity is selecting the valve the operating point to suit the interstage transformer available (you could specify an interstage for other valves/operating points, but that would be a bespoke design which would cost you much money).

The grid current does not affect the grid bias at the grid because the current exits the grid straight to ground with no dc resistance to negotiate.
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#26

Post by pre65 »

OK, but I thought the main advantage of cathode drive was that the valve was in A2 all the time (360 degrees), so called A3. :wink:
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#27

Post by Paul Barker »

Only if you bias it as such and we would then be starting from scratch a whole new design. That is not what I have designed. If you concluded the above I would suggest your reading is in advance of your ability to assimilate the knowledge into successful cogniciense.
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#28

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Paul Barker wrote:Only if you bias it as such and we would then be starting from scratch a whole new design. That is not what I have designed. If you concluded the above I would suggest your reading is in advance of your ability to assimilate the knowledge into successful cogniciense.
Most probably Paul. :wink: :lol:

I'll leave my 811a mutterings for another time, when (hopefully :) ) brain cells A & B have assimilated the required knowledge.
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#29

Post by IslandPink »

I'm looking at that last picture, trying to work out how the DC magnetisation is cancelled in the interstage . Both grids are grounded, both cathodes are positive, the current is going the same way in each half... but is it just down to which way you connect-up the secondary winding ?
Do you have that choice, which just inverts the driving signal , which is no problem ?
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#30

Post by Nick »

what the hell is with the A3. Only person i know who tried to invent that stupidity was Thorston i assumed for marketing purposes. The world does not need another class. In this case if its good enough for rdh its good enough for me.
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