Direct-coupled cathode/source follower driver in PP?

[Yvesm]

Originally posted by ray_moth
. . .
In any case, I wouldn't want to run in AB2 for any length of time, but it does seem to offer a bit of useful headroom for transients. Better than ugly clipping, anyway.

That's my faith !

TRUE AB2 calls for lower load and a much beefier psu !

Yves.

[oldeurope]

Hello Yvesm,

Originally #21 posted by Yvesm


That's my faith !

TRUE AB2 calls for lower load and a much beefier psu !

Yves.

I am sorry to tell you that this is not true.
First a transient is not able to change the charge of C2/C4
because the pentode has 82KΩ output resistance.
Second clipping takes place at the grid of V1B/V2B
instad at the output pentodes grid.

The advantage is more output power because of class AB2.
If you don't want AB2, you don't need the cathode followers.
No more headroom, you can choose between clipping at the grids
of the cathode follower and the grid of the output tube.

Kind regards,
Darius

[Bandersnatch]

Originally posted by oldeurope


How?

Kind regards,
Darius

Getting enough voltage swing, and for a grid-current capable cathode follower, delivering power is just like designing a stage for RC coupling. Account for the load, and draw the line across the curves... Now this line won't be a straight one, but putting it onto the proper set of curves( as in don't plan to drive an 813 grid 80V positive with a 12AX7 CF ) is not some black magic art.
cheers,
Douglas

[oldeurope]

Originally #23 posted by Bandersnatch


Getting enough voltage swing, and for a grid-current capable cathode follower, delivering power is just like designing a stage for RC coupling. ...
cheers,
Douglas

Hi Douglas

Thanks, but I think we misunderstood.
My question is how to prevent "blocking distortion" caused by charging C2/C4
when the grid of V1B/V2B takes current. See schematic #7

Kind regards,
Darius

[Bandersnatch]

Originally posted by oldeurope


Hi Douglas

Thanks, but I think we misunderstood.
My question is how to prevent "blocking distortion" caused by charging C2/C4
when the grid of V1B/V2B takes current. See schematic #7

Kind regards,
Darius

hey-Hey!!!,
That is the answer I gave. Make sure, by the usual and traditional means that the stage never needs the positive grid drive. It is not a given that it will be required. At some point the amp will run out of power, that is inescapeable. It is desired that this power limit be one developed in the output stage( usually anyway ).

That requires setting the operating conditions and selecting device parameters to fill the need. You'd need to do the same thing if we had a MOSFET there, save the design would keep it away from gate-source saturation voltage, yes?
cheers,
Douglas

[SY]

You'll get the blocking distortion at the grid of the cathode follower.

Any competent power amp design clips first at the output stage grids. So if the output stage needs (say) 70V p-p before getting to grid current, having a driver and CF that can swing 140V p-p gives one 6dB of overload margin before there's any blocking in the driver. And that's a very easy figure to obtain. With that much overload margin, the output stage distortion will be so gross that the CF is the least of your worries.

For a very detailed dissection of this issue, I'd highly recommend you read the Crystal Palace design brief in Morgan Jones's "Valve Amplifiers," 3rd ed. MJ has not been enthusiastic in the past about taking the CF beyond anti-blocking and going for AB2, but I suspect that when the 4th edition emerges, there might be some change in his thinking. In my own experience, it all depends on output tubes- I have gotten excellent results with 6L6 types in AB2.

[oldeurope]

Hi Douglas

An AB2 output tube driven n Volts positive by a cathode follower is equal to
AB1 with increased g2 Voltage. The additional g2 voltage is n Volts multiplied by µ(g2).
This is the reason why AB2 is not necessary in a pentode topology.
You don't need a cathode follower to drive a pentode.

Make sure, by the usual and traditional means that the stage never needs a grid drive voltage
more than say minus 5V. Same result, without follower.
In this case add 5V x µ(g2) to the traditional g2 DC level.
You don't need a follower, that's fine isn't it?

Kind regards,
Darius

[oldeurope]

Originally posted by SY


Any competent power amp design clips first at the output stage grids. So if the output stage needs (say) 70V p-p before getting to grid current, having a driver and CF that can swing 140V p-p gives one 6dB of overload margin before there's any blocking in the driver. And that's a very easy figure to obtain. With that much overload margin, the output stage distortion will be so gross that the CF is the least of your worries.

...

Hi SY,
I think this will kill the output tube.

Kind regards,
Darius

[SY]

What will kill the tube? Pentode AB2? See the bottom of 4-57 and the top of 4-58.

http://frank.pocnet.net/sheets/049/6/6L6GB.pdf

If you're implying that having overload margin in the driver will kill the output tube, I'd love to see some actual evidence of that.

[Bandersnatch]

Originally posted by oldeurope
Hi Douglas

An AB2 output tube driven n Volts positive by a cathode follower is equal to
AB1 with increased g2 Voltage. The additional g2 voltage is n Volts multiplied by µ(g2).
This is the reason why AB2 is not necessary in a pentode topology.
You don't need a cathode follower to drive a pentode.

Make sure, by the usual and traditional means that the stage never needs a grid drive voltage
more than say minus 5V. Same result, without follower.
In this case add 5V x µ(g2) to the traditional g2 DC level.
You don't need a follower, that's fine isn't it?

Kind regards,
Darius

What you describe is a means to remove the need for g1 current through careful design. One of my first steps along this design path is to eliminate the possibility of cut-off and anything associated with AB/B operation. Plate loading, B+ and plate dissipation deliver me the line of possibility...

If one needs more power, get bigger plates...and plate size can comfortably overwhelm the ability of a TX winder to deliver an OPT that can simultaneously attain this high ouput while not throwing away the low-power sonics. Let alone having efficient speakers not to sanely need much power...heh-heh-heh.

The 'stage __ will clip before the finals' being a bad thing is a bit of a red herring. The amp will clip at max power. Why it is clipping is not important. It may be that it is power limited by its driver, but that does not automatically qualify it as a 'bad' amp.
cheers,
Douglas