CCS in the output stage

[alexmoose]

Hello all I found something cool that I want to impliment in my new amp, however I need to figure out how to do it properly I want to break-out of the traditional RC coupling circuit, and see if I can get some help from good old "sand" (semiconductors). the set-up for this amp will be PP El34s

I was poking around on Tubelab, looking at the powerdrive circuit, and was wondering if there was a way to adapt it to work with pentodes instead of triodes, or anything similar to regulate the current between the input on the power pentode and the ground.

does anyone know how to do this properly?

does anyone eles think that Tubes, and Transistors can live side-by-side in peace?

[Tweeker]

Mosfet source followers could readily be used to drive pentodes and allow A2/AB2. However, the power gains from doing this are not as great as they are for triodes. Also, the linearity of some pentodes and beam tubes in the grid current region is not great.

Mosfet follies has more.

[ray_moth]

It's been done lots of times, either to run with grid current (class A2 or AB2), or simply to prevent blocking if the grid of the OP tube goes positive. I don't personally think EL34s do well in Class AB2 but some other tubes (e.g. 807 or 6146) do.

In any case, using a tube cf or MOSFET sf to drive the output tubes doesn't necessarily mean that you're going to run all the time with grid current, just that you don't want the amp to distort horribly if it happens momentarily. You could look at it as a bit of extra headroom.

[tubelab.com]

It's been done lots of times, either to run with grid current (class A2 or AB2), or simply to prevent blocking if the grid of the OP tube goes positive. I don't personally think EL34s do well in Class AB2 but some other tubes (e.g. 807 or 6146) do.

I don't know if Ray has actually tested this, or if he is capable of reading my mind from far away. I am currently working on an EL-34 SE amp and I found that PowerDrive did not make enough of a difference to warrant the added complexity. I found no power benefit, and the amp doesn't exhibit any overload problems without PowerDrive. I am running the EL-34 at 420 volts, you may get different results at a lower supply voltage.

I am using the IXYS CCS chip to load the driver tube (12AT7) but I couple it into the grid of the EL-34 with a capacitor and use cathode bias. That way there is no negative supply. Keep the grid leak resistor value low (120K) and the stopper as high as possible to avoid blocking distortion.

I haven't tried the 807 or the 6146.

The hookup for a pentode is the same as a triode. Keep in mind that the power supplies for the PowerDrive need to have enough headroom to cover the entire range of voltage swing seen on the grid of the output tube. For the average output pentode a -100 volt (or larger) and a +50 volt supply should be sufficient.

does anyone eles think that Tubes, and Transistors can live side-by-side in peace?

Of course they can, it is their owners that can't get along!

[Johan Potgieter]

Originally posted by tubelab.com
Of course they can, it is their owners that can't get along!

Capital! Wish I had said that!