Screen Grid Regulation Requirements

[dsavitsk]

I am thinking about putting together a little EL84 p-p power amp. One thing I am unsure about is the voltage regulation for the screen grid supply. In his RLD design, regarding the screen grid supply, SY notes "In this case, the requirements of ultra-low noise and ultra-low source impedance are not particularly important," however, elsewhere, Tubelab has suggested "The screen grid supply needs to be stiff, and is often regulated."

On my amp, I am using a linear regulator at about 320V for B+. I am also adding jumpers to switch the tubes from triode mode, where a resistor will connect the screen to the plate, to Gm (pentode) mode, where the screens will have their own regulator at around 280V. Presumably, the same resistor that ties the screens to the plate in triode mode will act as a stopper in Gm mode, which is the basis for the question. This resistance will be in series with the screen regulator, which suggests that a low Z regulator can't be a big deal. On the other hand, since there is a single regulator for both tubes, if the Z is too high, this could result in crosstalk, or something. I am thinking a small CCS + Zener to ground is a good option, but I am concerned that the Z is too high.

More info, in case it is relevant, input stage is a long tailed pair (12A_7 based), output stage biasing can be switched between a CCS (for Class A) and a low Z shunt reg (for AB) and there is a spot to add feedback in a way similar to how it is done in the El Cheapo -- basically I am working an PCB that can be used for a bunch of different EL84 topologies to make comparing them easier.

Thoughts?

[dcgillespie]

Class A operation always minimizes power supply requirements regarding its output impedance, so your screen regulator will have the least effect in that mode. For class AB however, regulated screens for pentode operation is the only way to go. Distortion is minimized, while power output is maximized. The impact of any typical value screen stability resistors used in any mode of operation will be nill on actual performance.

Dave

[SY]

Originally Posted by dsavitsk

Presumably, the same resistor that ties the screens to the plate in triode mode will act as a stopper in Gm mode, which is the basis for the question. This resistance will be in series with the screen regulator, which suggests that a low Z regulator can't be a big deal.

You have my logic completely. If the Z is low compared with the screen resistor, your work is done. I don't think George and I are saying anything different: you want a decent amount of regulation, but the difference between a few ohms and a few milliohms is lost when there's several hundred ohms of stopper resistor.