Grid stoppers in class AB2 push-pull arrangenets?

[Kay Pirinha]

Hi,

for obvious reasons a class AB2 output stage requires a very low inpedance driver to ensure low distortion despite of grid current flowing at some part of the signal cycle. But power tubes also require grid stopper resistors to prevent oscillations. How could we bring these requirements together? Would ferrite beads, soldered directly to the grid lugs of the sockets, instead of resistors, be a solution?

[DF96]

Ferrite beads might work. Lowish value grid stopper resistors should not do much harm as grid current in AB2 won't be huge. You could try anode stoppers instead.

[Michael Koster]

Originally Posted by Kay Pirinha

Hi,

for obvious reasons a class AB2 output stage requires a very low inpedance driver to ensure low distortion despite of grid current flowing at some part of the signal cycle. But power tubes also require grid stopper resistors to prevent oscillations. How could we bring these requirements together? Would ferrite beads, soldered directly to the grid lugs of the sockets, instead of resistors, be a solution?

I measured 50mA grid current peak driving a small transmitting tube into class A2. Even a a 100 ohm resistor would drop 5 volts. Something like this ferrite bead would stop RF oscillation while being transparent at audio frequencies.

[tubelab.com]

I measured 50mA grid current peak driving a small transmitting tube into class A2.

I have seen 250 mA on an 833A. I tend to use a small carbon comp resistor, about 10 ohms with a ferrite bead over each lead. The 6 hole ferrite that you pictured is also a good possibility, but I haven't seen them lately.

[SY]

Try the old ham radio trick- wind 15 turns or so of enameled wire around a 2W carbon comp resistor.

[SpreadSpectrum]

I have just made the leads really short, like mount a mosfet to drive the tube right next to the tube socket on the chassis. I drove triode-connected KT88s in AB2 like this with no oscillations (they are pretty high transconductance tubes, too). It's kind of cheesy to solder to a naked TO-220 package like that. I've found a really cheap PCB manufacturer and am going to have some small boards made to put a source follower driver right next to the grid of a 6L6-pinout tube. Grid stopper optional.

[ElectricMan]

I had five Fisher 90-A monster amps

I was going to be a "Rock Star" and sold all I had

Veelley dumbsky kid!

These amps had,

four EL34s, two 5r4 rects, class AB2, massive transformers, and a sooper wild driver stage that was a 12ax7 to a 12au7, one pair of EL34s biased higher than the other pair, three different feedback paths, very good design

You can look at the schemo for a Fisher 500-C receiver etc. and some of the other companies back then designs for AB2 amps for guidance.

The circuit around the driver tube is very complex

I think Fisher was the god of AB2 design

The 90-A amps could tic out 120 watts peak, and huge bass

Always good to see what the masters did, if anything applies

[Michael Koster]

Originally Posted by SY

Try the old ham radio trick- wind 15 turns or so of enameled wire around a 2W carbon comp resistor.

I can see the color bands now... yellow violet black silver... these were used on plate cap leads, but why not low impedance grid stoppers.

Personally, I have been lucky and used a 6 inch piece of wire with no problems.

[tubelab.com]

I think Fisher was the god of AB2 design

Fisher didn't have mosfet followers. There are a few of us here that can extract serious power from mere mortal tubes. When you have nothing better to do than read 600+ posts we did an AB2 design from the beginning to finished amp from halfway around the world.

6L6GC AB2 Amp