Grid stopper resistor required (KT66)?

[kevinkr]

I use metal films as grid stoppers, and have observed no problems with oscillations up into the VHF range, > 100MHz even in cases where oscillation was observed prior to installation of the stopper resistor.

Carbon composition are lower inductance, but most modern metal film resistors have sufficiently low inductance in the values typically used for grid stoppers that I have never observed a problem.

I use grid stoppers with all high transconductance vhf triodes/pentodes as well as most power pentodes like the EL84 which I have observed merrily oscillating away at >100MHz in some power supply regulators I had recently designed.

Checking to just a few MHz won't tell you much about HF/VHF parasitics, you need a fast scope to be sure or just throw in the grid stoppers as insurance against mischief and less than optimum layout situations. Most cases grid and screen stoppers prevent problems before they start.

 

[zigzagflux]

you need a fast scope to be sure or just throw in the grid stoppers as insurance against mischief and less than optimum layout situations.

Errr, ain't got one of those. 20MHz only kind sir.

What about transformer coupling with short leads to the grids? Would you still install grid stoppers?

 

[Lowrider]

Life is too short, take a chance like I did...

 

[kevinkr]

Errr, ain't got one of those. 20MHz only kind sir.

What about transformer coupling with short leads to the grids? Would you still install grid stoppers?

I'm noted for being a little over the top, but yes I would, and do use grid stoppers in this situation. It isn't a bad question to ask though as to whether or not this is being applied somewhat mindlessly - in the case where you don't have a high speed scope available and in particular are using high transconductance types I would be inclined to just play it safe and use that resistor.

 

[jnb]

You'll likely see 100MHz on a 20MHz scope - it will show lower in level than it should, though. The timebase also won't be too helpful so you'll have to live with the trace being squished.