is it really required to use carbon comp for grid stoppers?

[jarthel]

i've read various discussions in AA saying grid stoppers must be carbon comps. But i've seen response audio using riken carbon film as mod to ASL amps. i've seen other articles (mostly diy homepages) using metal films as grid stoppers.

does that mean these people are doing it wrong? I asked since carbon comps are unavailable locally. that means I have to go ebay usa or places that sell carbon comps (thlaudio, handmade and etc). and I only need 1 resistor.

thank you.

[Brian Beck]

Another topic where everyone has an opinion. A grid-stopper is used to isolate/damp any inductive reactance in the grid circuit that could interact with the Miller capacitance (which is itself a form of negative feedback). The inductive reactance can come from the grid wiring itself. Essentially we’re lowering the Q of the resonance circuit that results. An undamped resonance can create peaks in the ultrasonic response, or even oscillation, especially in high gm tubes. So you want to use a low inductance (and low shunt capacitance) resistor for a grid-stopper to avoid making the problem even worse. Carbon comps have their problems when used elsewhere, but here they behave like a resistor out to VHF and even UHF frequencies, which is desirable. For obvious reasons, wire-wounds should not apply for grid-stopper duty. Other resistor technologies such as metal film and carbon film will probably work most of the time, depending on how inductive they are (spiral laser cuts etc.)

Ferrite beads can be used too, since they become lossy and mimic a low-value resistor at RF frequencies, but the ferrites are non-linear, and that gives some people a concern (although the effect happens at many tens of megahertz).

[Tweeker]

Cathode followers can be a touchier case than some. Grid stopper is more effective with lower inductance, keep leads short, solder directly to socket if possible.

[dhaen]

Hmm,

I see a contradiction in the argument here:
As a grid-stopper, lets use a non inductive resisitor
or an inductor (ferrite bead)...

You might argue that a ferrite bead is a rather lossy inductor (which is good), but then again, so is spiral cut resistor....

[Brian Beck]

A ferrite bead (of the kind we're suggesting here) becomes more resistive than inductive over a wide range of RF frequencies. People mistake this part as an inductor when it really acts more like a "short" at audio frequencies and a small resistor at RF (paradoxically). You will see them used extensively in tube test equipment, even tube testers, for this reason. The inductive component of the ferrite bead, especially with a simple straight pass-through of the wire (which makes a single turn) can be quite small.

I'm sure it's possible to find a combination of ferrite bead, tube, circuit and layout that will oscillate, just as it possible to find a combination with a resistor that will also oscillate. There are many variables, and experimentation with grid stopping is always the best proof. If you have a wide-bandwidth scope, run 10kHz (or higher) square waves through the amp while varying the amplitude. Also try sine wave sweeps out to the megahertz region, looking for peaking in amplitude response or bursts of oscillation on portions of the waveforms.

[poobah]

The primary property of ferrite beads is resuctance. It does not lend itself well to conventional analysis. But, an assortment of the little guys is very handy for solving all sorts of high frequency and noise problems. Empirical trial and error is the best way to find the right one... the results are often incredible.

I use a ton of these things to keep little signals clean in the presence of big ones.

[Brian Beck]

I found a short explanation from Murata that might help:

Murata ferrite bead explanation

BTW, I'm not pushing ferrite beads as audio grid stoppers, just suggesting their possible consideration along with simple resistors. They have the advantage of less audio bandwidth reduction compared to simple grid stopper resistors (especially larger ones), but the non-linearity is worrisome. I haven't spent enough time making listening comparisons to say for sure. I usually use carbon comps myself, as small as I can get away with. Often that means NO grid stopper at all. Keep lead lengths short and grounds beefy and close by. For cathode followers, Tweeker’s points are well taken. In addition, make sure to bypass the plate of a CF to ground with a small, decent quality cap to keep the plate shorted at RF frequencies.

[dhaen]

Trolling away on the crest of a wave ...

So a ferrite bead is a lossy inductor then?

[Brian Beck]

Originally posted by poobah
The primary property of ferrite beads is resuctance.

I suspect you meant "reluctance", or perhaps you've invented a new term to decribe what you hear with ferrite beads?

[Brian Beck]

Originally posted by dhaen
So a ferrite bead is a lossy inductor then?

Yes, very lossy.