How many grid stoppers do I need ?

[nigelwright7557]

I understand the need for a grid stopper to stop RF pick up.
I have a mixer with two valves so do I need a grid stopper on every grid or just the first one in the mixer?
One of my stages is a cathode follower, does this need a grid stopper too ?

 

[BinaryMike]

We're often concerned about RF oscillation within the amplifier more than external RFI sources. My personal approach to this problem is to own a scope with 300~500MHz bandwidth so spurious oscillations cannot hide from me and I can avoid grid stopper overkill. Be aware that touching a probe to some circuits can stop an oscillation, so it's wise to use the probe more like an antenna when troubleshooting. I have a 3GHz RF spectrum analyzer as well, but I've never confirmed a spurious oscillation in an audio amplifier in the GHz range.

 

[nigelwright7557]

I am using two 12AU7's in series in a mixer/pre amp.
Three of the stages are output from anode and the other stage is a cathode follower.
Due to price I am just using a plastic enclosure so will have to watch out for outside radiation affecting the circuit.

 

[Wavebourn]

None, if their absence does not cause oscillations.

 

[gingertube]

12AU7 gm is 2.2mA/V
Recommended minimum grid stop is 8/gm or 3k6 in this case.
Just bung 3K9 gridstops on each triode including the cathode follower.
They may not be needed but will not do any harm.
Cheers,
Ian

 

[artosalo]

I have experienced grid (g1) stoppers necessary only with some high gm pentodes.
Never with medium mu triodes.

 

[JMFahey]

IF you have internal oscillation problems firstyou need to improve separation, shielding and grounding rather than applying a band aid solution.

At least in my experience, the main utility of grid stoppers resides in stopping external RF interference, mainly Radio stations.

 

[DF96]

The primary purpose of a grid stopper is to stop parasitic RF oscillation. For this it is not a "band aid" but the correct tool for the job; it adds positive resistance to overcome the negative resistance seen at a valve port. Only when unusually high values are needed can it be seen as a substitute for good layout. Note that grid stoppers may increase the risk of feedback loop instability, by adding phase shift.

A secondary purpose is to filter out incoming RF, but a proper filter may be better.

 

[Wavebourn]

I have experienced grid (g1) stoppers necessary only with some high gm pentodes.
Never with medium mu triodes.

Sometimes a cap from grid to the ground is better than a stopper.

 

[nigelwright7557]

There is no feedback in the mixer so phase shift isn't a problem.
I have added a 10k to each grid input right at the valve.
I have included the layout below minus the copper pours.
The ground line is in violet to highlight it.

An externally hosted image should be here but it was not working when we last tested it.

 

[Pegaso]

Sometimes a cap from grid to the ground is better than a stopper.

same Audio Note and others brands have 220pf grid to ground

 

[DF96]

For a cathode follower the best place for a stopper may be the cathode.

 

[hego]

I have experienced grid (g1) stoppers necessary only with some high gm pentodes.
Never with medium mu triodes.

Never???

For my experience, the grid-stoppers resistors haven a other propietie: dont block the prevoius cap . The sound is ever quick !!!

I use them in all my tubes for this reason.

Cheers