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Old 19th May 2017, 11:05 AM   #1
bigwill is offline bigwill  United Kingdom
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Default Success or problems with ferrite beads / rings as stoppers?

Say for construction reasons it was awkward or inelegant to solder a resistor directly up against a valve base, do ferrite beads generally work instead? I see it recommended a lot but I don't see it done often. The valves in question are high GM RF types.

I have some tiny toroidals with (presumably lossy) cores rated for EMI suppression. A couple of turns through them gives a measured inductance of around 10uH at 200Khz. Ideally above a certain frequency, a resistive lossy component would dominate, dampening any parasitic resonances.


Does anyone have any first hand experience with this working, or perhaps even making things worse? Any data points would be appreciated.


My confession is I absolutely hate having unsupported resistors poking out of valve sockets, which are then soldered to a wire. I would love to avoid this in a particular scenario where there is nowhere to secure a resistor.
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Old 19th May 2017, 11:59 AM   #2
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If there was an advantage over carbon resistor, I am sure they would have been used from the onset of valves. If you use a 1/8W resistor and sleeve it, there is no weight and no need for support.
Inductors cause HF issues and parasitics, so it seems logical that ferrite beads will also.
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Old 19th May 2017, 12:02 PM   #3
bigwill is offline bigwill  United Kingdom
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Originally Posted by JonSnell Electronic View Post
If there was an advantage over carbon resistor, I am sure they would have been used from the onset of valves. If you use a 1/8W resistor and sleeve it, there is no weight and no need for support.
Inductors cause HF issues and parasitics, so it seems logical that ferrite beads will also.
I fully admit to being awkward here I will probably end up just using resistors. The problem with the tiny ones is their leads bend too much and may end up snapping off - I'd rather use a 1/2w or even 1w as they are physically much studier, and of course heatshrink sleeve them.

For what it's worth the audio amp I am working on (not my design) used high slope RF valves all over the place with NO stoppers and long wire runs.
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Old 19th May 2017, 12:06 PM   #4
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I have used beads as stoppers in many situations.

Using them requires fairly careful design and selection, though.

The usual beads - in the forms of a hollow cylinder that is designed to slip over the lead of a component - can work well on some kinds of FETs, but the frequency range that they work over starts higher than chip-beads or multi-turn leaded beads, and will not be much help with most valves.

The slip-on beads require careful mounting. If they are allowed to move & vibrate around, (some types, at least) can generate noise voltages on the conductor! Very unpleasant. Fix them in place with something.

General considerations:

Think about what frequency range you want to suppress. If you run your circuit with no stoppers, can you provoke oscillation? If so, measure the frequency, and select a bead that offers resistive loss in that band.

Low frequency (<1MHz) oscillation is not a job for beads, and might well be down to poor construction (anode or grid wiring is too long, bad ground layout, supply cap too far away, etc).

Chip beads like Murata BLM21AG series are quite effective at HF, and do a good job of general oscillation suppression - but you need a PCB, or some cut FR-4 to mount them on.

But with grids where no current is expected, carbon composition resistors give trouble-free performance and are probably less hassle.
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Old 19th May 2017, 12:29 PM   #5
bigwill is offline bigwill  United Kingdom
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Quote:
Originally Posted by Rod Coleman View Post
I have used beads as stoppers in many situations.

Using them requires fairly careful design and selection, though.

The usual beads - in the forms of a hollow cylinder that is designed to slip over the lead of a component - can work well on some kinds of FETs, but the frequency range that they work over starts higher than chip-beads or multi-turn leaded beads, and will not be much help with most valves.

The slip-on beads require careful mounting. If they are allowed to move & vibrate around, (some types, at least) can generate noise voltages on the conductor! Very unpleasant. Fix them in place with something.

General considerations:

Think about what frequency range you want to suppress. If you run your circuit with no stoppers, can you provoke oscillation? If so, measure the frequency, and select a bead that offers resistive loss in that band.

Low frequency (<1MHz) oscillation is not a job for beads, and might well be down to poor construction (anode or grid wiring is too long, bad ground layout, supply cap too far away, etc).

Chip beads like Murata BLM21AG series are quite effective at HF, and do a good job of general oscillation suppression - but you need a PCB, or some cut FR-4 to mount them on.

But with grids where no current is expected, carbon composition resistors give trouble-free performance and are probably less hassle.
Many thanks for the reply.

The amp is currently in bits for restoration and as I reassemble it I'm just struck by some questionable layout choices. As far as I remember the amp had no oscillation issues but probably only due to pure luck. If you saw the wiring you would understand.

The amplifier is loop stable but who knows what it could have done in the Mhz region local to the driver stages.

I am probably causing myself more trouble by going with beads and will just have to bite the bullet and use some sensible valued stoppers. Resistors poking out of B9A sockets drive me mad though
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Old 19th May 2017, 12:50 PM   #6
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In most cases I would use a 1/2W metal resistor attached to a single tag, so that it's mechanically secure at both ends.

Sent from my phone. Please excuse any typpos.
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Old 19th May 2017, 01:54 PM   #7
DF96 is offline DF96  England
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Quote:
Originally Posted by bigwill
and of course heatshrink sleeve them
Why?

Small ferrite beads are often used for suppressing VHF/UHF oscillation in HF equipment; they add some resistance but possibly too little for most valves. They may help with some UHF valves, but I suspect the reason resistors are almost always used for audio circuits is that resistors are the best choice.
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Old 19th May 2017, 02:24 PM   #8
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I never use gri stoppers until I detect they are really necessary. I saw many "designers" who put grid stoppers elsewhere and don't know if the circuit really needs it, or they generate further problems.
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Old 19th May 2017, 02:32 PM   #9
kodabmx is offline kodabmx  Canada
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I generally use 1/2W 300R MF resistors, I've soldered them to the socket at the length of the lead without issue. Never had an audible oscillation. Needed 47R plate stoppers on a PPP 6P3S amp though.

I've used ferrite beads but only for a phono stage where the added gain and length of interconnect make it a tuned antenna/radio circuit. Without them I could pick up FM radio with the phono stage.
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Old 19th May 2017, 04:48 PM   #10
bigwill is offline bigwill  United Kingdom
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I fully agree with the sentiment that a normal resistor is the best way to go. The only reason I enquired about the beads is because in a particular situation it may have been neater to use one. If it was my design I would have gone with a resistive stopper and a rigid way to mount it from the start.
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