Question about grid stopper values

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RhythMick
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#1 Question about grid stopper values

Post by RhythMick »

Reading Merlin Blencowe's website and book about grid stopper value calculation. This page is written primarily with regard to guitar amps, but I think the principles apply to hifi ?

http://www.valvewizard.co.uk/gridstopper.html

As always I find his explanations clear, but if I look at the D3a in triode mode for example...

Cin = Cgk + Cga * (1 + A) = 6.7 + 1 * (1 + 81) = roughly 100pF allowing for strays

However, to achieve a low pass filter with F(-3db) = 20KHz I would need a 74K grid stopper ? Is that right ? I'm fairly sure I've seen suggestions of 100R up to 1K, but nothing like that high.

I could follow his suggestion of putting additional capacitance in parallel with Miller and lower the grid stopper, but this seems odd to me.

What have I missed ?
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Nick
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#2 Re: Question about grid stopper values

Post by Nick »

Why would you want to set the -3dB at 20kHz, the goal is to prevent unwanted HF oscillation. There is some distance between 20kHz and RF.
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izzy wizzy
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#3 Re: Question about grid stopper values

Post by izzy wizzy »

I've never calculated grid stoppers. For d3a I use 680R Allan Bradley carbon. Probably could use lower, down to 330R maybe. For some reason I settled on 680R. 220 to 330 for most other less hot valves.
RhythMick
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#4 Re: Question about grid stopper values

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Nick wrote: Mon Aug 02, 2021 10:47 pm Why would you want to set the -3dB at 20kHz, the goal is to prevent unwanted HF oscillation. There is some distance between 20kHz and RF.
Fair point, I was following Merlin's approach. What would be a sensible cut off?
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#5 Re: Question about grid stopper values

Post by RhythMick »

izzy wizzy wrote: Tue Aug 03, 2021 1:39 am I've never calculated grid stoppers. For d3a I use 680R Allan Bradley carbon. Probably could use lower, down to 330R maybe. For some reason I settled on 680R. 220 to 330 for most other less hot valves.
Thanks. On a related question, why carbon comp for grid stoppers? Is it because they are less inductive?
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#6 Re: Question about grid stopper values

Post by RhythMick »

I happen to have some carbon comp 820R handy so I could use those.

That would put the cut off frequency just shy of 2MHz...?
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Nick
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#7 Re: Question about grid stopper values

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RhythMick wrote: Tue Aug 03, 2021 12:02 pm I happen to have some carbon comp 820R handy so I could use those.

That would put the cut off frequency just shy of 2MHz...?
That would be more like it IMHO. Though you do need to consider thermal noise.
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Nick
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#8 Re: Question about grid stopper values

Post by Nick »

RhythMick wrote: Tue Aug 03, 2021 11:59 am
izzy wizzy wrote: Tue Aug 03, 2021 1:39 am I've never calculated grid stoppers. For d3a I use 680R Allan Bradley carbon. Probably could use lower, down to 330R maybe. For some reason I settled on 680R. 220 to 330 for most other less hot valves.
Thanks. On a related question, why carbon comp for grid stoppers? Is it because they are less inductive?
Well that seems to be the common reason given, not sure it matters, why would a bit of inductance be a bad thing in this situation anyway. Personally I don’t think it matters.it will if you are switching a mosfet bridge at 1Ghz, but we are not.
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#9 Re: Question about grid stopper values

Post by IslandPink »

From practical experience it will matter if you put a cheap blue metal film resistor on there, they sound like crap !
I have used carbon comp mostly but I think carbon films have worked OK too.
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#10 Re: Question about grid stopper values

Post by izzy wizzy »

Weirdly there is a sound difference. I've used cc neohm things and Allen Bradley and prefer the abs. It's only small though.

I've read the spiral cut in films lead to inductance issues but as Nick says, why would that be a problem. Unless the problems is horrible sound due to oscillations which might account for what Mark said about the sound.

It'd be nice to have some definitive answers on this but I've not found any.
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Nick
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#11 Re: Question about grid stopper values

Post by Nick »

Generally small metal film resistors have a inductance of less than 1uH. Not saying they won't sound bad for other reasons but I cant see the inductance being the cause at the frequencies we work at. I find Takman metal films are generally transparent enough If CC noise is a worry.
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izzy wizzy
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#12 Re: Question about grid stopper values

Post by izzy wizzy »

My understanding was that parasitic oscillation occurs at very high frequencies compared to the ones we are interested in so RF frequencies. Which is a very broad band and not very specific but somewhere in the tens of MHz. That's why I thought the inductance did matter. Also why it's very hard to see on regular scopes. Seem to remember Lynn Olson talked about some technique to pick them up. It's way beyond my equipment.

I think some use much higher values of film resistors to overcome this issue where a lower value of CC can be used. Just some stuff that's soaked in over the years. Would be great if there was some numbers we could sling at the problem. Always seems so wishy washy.

Some go as far to ferrite bead the heaters, similar on the plate, specific caps to ground (not direct 0V) on the heaters (I tried that bit; not sure if it did anything).
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Nick
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#13 Re: Question about grid stopper values

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Which is a very broad band and not very specific but somewhere in the tens of MHz. That's why I thought the inductance did matter.
Its hard to find actual values for inductance, but I found a 2-300nH for metal film, though some like PRO2 state non inductive design.I guess the best would be surface mount types soldered to the valve pins, but then the pins and the internal connectors are going to add more inductance.

Ok, so what’s the reactance of 1uH at 100Mhz? 600 ohm. That will maybe matter, but I would have thought for use as a grid stopper inductance would help to form a second order low pass.

I guess I am saying, yes, it may matter, but does it matter that it matters :-)
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#14 Re: Question about grid stopper values

Post by RhythMick »

:D
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izzy wizzy
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#15 Re: Question about grid stopper values

Post by izzy wizzy »

Yup. Couldn't agree more. But then I know way less about RF than audio other than it's bloody tricky stuff. Like a benign piece of wire for us is all kinds of things at RF frequencies even depending on where it is in relation to other things. I'd love to know. Might be time to hit the books.
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