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Grid stopper resistor question
Grid stopper resistor question
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Old 12th September 2019, 04:03 PM   #1
sumotan is offline sumotan  Indonesia
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Default Grid stopper resistor question

Hi Guys,
My amp input has a 1k grid stopper on the 12ax7. Is it ok if I replace it with a 1.3k resistor instead ? Why so cause I want to try Shinkoh which is only available in 1.3k value.

Thks
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Old 12th September 2019, 04:08 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Grid stoppers are best if they are carbon composition. The value is not critical.
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Old 12th September 2019, 04:11 PM   #3
sumotan is offline sumotan  Indonesia
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Thank you kindly Jon
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Old 12th September 2019, 04:15 PM   #4
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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The so called "grid stoppers" make a low pass filter with the resistor value itself and the inout capacitances of the triode (Grid to cathode, to earth and to plate) plus the miller capacitance, which depends on the stage gain. So, the higher the resistance value, the lower the amp frequency response and worse are the transients. I never use them unless I am fully convinced of the necessary of them. Sometimes, small inductors (some few microhenry) will also do the job, with a double pole in the frequency roll off.
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Old 12th September 2019, 11:46 PM   #5
PRR is offline PRR  United States
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1k (even 1.3k) is not much of a low-pass. And the grid is driven from typically higher impedance than that.

The problem is: you think you are building an audio amplifier. But the tube doesn't know that, and has gain out to MHz. It has internal capacitances, and external wires which act as inductances. Capacitance and inductance is a tuned circuit. Tuned circuit in both plate and grid is a potential oscillator.

1k and 30pFd is 1.3MHz. So it is likely that a tuned circuit with these values will be low Q due to R becoming greater than Z(c). The low-Q tuned circuit is much less likely to support oscillation.
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Old 13th September 2019, 07:21 AM   #6
jhstewart9 is offline jhstewart9  Canada
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The Carbon Composition resistor in the grid lead was often called a Swamping Resister since it lowered the RF Q of the cct. Needs to be 1/2W connected as close to the tube socket as possible.
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Old 13th September 2019, 09:55 AM   #7
DF96 is offline DF96  England
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Grid stoppers don't have to be carbon composition. In some cases a little inductance will acually help the stopper do its job; sometimes a lossy inductor will do a better job.

Grid stoppers don't have to be expensive even if you believe in expensive resistors elsewhere in the amplifier. If you can 'hear' the grid stopper than there is something seriously wrong with your amp design.
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Old 13th September 2019, 10:14 AM   #8
sumotan is offline sumotan  Indonesia
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Your comment that grid stopper resistor does not affect sound got me curious DF96.
Why would it not be when this resistor is directly in the signal path.

Thanks
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Old 13th September 2019, 11:18 AM   #9
pelanj is offline pelanj  Czech Republic
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I bought a DIY amp that was basically a rat's nest wiring - one of the channels seemed to be lower in level than the other. With a scope I found out that the bloody thing is oscillating at ca 600 kHz and removing feedback did not help (I thought there was wrong polarity). I installed 1k grid stoppers on all 4 tubes and voila, the amp works perfectly. So the moral of the story is - I will never have any amp without grid stoppers.

I am with DF96 - the small resistors behave like linear resistance way higher than audio frequencies and since it is linear, you cannot hear any difference - at least that my idea of it.
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Old 13th September 2019, 11:44 AM   #10
SemperFi is offline SemperFi  Wake Island
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Quote:
Originally Posted by sumotan View Post
Your comment that grid stopper resistor does not affect sound got me curious DF96.
Why would it not be when this resistor is directly in the signal path.

Thanks
It is probably that when no signal current goes through the component, the component contributes nothing to the signal. Grid stoppers in HiFi amps have no current through them since the grid is never positive. ClassA2 and AB2 does draw current and some people claim to hear different resistor types.
Musical instrument amplifiers are often over driven (mostly guitar) and ni those cases carbon comp is usually preferred, not just for their great pulse- and overload-tolerance, and non inductive properties, but also for sound.

Substituting grid stoppers with inductance is chancing it. The inductor may have high impedance at the frequency where the tube likes to oscillate, but what if you use an inductor with self resonance below the frequency where the tube can oscillate? It may very well get worse. The whole point of 'place grid stopper right at the pin' is to introduce as little stray inductance as is physically possible. Carbon comps are the best for this purpose. Since no current is passed, their so called excessive noise is not an issue, they give no more noise than the best metal film.
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