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Grid stopper resistor question
Grid stopper resistor question
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Old 15th September 2019, 09:36 AM   #21
JMFahey is offline JMFahey  Argentina
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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.

Just do the Math

* 1k or 1k3 in series with a zillion ohm impedance does little to nothing.

* 1k or 1k3 in series with a 30pF cuts off frequencies above 5.3MHz or 4MHz so way way above Audible range.

Originally Posted by dotneck335 View Post
I am still not getting why the carbon comps are BETTER as a grid stopper resistor than a metal film one. WHAT makes them SOUND better?
Nobody said they sound better
I guess they are better as grid stoppers because in principle they are less inductive than other types and have been favored by Hams for decades now.
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Old 15th September 2019, 10:40 AM   #22
Ultima Thule is offline Ultima Thule  Europe
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How about a good old carbon film, it's non-metallic, the resistive material that is.

Out of curiosity I am wondering too why carbon composition would be a preferable grid stopper, I am aware of its inherent noticeable distortion component which may attract some people for it's subjectively audible niceness, but I would rather thinking of what advantages and properties it has in curbing oscillation and attenuating incoming RF pollution from the ether.
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Old 15th September 2019, 11:17 AM   #23
RajkoM is offline RajkoM  Bosnia and Herzegovina
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Anatoly and Mr Kronye are right.
Only if necessary, install a grid stopper. Then put on that resistor that you like the sound of.
Personally, in tube diy, I always use a Caddock MP915 or MP930 (with ot w/o heathsink) NI metal film resistor.
They are quite cheap at my dealer.

Best diying.
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Old 15th September 2019, 12:16 PM   #24
jhstewart9 is offline jhstewart9  Canada
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One K CC resister thermal noise is in the microVolts, orders of magnitude less than the equivalent noise resistance of the 12AX7. Do the math, all covered in great detail in text books of the vacuum toob age. If there are noise problems you can hear from a power amp with the gain set to zero, better start looking elsewhere for the problem.

A low noise front end requires a Hi-G triode, run a high plate current. A cascode setup works well, low noise & reasonable gain. Usually low noise is only needed for low level sources like magnetic phono pickups.
12AY7 is a low noise alternative to the 12AX7. Pin for pin plugin in, adjust the bias.
Look out for microphonics, the ancient toobs favored by some make good microphones, create lots of headaches for the adventurous.
The cascode example is from the 50's. I tried it, dead quiet.
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Old 15th September 2019, 02:02 PM   #25
kodabmx is offline kodabmx  Canada
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Grid stopper resistor question
FWIW I almost always use 1% MF types of between 180 and 1000 ohms for the grid stopper. I also use metal oxide types for cathode/plate stoppers where necessary (current sharing of parallel tubes mainly), between 10 and 47 ohms. Carbon comp resistors are great because they are easy to see the colour bands on with my eyes, and they "look cool" from a nostalgia perspective, but they aren't great in a humid environment and they cost 100 times more than MF.
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Old 15th September 2019, 04:55 PM   #26
petertub is offline petertub  Sweden
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Grid stopper resistor question
metal and carbon film resistors are spiral formed, this is a intrinsic part of the build and calibrate process.

Carbon composition is massive and has no built-in spiral, thus less inductance.
For use as grid and scree stoppers they should be as little inductive as possible and mounted as close to the grid as possible.
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Old 15th September 2019, 11:19 PM   #27
Wavebourn is offline Wavebourn  United States
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Grid stopper resistor question
Lossy inductance is another way to go. Inductance is enough to stop oscillation, but too small to influence properties on working frequencies. Such stoppers, with few turns through lossy ferrite cores, are used in radio frequency gear.
Single Ended Class-A-Holism is addictive!!!
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Old 15th September 2019, 11:37 PM   #28
jhstewart9 is offline jhstewart9  Canada
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Default Ancient Low Level Toob for Low Microphonics

For those who still want to use a relic in the amp front end the 264 or 864 will fill the low microphonics requirement. Built from the bottom up to be free of microphonics..
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Old 16th September 2019, 02:27 AM   #29
H713 is offline H713  United States
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As has been stated before, CC resistors are as close to non-inductive as you are likely to see. I've never had problems using a metal film resistor as a grid stopper, but that's not to say it can't cause problems. Increasing the value from 1K to 1.3K will probably have a negligible effect in all likelihood. Lots of people just throw a 1K grid stopper on every tube "just to be safe". I agree with Wavebourn- It's better to see if it is actually necessary than to just throw them in everywhere.

There's another point that was mentioned only briefly, and I think deserves a bit more attention. CC resistors handle surge currents (and high voltage) considerably better than other types like metal film and even wirewound. Metal film resistors can instantly open as the laser-cut metal film vaporizes, and I have yet to see a CC resistor do this. Wirewound resistors can also do this, and I have seen it on multiple occasions. If you look at most discharge sticks for very high voltages (10kV+), you will see that (if properly made), they use big carborundum resistors for this very reason. In grid stopper duty, this is a moot point since the current through it is negligible.
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Old 16th September 2019, 02:33 AM   #30
nigelwright7557 is offline nigelwright7557  United Kingdom
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I think its better to include grid stoppers on the pcb first rather than have to cut and bodge the pcb later if RF is a problem.
At 1p each its not going to break the bank.
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