Grid stopper resistor for 12B4A? - diyAudio
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Old 15th May 2009, 05:48 AM   #1
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Default Grid stopper resistor for 12B4A?

Hi all!. I am only weeks away from having all the parts I need to biuld my 12B4A amplifier but I still have a few questions. I had planned on using 1K grid stopper resistors on the 12B4As. Is this too high a value? The tube will have a B+ of 215v and a current of 27mA. Thanks.
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Old 15th May 2009, 10:29 AM   #2
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It will be fine. The gain of a 12B4 is pretty low, so the Miller capacitance will be small. The most important thing is the physical placement.
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Old 15th May 2009, 06:03 PM   #3
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In other words keep the leads real short?
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Old 15th May 2009, 06:05 PM   #4
Salas is online now Salas  Greece
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Yes, and use carbon composition if possible too. Also the 12B4A has two grid points (2,7) and they both need stoppers in my experience.
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Old 16th May 2009, 01:18 AM   #5
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hey-Hey!!!,
I'd put a 1k carbon comp on each of the two grid pins. Total series resistance is then 500R.
cheers,
Douglas
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Old 16th May 2009, 03:18 AM   #6
ccschua is offline ccschua  Malaysia
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how about using Riken Resistor ? will it improve the sound to be more 'warm'
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Old 16th May 2009, 06:21 AM   #7
mikvous is online now mikvous  Greece
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Hi all,
My experience,could not stop them oscillating with stoppers smaller than 2.7 k at both pins.It mostly depends on the layout .Operating point seems to affect also.I run them with B+ 325V ,Ra 6.8K, Rk 1.5K,aprox 20mA.
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Old 16th May 2009, 03:23 PM   #8
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Quote:
Originally posted by ccschua
how about using Riken Resistor ? will it improve the sound to be more 'warm'

Not a good idea. Carbon composition IS warm sounding and it has the virtue of being both non-metallic and non-inductive. The spiral LASER trimming used in all Carbon film parts, including the "super-duper" Rikens, implies an unwanted inductance.
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Old 16th May 2009, 04:47 PM   #9
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Quote:
Originally posted by mikvous
Hi all,
My experience,could not stop them oscillating with stoppers smaller than 2.7 k at both pins.It mostly depends on the layout .Operating point seems to affect also.I run them with B+ 325V ,Ra 6.8K, Rk 1.5K,aprox 20mA.
hey-Hey!!!,
I've got mine running in a LTP, B+ of 480, plate loads of 20k and ~40 mA of cathode current per pair. The LTP is about the most oscillation-prone circuit I've seen, and with 620R on either pin 2 or 7 it is *NOT* singing anywhere a 100 MHz scope can see.
cheers,
Douglas
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Old 16th May 2009, 05:19 PM   #10
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How about the most transparent grid stop resistor of all?

None.  They're cheap, too!

If it oscillates, then retrench. I have never had a circuit that did not have more than 6dB of NFB oscillate.  My methodology is to build without the things, and test. If it oscillates, start putting grid stops on there starting with 100Ω or so, and test out.

You a experimenter.  So, experiment.  When I performed this experiment, I felt that the degradation of the sound with any grid stop resistor made it very attractive to at least try first, and see if the circuit was gonna get frisky.

Aloha,

Poinz
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