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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Grid stopper resistor for 12B4A?

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G

Member
Joined 2002
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.
 
mikvous said:
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
 
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
AudioTropic
 
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