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In Reply to: RE: Grid Stopper Resistors. posted by sony6060 on January 07, 2017 at 11:42:35
High transconductance, not voltage gain (mu), is what drives oscillation risk.
In my experience, 1-2mA/v rarely needs a grid stopper, 5mA/v (like a 2A3) definitely needs one, and over 10mA/v may need more than just a grid stopper.
Follow Ups:
That's good info, Paul. What surprises me is how many older manufacturers didn't use stoppers at high transconductance outputs like 6BQ5s. Engineers don't always get these things right, even when they're paid to do so. :)
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Buy Chinese. Bury freedom.
Edits: 01/10/17
My most memorable incident occurred ~15 years ago, with a 2A3 amp I had just built. Everything looked good, but it had a distorted buzzy sound and the voltage measurements made no sense - they were not even self-consistent. I had spent an hour or two in the lab trying to figure out what the problem was, before I noticed the soldering iron. It had an LED array showing the temperature, and the LEDs were flickering. I shut off the amp and the LEDs stabilized, but they resumed when I switched the amp back on. The amp was apparently making enough radio frequency noise to cause the flickering. Adding a 1K grid stopper immediately fixed the problem, the sound was great, hum/buzz gone. Lesson learned!
Sounds as though you built a 2A3 transmitter! I've had odd experiences with oscillations too. It pays to take precautions.
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Buy Chinese. Bury freedom.
Ok Paul. The 6SN7 transconductance is about 2700 & the 6V6GT is 5000. I will add grid stoppers to the 6V6. The 6SN7 does not need grid stoppers.
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