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Grid Stopper Resistors.

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Posted on January 7, 2017 at 11:42:35
sony6060
Audiophile

Posts: 1465
Location: USA
Joined: August 8, 2014
 Ultra high gain tubes need grid stopper resistors. Typical value is 1K carbon comp resistors.

What about low gain such as a 6SN7 tube & would a grid stopper alter the sound? Or 6V6GT power tubes in tetrode or triode connection. Tetrode has about 6db negative feedback, triode no feedback.

 

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RE: Grid Stopper Resistors., posted on January 7, 2017 at 12:01:51
Tre'
Industry Professional

Posts: 17297
Location: So. Cal.
Joined: February 9, 2002
 "What about low gain such as a 6SN7 tube & would a grid stopper alter the sound?"

If a grid stop resistor alters the sound (other than stopping RF oscillation) then it's value is too high.

The value of a grid stop resistor and the value of the tube's Miller capacitor forms a low pass filter.

The -3db point of that filter should be at least 200kHz. That will stop RF but let all the music pass without shifting the phase.

In practice you only what to use enough resistance to stop the oscillation.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Grid Stopper Resistors., posted on January 8, 2017 at 08:39:57
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 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.

 

RE: Grid Stopper Resistors., posted on January 8, 2017 at 09:03:35
sony6060
Audiophile

Posts: 1465
Location: USA
Joined: August 8, 2014
 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.

 

RE: Grid Stopper Resistors., posted on January 8, 2017 at 15:52:51
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 In my se amps with 6sn7 driver tube, i don't use a grid stopper. There doesn't seem to be any problem with oscillation or RFI.

 

RE: Grid Stopper Resistors., posted on January 10, 2017 at 05:41:03
Triode_Kingdom
Audiophile

Posts: 10045
Location: Central Texas
Joined: September 24, 2006
 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. :)


 

RE: Grid Stopper Resistors., posted on January 10, 2017 at 10:26:57
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 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!

 

RE: Grid Stopper Resistors., posted on January 10, 2017 at 22:33:43
Triode_Kingdom
Audiophile

Posts: 10045
Location: Central Texas
Joined: September 24, 2006
 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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