Grid resistors to power tubes

[dude]

Just curious about values, some older amps didn't even have them. How does the value effect tone, say between 1K5 and 5K6? Probably nothing...


al 

[jjasilli]

http://www.aikenamps.com/index.php/grid-resistors-why-are-they-used


http://www.aikenamps.com/index.php/what-is-blocking-distortion

[tubeswell]

47k is okay for commonly used output tube types (6V6, 6L6, EL34)

[dude]

According to Aiken, grid stoppers on the power tunes of 6L6's are usually 1K5 and Marshalls 5k6 and that difference is not going to cause any oscillations. Basically, no I won't notice any difference in tone, highs or distortion. I used 5K6 in a 6V6 plexi conversion, as I may want to try EL34's, since the AO43 Hammon iron I'm using ran a 5U4 and I'm running a 5AR4, I should be good with the filament draw, as far as the draw from the EL34's, I'll have to keep a watch on the PT giving off heat.


Thanks for the links,
al 

[jjasilli]

Sometimes large grid stoppers are used with EL84's to address a brittle or "glassy" tone. 


The grid stopper resistance value works with the tube's internal Miller capacitance to form an RC filter which reduces hi's.  It can be used to kill RF noise; tame hi's in the audio range; or address blocking distortion. 



Operationally, the grid stopper value adds to the grid leak value; the sum total should not exceed the tube specs for G1 resistance to ground.


I think they're used to correct a problem, rather than for actual tone shaping.

[PRR]

If you OVERdrive a tube, with R-C coupling, the forward bias grid conducts like a rectifier. This makes a DC voltage which biases the tube more-Off. With hard flailing it will go to cut-off, "grid block", and go silent for a bit after a hard transient.

Williamson pointed out that a series resistor 0.2X to 0.5X the grid bias resistor decreases the de-biasing by roughly balancing charges. In hi-fi with occasional grid clip this is more polite. For Pentodes, and even Williamson's triodes, the loss of highs due to grid capacitance is not large for the usual resistor value, 56K.

In gee-tar we DO want the ability to grid-clip and flirt with grid-block. Since amount of overdrive is at the player's fingertips and (in)discretion, we do not need or want an "ideal" balance. Values like 1K to 10K seem to be useful. There may be some designer's-taste to this, because as noted different brands tend to different values.

The EL84 is about 1.8X as prone to grid-blocking because it has 1.8X the sensitivity of the other bottles, and may need special consideration.