Value of grid leak resistor

[Klimon]

How do you choose the value of the grid leak resistor? With 'common' tubes it isn't not such a problem, aka just do what all the others do, but I'm building a few amps with more uncommon tubes as 6ck4 (seems to work fine with 470k) and 7236 (haven't build it yet). A higher value makes the use of a smaller value coupling cap possible (470k / 0,1µ seems to be a commonly seen combo) but I've just read about thermal runaway requiring to lower the grid leak R with some tubes.

Would it be a good rule of thumb to start with 470k and lower the value if the tube start to draw excessive current or starts hotplating? Does the value of grid resistor affect the sound or is this negligable?

Thanks for your thoughts,

Simon

[Zen Mod]

Originally posted by Klimon
How do you choose the value of the grid leak resistor? With 'common' tubes it isn't not such a problem, aka just do what all the others do, but I'm building a few amps with more uncommon tubes as 6ck4 (seems to work fine with 470k) and 7236 (haven't build it yet). A higher value makes the use of a smaller value coupling cap possible (470k / 0,1µ seems to be a commonly seen combo) but I've just read about thermal runaway requiring to lower the grid leak R with some tubes.

Would it be a good rule of thumb to start with 470k and lower the value if the tube start to draw excessive current or starts hotplating? Does the value of grid resistor affect the sound or is this negligable?

Thanks for your thoughts,

Simon

web is full with toob databases;
your question is certainly covered in every decent data sheet,so its easier way to find data for exact tube.

anyway- 100K to 470K is common range for signal tubes,and you must be cautious only with power tubes

[ray_moth]

A wide range of values is possible with many tubes, but there are exceptions.

For OP beam tetrodes using fixed bias, you are normally advised to use 100k or less, sometimes much less, depending on the tube type. With cathode bias, the grid resistor can be of higher value. Pentode OP tubes have similar rules, except that their grid resistors can generally be higher in value than for tetrodes. As Zen Mod says, look in the tube data sheets.

Be careful of using too high a grid resistor with triodes having high miller capacitance, such as 6SL7 and 12AX7/ECC83. It's best to keep to 100k or less with these tubes, to prevent HF roll-off within the audible frequency range.

[Klimon]

Okay, we'll do. I'm aware of datasheets providing info on grid leak resistors but from what I remember they are recommending values much higher than what seems to be common practice.

Simon

[kevinkr]

I've used 6CK4 extensively in some of my older designs and have never had a problem with these tubes. I generally recommend 100K - 220K grid resistors with these tubes, particularly if run at >300V and fixed bias. With cathode bias 470K should be ok, but I'm always more comfortable with lower grid resistances.

Great sounding tube btw...