Preamp 6v6 grid leak resistor value? - diyAudio
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Old 30th August 2009, 04:20 AM   #1
jetbat is offline jetbat  United States
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Default Preamp 6v6 grid leak resistor value?

Hi, I need some help with 6v6 grid leak resistor values for use in a preamp. I got the idea using the 6v6 as a preamp tube from this thread.

6V6 line preamp

With this he is using a 100k pot for the grid leak I think. I am not using a pot in that position so I need to figure out what I can use.

Right now I am using the tube with fixed bias (-20V) and it works fine.
(see picture 1)
Now I want to try it with cathode bias but I'm not sure on the proper value of the grid lead resistor. Picture 2 is what I am thinking of trying.

Also I must say this is for a guitar amp. That is why I have the bias cold so I get some compression and distortion.

Looking a the datasheet, I am not sure which value to use. (see picture 3) Should I use the 500k or the 2.2meg recommendation for the upper limit.

Thanks
Scott
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Old 30th August 2009, 06:11 AM   #2
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I would think 500K would be fine. Sometimes you have to be careful using higher value grid leaks,even when they are suggested on the datasheet,it sometimes depends on the tube.
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Old 30th August 2009, 09:09 AM   #3
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So long as you're using a cathode resistor of 150 ohms or more, you can theoretically go to the 2.2M limit. But if the 6V6 is old and gassy, you may get bias stability problems.

in a guitar amp, set the gridleak R in combination with the coupling cap! The Fender/Marshall approach is 22nF/630V cap and about 220K gridleak. you could use 22n and 470K or 100n/100K according to what you have in a junkbox. All else being equal, a LOW value of gridleak is better, to counter the gas problem. The low values of coupling cap are used in guitar amps to limit the rumbly bass (especially in overdrive channels).

If the 6V6 is connecting to the pickup though (bad idea, gain too low for guitar) you need 1M gridleaks, or the pickup sounds dull.
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Old 30th August 2009, 09:49 PM   #4
jetbat is offline jetbat  United States
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Thanks for the replies.

There is a 12ax7 tube in front of the 6v6 to drive it, so there is plenty of gain. I'm using .047uf coupling caps between the 12ax7 stages and the 6v6. I put a .1uf (what I had laying in front of me at the time) before the 6v6 to isolate the negative bias from a volume pot that I am going to remove. The low end is pretty thick but it sounds good. I was going to try .022uf caps and see how that sounds. As for the grid leak, I'll try the 220k since that is already in there.
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