grid leak/ground resistor for 12ax7

[illuminatiNPS]

So I am working on this project that calls for a 12ax7 with no grid to ground. the cathode is biased at 1.5k. After reading up on "grid leak"/ground resistors I found to two theorys. 1) If it is not there it can cause the grid voltage to overtake the cathode and therefore turning the grid into a clipping stage like a diode. 2) It doesn't matter if its there or not if the input jack to the pedal is grounded. My question is being that normally 1M is used, what would be the difference tonewise or gainwise if it is pulled out. This is a high gain pedal that uses a tube. Does not having give more gain??

[merlinb]

It may give slightly more gain if it is not fitted, since it is not loading the pickup. Possibly not much difference though, especially if it's a strat with the usual 250k volume/tone pots fitted.

It is decidedly dogy practice to allow the grid to get its reference from a long cable with a guitar at the other end of it. At the very least I would fit a leak of several megohms close to the tube.

[illuminatiNPS]

definitly not a strat being used. more like an esp with emg 81's. the pedal is techincally a modded big muff with a tube gain stage.  i have one built with a grid leak resistor and it has loads of gain. i wonder by pulling it like the schem says will change it much.

[amptramp]

You would not get much of an increase from eliminating the resistor but you should be aware that a 12AX7 is like two triode sections of a 6AV6 which has been used in radios for ages.  In radio use, the cathode is grounded and the bias comes from contact potential, the impingement of electrons from the cathode on the grid.  In fact, the tube is adequately biased for small signals with 10 megohms on the grid and no cathode resistance and I have seen versions using as little as 6.8 megohms.  Without the 1 meg resistor, the bias point will change slightly with control settings or plugging in different guitars.

I would leave it in so you can perform tests without having to plug in a guitar.

[illuminatiNPS]

well thats the plan. i have one with a 1M grid to ground, and I am building a second one without. I hope they sound the same. here is the first:https://soundcloud.com/nickspera/Biggermuffpedal

[PRR]

> what would be the difference tonewise or gainwise if it is pulled out.

So pull it out! (Or put it in?) There's no "12AX7 leash law". You have the tools. You have the parts. It is a 12 cent experiment!


In some designs the grid leak is omitted because the designer ASS-umes you will plug your guitar into it. Guitar is about a 5K-50K DC leak. When the guitar is not plugged, these designs often use a shorting jack so the grid leak is zero. Everybody happy. (Except now we cascade pedal to pedal and not all have "nice" outputs, with bleed resistors.)

BTW, there is no specified maximum for 12AX7 grid resistance. 10Meg will shift the bias enough to notice, and "open" might shift it too far for good output, but open is legal and safe (below 300V or with 100K to higher voltage). The 12AX7 will not go to a dangerous point. (This is not true for all tubes. An open-grid 12AU7 can find itself run very hot. Open-grid power tubes can melt in a minute.)

_I_ would throw 3Meg on there. This will NOT affect guitar tone (as long as you don't know it is there; have a friend blind A/B it for you).